Screen-space Water Rendering

The latest in the series of tech art / rendering problems I’ve been working on is finding a good solution to rendering water. Specifically, rendering narrow, fast-moving particle-based streams of water. Over the past week, I think I’ve gotten some good results, so I’m going to write it up here.

I don’t really like voxelized / marching cubes approaches to water rendering (see, for example, Blender’s fluid simulation rendering). When the water volume is on a similar scale to the underlying grid used to render, the motion is noticeably discrete. This can be addressed by increasing the grid resolution, but for a narrow stream over relatively long distances this simply isn’t practical in realtime without serious compromises in runtime and memory usage. (There is some precedent using sparse octree structures in voxel rendering, which improves this. I’m not sure how well that works for dynamic systems, and in any case it’s more complexity than I want to deal with.)

The first alternative I looked at was Müller’s “Screen Space Meshes”. This involves rendering the water particles to a depth buffer, smoothing it, identifying connected patches of similar depth, and then building a mesh from the result using marching squares. This is probably *more* feasible now than it was in 2007 (as you can reasonably build the mesh in a compute shader), but still more complexity and cost than I’d like.

Finally, I stumbled upon Simon Green’s 2010 GDC presentation, “Screen Space Fluid Rendering For Games”. This begins the same way Screen Space Meshes does: render particles to a depth buffer and smooth it. But rather than build a mesh, use the resulting buffer to shade and composite the liquid over the main scene (by explicitly writing depth.) This was what I decided to implement.

Setup

The past few projects in Unity have taught me not to fight its rendering conventions. So, the fluid buffers are rendered by a second camera, with lower camera depth so that it renders before the main scene. Any fluid system lives on a separate render layer; primary camera excludes the fluid layer, while the second camera only renders the fluid. Both cameras are then parented to an empty object to keep them aligned.

This setup means I can render pretty much anything to the fluid layer, and it should work as expected. In the context of my demo scene, this means that multiple streams and splashes from subemitters can merge together. This should also allow for mixing in other water systems, like heightfield-based volumes, which will then be rendered consistently. (I have not tested this yet.)

The water source in my test scene is a standard particle system. No actual fluid simulation is being performed. This in turn means that particles overlap in a not-quite-physical way, but the final shading looks acceptable in practice.

Fluid Buffer Rendering

The first step in this technique is rendering the base fluid buffer. This is an offscreen buffer that contains (at this point in my implementation): fluid thickness, screen-space motion vector, and a noise value. We also render a depth buffer, explicitly writing depth from the fragment shader to turn each particle quad into a spherical (well, ellipsoidal) blob.

The depth and thickness calculations are straightforward:

frag_out o;

float3 N;
N.xy = i.uv*2.0 - 1.0;
float r2 = dot(N.xy, N.xy);
if (r2 > 1.0) discard;

N.z = sqrt(1.0 - r2);

float4 pixel_pos = float4(i.view_pos + N * i.size, 1.0);
float4 clip_pos = mul(UNITY_MATRIX_P, pixel_pos);
float depth = clip_pos.z / clip_pos.w;

o.depth = depth;

float thick = N.z * i.size * 2;

(The depth calculation can be simplified, of course; we only need z and w from the clip position.)

We’ll come back to the fragment shader for motion vectors and noise in a bit.

The vertex shader is where the fun starts, and where I diverge from Green’s technique. The goal of this project is rendering high-speed streams of water; this is possible with spherical particles, but takes an inordinate number of them to produce a continuous stream. Instead, I stretch the particle quads based on velocity, which in turn stretches the depth blobs to be elliptical, not spherical. (Since the depth calculation is based on UV, which is unchanged, this Just Works.)

Experienced Unity developers may be wondering why I’m not using the built-in Stretched Billboard mode provided by Unity’s particle system. Stretched Billboard stretches unconditionally along the velocity vector in world space. In general this is fine, however it causes a very noticeable problem when the velocity vector is aligned to (or very close to) the camera’s forward vector. The billboard stretches *into* the screen, very clearly highlighting its 2D nature.

Instead, I use a camera-facing billboard and project the velocity vector onto the plane of the particle, using that to stretch the quad. If the velocity vector is perpendicular to the plane (into or out of the screen), the particle stays unstretched and spherical, exactly as it should, while velocity across the screen causes the particle to stretch in that direction, as expected.

Long explanation aside, it’s quite a simple function:

float3 ComputeStretchedVertex(float3 p_world, float3 c_world, float3 vdir_world, float stretch_amount)
{
    float3 center_offset = p_world - c_world;
    float3 stretch_offset = dot(center_offset, vdir_world) * vdir_world;

    return p_world + stretch_offset * lerp(0.25f, 3.0f, stretch_amount);
}

In order to calculate the screenspace motion vector, we actually compute two sets of vertex positions:

float3 vp1 = ComputeStretchedVertex(
    vertex_wp,
    center_wp, 
    velocity_dir_w, rand);
float3 vp0 = ComputeStretchedVertex(
    vertex_wp - velocity_w * unity_DeltaTime.x,
    center_wp - velocity_w * unity_DeltaTime.x,
    velocity_dir_w, rand);

o.motion_0 = mul(_LastVP, float4(vp0, 1.0));
o.motion_1 = mul(_CurrVP, float4(vp1, 1.0));

Note that, since we are calculating motion vectors in the main pass, rather than the motion vector pass, Unity will not provide the previous or non-jittered current view-projection for you. I attached a simple script to the relevant particle systems to fix this:

public class ScreenspaceLiquidRenderer : MonoBehaviour 
{
    public Camera LiquidCamera;

    private ParticleSystemRenderer m_ParticleRenderer;
    private bool m_First;
    private Matrix4x4 m_PreviousVP;

    void Start()
    {
        m_ParticleRenderer = GetComponent();
        m_First = true;
    }

    void OnWillRenderObject()
    {
        Matrix4x4 current_vp = LiquidCamera.nonJitteredProjectionMatrix * LiquidCamera.worldToCameraMatrix;
        if (m_First)
        {
            m_PreviousVP = current_vp;
            m_First = false;
        }
        m_ParticleRenderer.material.SetMatrix("_LastVP", GL.GetGPUProjectionMatrix(m_PreviousVP, true));
        m_ParticleRenderer.material.SetMatrix("_CurrVP", GL.GetGPUProjectionMatrix(current_vp, true));
        m_PreviousVP = current_vp;
    }
}

I cache the previous matrix manually because Camera.previousViewProjectionMatrix gives incorrect results.

¯\_(ツ)_/¯

(This method also breaks render batching; it may make more sense in practice to set global matrix constants instead of per-material.)

Back in the fragment shader, we use these projected positions to calculate screenspace motion vectors:

float3 hp0 = i.motion_0.xyz / i.motion_0.w;
float3 hp1 = i.motion_1.xyz / i.motion_1.w;

float2 vp0 = (hp0.xy + 1) / 2;
float2 vp1 = (hp1.xy + 1) / 2;

#if UNITY_UV_STARTS_AT_TOP
vp0.y = 1.0 - vp0.y;
vp1.y = 1.0 - vp1.y;
#endif

float2 vel = vp1 - vp0;

(The motion vector calculations are taken almost verbatim from https://github.com/keijiro/ParticleMotionVector/blob/master/Assets/ParticleMotionVector/Shaders/Motion.cginc)

Finally, the last value in the fluid buffer is noise. I use a per-particle stable random value to select one of four noises (packed into a single texture). This is then scaled by the speed and one minus the particle size (so faster and smaller particles are noiser). This noise value is used in the shading pass to distort normals and add a layer of foam. Green’s presentation uses 3-channel white noise, while a followup paper (“Screen Space Fluid Rendering with Curvature Flow”) suggests octaves of Perlin noise. I use Voronoi/cellular noise at various scales:
_voronoi_packed_jg_4_8_16_32

Blending Problems (And Workarounds)

Here is where the first problems with my implementation emerge. In order to calculate thickness correctly, particles are blended additively. Because blending affects all the outputs, this means the noise and motion vectors are also additively blended. Additive noise is fine; additive motion vectors are *not*, and left as-is produce ridiculous results for TAA and motion blur. To counteract this, I multiply the motion vectors by thickness when rendering the fluid buffer, and divide by total thickness in the shading pass. This produces a weighted average motion vector for all overlapping particles; not exactly what we want (produces some weirdness where streams cross each other), but acceptable.

A more difficult problem is depth; in order to render the depth buffer correctly, we need both depth writes and depth tests active. This can cause problems if our particles are unsorted (as differences in rendering order can cause particles behind others to have their output culled). So, we tell Unity’s particle system to keep particles ordered by depth, then cross our fingers and hope systems render by depth as well. There *will* be cases when systems overlap (i.e, two streams of particles intersecting) that will not be handled correctly, which will result in a lower thickness than expected. This doesn’t seem to happen very often in practice, and doesn’t make a huge difference in appearance when it does.

The proper approach would likely be rendering the depth and color buffers completely separately — at the cost of now having to render two passes. Something to investigate when tuning this.

Depth Smoothing

Finally, the meat of Green’s technique. We’ve rendered a bunch of spherical blobs to the depth buffer, but water is not “blobby” in reality. So now, we take this approximation and blur it to more closely match how we expect the surface of a liquid to look.

The naive approach is to apply a Gaussian blur to the entire depth buffer. This produces weird results — it smooths distant points more than close ones, and blurs over silhouette edges. Instead, we can vary the radius of the blur by depth, and use a bilateral blur to preserve edges.

There’s just one problem: these adjustments make the blur non-separable. A separable blur can be performed in two passes, blurring horizontally and then vertically. A non-separable blur is performed in a single pass, blurring both horizontally and vertically at the same time. The distinction is important because separable blurs scale linearly (O(w) + O(h)), while non-separable blurs scale quadratically (O(w*h)). Large non-separable blurs quickly become impractical.

As mature, responsible engineers, we do the obvious thing: plug our ears, pretend the bilateral blur *is* separable, and implement it with separate horizontal and vertical passes anyway.

Green demonstrates in his presentation that, while this *does* generate artifacts in the result (particularly when reconstructing normals), the final shading hides them well. With the narrower streams of water I’m generating, these artifacts seem to be even less common, and don’t have much effect on the result.

Shading

At last, the fluid buffer is done. Now for the second half of the effect: shading and compositing with the main image.

Here we run into a number of Unity’s rendering limitations. I choose to light the water with the sun light and skybox only; supporting additional lights requires either multiple passes (expensive!) or building a GPU-side light lookup structure (expensive and somewhat complicated). In addition, since Unity does not expose access to shadow maps, and directional lights use screenspace shadows (based on the depth buffer rendered by opaque geometry), we don’t actually have access to shadow information for the sun light. It’s possible to attach a command buffer to the sun light to build us a screenspace shadow map specifically for the water, but I haven’t yet.

The final shading pass is driven via script, and uses a command buffer to dispatch the actual draw calls. This is *required*, as the motion vector texture (used for TAA and motion blur) cannot actually be bound for direct rendering using Graphics.SetRenderTarget(). In a script attached to the main camera:

void Start() 
{
    //...
    
    m_QuadMesh = new Mesh();
    m_QuadMesh.subMeshCount = 1;
    m_QuadMesh.vertices = new Vector3[] { 
        new Vector3(0, 0, 0.1f),
        new Vector3(1, 0, 0.1f),
        new Vector3(1, 1, 0.1f),
        new Vector3(0, 1, 0.1f),
    };
    m_QuadMesh.uv = new Vector2[] {
        new Vector2(0, 0),
        new Vector2(1, 0),
        new Vector2(1, 1),
        new Vector2(0, 1),
    };
    m_QuadMesh.triangles = new int[] {
        0, 1, 2, 0, 2, 3,
    };
    m_QuadMesh.UploadMeshData(false);

    m_CommandBuffer = new CommandBuffer();
    m_CommandBuffer.Clear();
    m_CommandBuffer.SetProjectionMatrix(
        GL.GetGPUProjectionMatrix(
            Matrix4x4.Ortho(0, 1, 0, 1, -1, 100), false));
    
    m_CommandBuffer.SetRenderTarget(
        BuiltinRenderTextureType.CameraTarget, 
        BuiltinRenderTextureType.CameraTarget);
    m_CommandBuffer.DrawMesh(
        m_QuadMesh, Matrix4x4.identity, m_Mat, 0, 
        m_Mat.FindPass("LIQUIDCOMPOSITE"));

    m_CommandBuffer.SetRenderTarget(
        BuiltinRenderTextureType.MotionVectors, 
        BuiltinRenderTextureType.Depth);
    m_CommandBuffer.DrawMesh(
        m_QuadMesh, Matrix4x4.identity, m_Mat, 0, 
        m_Mat.FindPass("MOTION"));   
}

The color and motion vector buffers cannot be rendered simultaneously with MRT, for reasons I have not been able to determine. They also require different depth buffer bindings. We write depth to *both* of these depth buffers, incidentally, so that TAA reprojection works properly.

Ah, the joys of a black-box engine.

Each frame, we kick off the composite render from OnPostRender():

RenderTexture GenerateRefractionTexture()
{
    RenderTexture result = RenderTexture.GetTemporary(m_MainCamera.activeTexture.descriptor);
    Graphics.Blit(m_MainCamera.activeTexture, result);
    return result;
}

void OnPostRender()
{
    if (ScreenspaceLiquidCamera && ScreenspaceLiquidCamera.IsReady())
    {
        RenderTexture refraction_texture = GenerateRefractionTexture();

        m_Mat.SetTexture("_MainTex", ScreenspaceLiquidCamera.GetColorBuffer());
        m_Mat.SetVector("_MainTex_TexelSize", ScreenspaceLiquidCamera.GetTexelSize());
        m_Mat.SetTexture("_LiquidRefractTexture", refraction_texture);
        m_Mat.SetTexture("_MainDepth", ScreenspaceLiquidCamera.GetDepthBuffer());
        m_Mat.SetMatrix("_DepthViewFromClip", ScreenspaceLiquidCamera.GetProjection().inverse);
        if (SunLight)
        {
            m_Mat.SetVector("_SunDir", transform.InverseTransformVector(-SunLight.transform.forward));
            m_Mat.SetColor("_SunColor", SunLight.color * SunLight.intensity);
        }
        else
        {
            m_Mat.SetVector("_SunDir", transform.InverseTransformVector(new Vector3(0, 1, 0)));
            m_Mat.SetColor("_SunColor", Color.white);
        }
        m_Mat.SetTexture("_ReflectionProbe", ReflectionProbe.defaultTexture);
        m_Mat.SetVector("_ReflectionProbe_HDR", ReflectionProbe.defaultTextureHDRDecodeValues);

        Graphics.ExecuteCommandBuffer(m_CommandBuffer);

        RenderTexture.ReleaseTemporary(refraction_texture);
    }
}

That’s the end of the CPU’s involvement, it’s all shaders from here on.

Let’s start with the motion vector pass. Here’s the entire shader:

#include "UnityCG.cginc"

sampler2D _MainDepth;
sampler2D _MainTex;

struct appdata
{
    float4 vertex : POSITION;
    float2 uv : TEXCOORD0;
};

struct v2f
{
    float2 uv : TEXCOORD0;
    float4 vertex : SV_POSITION;
};

v2f vert(appdata v)
{
    v2f o;
    o.vertex = mul(UNITY_MATRIX_P, v.vertex);
    o.uv = v.uv;
    return o;
}

struct frag_out
{
    float4 color : SV_Target;
    float depth : SV_Depth;
};

frag_out frag(v2f i)
{
    frag_out o;

    float4 fluid = tex2D(_MainTex, i.uv);
    if (fluid.a == 0) discard;
    o.depth = tex2D(_MainDepth, i.uv).r;

    float2 vel = fluid.gb / fluid.a;

    o.color = float4(vel, 0, 1);
    return o;
}

Screenspace velocity is stored in the green and blue channels of the fluid buffer. Since we scaled velocity by thickness when rendering the buffer, we divide the total thickness (in the alpha channel) back out, to produce the weighted average velocity.

It’s worth noting that, for larger volumes of water, we may want a different approach to dealing with the velocity buffer. Since we render without blending, motion vectors for anything *behind* the water are lost, breaking TAA and motion blur for those objects. This isn’t an issue for thin streams of water, but may be for something like a pool or lake, where we want TAA or motion blur for objects clearly visible through the surface.

The main shading pass is more interesting. Our first order of business, after masking with the fluid thickness, is reconstructing the view-space position and normal.

float3 ViewPosition(float2 uv)
{
    float clip_z = tex2D(_MainDepth, uv).r;
    float clip_x = uv.x * 2.0 - 1.0;
    float clip_y = 1.0 - uv.y * 2.0;

    float4 clip_p = float4(clip_x, clip_y, clip_z, 1.0);
    float4 view_p = mul(_DepthViewFromClip, clip_p);
    return (view_p.xyz / view_p.w);
}

float3 ReconstructNormal(float2 uv, float3 vp11)
{
    float3 vp12 = ViewPosition(uv + _MainTex_TexelSize.xy * float2(0, 1));
    float3 vp10 = ViewPosition(uv + _MainTex_TexelSize.xy * float2(0, -1));
    float3 vp21 = ViewPosition(uv + _MainTex_TexelSize.xy * float2(1, 0));
    float3 vp01 = ViewPosition(uv + _MainTex_TexelSize.xy * float2(-1, 0));

    float3 dvpdx0 = vp11 - vp12;
    float3 dvpdx1 = vp10 - vp11;

    float3 dvpdy0 = vp11 - vp21;
    float3 dvpdy1 = vp01 - vp11;

    // Pick the closest
    float3 dvpdx = dot(dvpdx0, dvpdx0) > dot(dvpdx1, dvpdx1) ? dvpdx1 : dvpdx0;
    float3 dvpdy = dot(dvpdy0, dvpdy0) > dot(dvpdy1, dvpdy1) ? dvpdy1 : dvpdy0;

    return normalize(cross(dvpdy, dvpdx));
}

This is the expensive way to do view position reconstruction: take the clip-space position, and unproject it.

Once we have a way to reconstruct positions, normals are easy: calculate position for the adjacent points in the depth buffer, and build a tangent basis from there. In order to deal with silhouette edges, we sample in both directions, and pick the closer point in view space to use for normal reconstruction. This works *surprisingly well*, only failing with very thin objects.

It does mean we do five separate unprojections per pixel (the current point and its four neighbors). There is a cheaper way; this post is getting quite long as it is so I’ll leave that for another day.

The resulting normals:
normals

I distort this calculated normal with the derivatives of the fluid buffer’s noise value, scaled by a strength parameter and normalized by dividing out thickness (for the same reason as velocity):

N.xy += NoiseDerivatives(i.uv, fluid.r) * (_NoiseStrength / fluid.a);
N = normalize(N);

Finally, it’s time for some actual shading. The water shading has three main parts: specular reflection, specular refraction, and foam.

The reflection term is standard GGX, lifted wholesale from Unity’s standard shader. (With one correction: using the proper F0 for water of 2%.)

Refraction is more interesting. Correct refraction requires raytracing (or raymarching, for a close estimate). Fortunately, refraction is less intuitive than reflection, and being incorrect is not as noticeable. So, we offset the UV sample for the refraction texture by the x and y of the normal, scaled by thickness and a strength parameter:

float aspect = _MainTex_TexelSize.y * _MainTex_TexelSize.z;
float2 refract_uv = (i.grab_pos.xy + N.xy * float2(1, -aspect) * fluid.a * _RefractionMultiplier) / i.grab_pos.w;
float4 refract_color = tex2D(_LiquidRefractTexture, refract_uv);

(Note the aspect correction; not necessarily *needed* — again, this is just an approximation — but simple enough to add.)

This refracted light passes through the liquid, and so some of it gets absorbed:

float3 water_color = _AbsorptionColor.rgb * _AbsorptionIntensity;
refract_color.rgb *= exp(-water_color * fluid.a);

Note that _AbsorptionColor is defined exactly backwards from what you may expect: the values for each channel indicate how much of it is *absorbed*, not how much is let through. So, an _AbsorptionColor of (1, 0, 0) results in teal, not red.

The reflection and refraction are blended with Fresnel:

float spec_blend = lerp(0.02, 1.0, pow(1.0 - ldoth, 5));
float4 clear_color = lerp(refract_color, spec, spec_blend);

Up to this point I’ve (mostly) played by the rules and used physically-based shading.

While it’s decent, there’s a problem with this water. It’s a bit hard to see:
no_foam

Let’s add some foam to fix that.

Foam appears when water is turbulent, and air mixes into the water forming bubbles. These bubbles cause a variety of reflections and refractions, which on the whole create a diffuse appearance. I’m going to model this with a wrapped diffuse term:

float3 foam_color = _SunColor * saturate((dot(N, L)*0.25f + 0.25f));

This gets added to the final color using an ad-hoc term based on the fluid noise and a softer Fresnel term:

float foam_blend = saturate(fluid.r * _NoiseStrength) * lerp(0.05f, 0.5f, pow(1.0f - ndotv, 3));
clear_color.rgb += foam_color * saturate(foam_blend);

The wrapped diffuse has been normalized to be energy conserving, so that’s acceptable as an approximation of scattering. Blending the foam color additively is… less so. It’s a pretty blatant violation of energy conservation.

But it looks good, and it makes the stream more visible:
with_foam

Further Work and Improvements

There are a number of things that can be improved upon here.

– Multiple colors. Currently, the absorption is calculated only in the final shading pass, and uses a constant color and intensity for all liquid on the screen. Supporting different colors is possible, but requires a second color buffer and solving the absorption integral piecewise per-particle as we render the base fluid buffer. This is potentially expensive.

– Complete lighting. With access to a GPU-side light lookup structure (either built manually or by tying into Unity’s new HD render pipeline), we could properly light the water with arbitrary numbers of lights, and proper ambient.

– Better refraction. By using blurred mipmaps of the background texture, we can better simulate the wider refraction lobe for rough surfaces. In practice this isn’t very helpful for small streams of water, but may be for larger volumes.

Given the chance I will continue tweaking this into oblivion, however for now I am calling it complete.

Texture-space Decals

I wanted to write up an interesting method of projected decal rendering I’ve been working on recently.

decal_many

The basic idea is (relatively) straightforward. To place a decal onto a mesh:
– Render the decal into a texture, projected onto the mesh’s UVs.
– Render the mesh using the decal texture as an overlay, mask, etc.

Naturally, the details get a bit more complex.

(I implemented this in Unity; unlike the rest of this blog, code samples will be using C# and Cg.)

Texture-space rendering

Rendering into texture space is not a new technique, but it is rather uncommon. (The canonical runtime usage is subsurface scattering: render diffuse lighting into a texture, blur it, and sample the result in the main shading pass.)

It is simpler than it may seem: we base the vertex shader output on the mesh UV coordinates, rather than positions:

struct appdata
{
    float4 vertex : POSITION;
    float2 uv : TEXCOORD0;
};

struct v2f
{
    float4 vertex : SV_POSITION;
};

v2f vert (appdata v)
{
    v2f o;
    // Remap UV from [0,1] to [-1,1] clip space (inverting Y along the way.)
    o.vertex = float4(v.uv.x * 2.0 - 1.0, 1.0 - v.uv.y * 2.0, 0.0, 1.0);
    return o;
}

(The Y coordinate of UVs is inverted here; this may vary depending on your renderer.)

In order to project the decal into the texture, we add a matrix uniform for the projection and a texture sampler for the decal texture itself. The vertex shader then projects the vertex position using this matrix, which gives us a UV coordinate for sampling the decal texture.

(Yes, input UVs become output positions and input positions become output UVs here. It’s a bit confusing.)

The fragment shader takes this projected UV coordinate, and performs a range check. If every component is within the range [0, 1], then the we sample the decal texture. Otherwise, the fragment is outside the projection and the shader discards the fragment.

struct appdata
{
    float4 vertex : POSITION;
    float2 uv : TEXCOORD0;
};

struct v2f
{
    float3 uvw0 : TEXCOORD0;
    float4 vertex : SV_POSITION;
};

sampler2D _MainTex;
float4 _MainTex_ST;
float4 _Color;

float4x4 _DecalProjection0;

v2f vert (appdata v)
{
    v2f o;
    // Remap UV from [0,1] to [-1,1] clip space (inverting Y along the way.)
    o.vertex = float4(v.uv.x * 2.0 - 1.0, 1.0 - v.uv.y * 2.0, 0.0, 1.0);

    float4 world_vert = mul(unity_ObjectToWorld, v.vertex);
    // We assume the decal projection is orthographic, and omit the divide by w here.
    o.uvw0 = mul(_DecalProjection0, world_vert).xyz * 0.5 + 0.5;
    return o;
}

bool CheckBox(float3 uvw)
{
    return !(uvw.x < 0.0 || uvw.y < 0.0 || uvw.z  1.0 || uvw.y > 1.0 || uvw.z > 1.0);
}

fixed4 frag (v2f i) : SV_Target
{
    bool in0 = CheckBox(i.uvw0);

    // Don't touch this pixel if we're outside the decal bounds.
    if (!in0) discard;

    // In this instance, _MainTex contains an alpha mask for the decal.
    return tex2D(_MainTex, i.uvw0.xy).a * _Color;
}

(While this shader uses the world-space position, using object-space positions will give better results in practice, as it will maintain precision for objects far away from the world origin.)

On the CPU side, we drive the decal rendering from script:

public void SplatDecal(Material decal_material, Matrix4x4 decal_projection)
{
    int pass_idx = decal_material.FindPass("DECALSPLATPASS");
    if (pass_idx == -1)
    {
        Debug.LogFormat("Shader for decal splat material '{0}' does not have a pass for DecalSplatPass.", decal_material.name);
        return;
    }

    RenderTexture active_rt = RenderTexture.active;
    m_DecalTexture.MarkRestoreExpected();
    Graphics.SetRenderTarget(m_DecalTexture);
    decal_material.SetMatrix("_DecalProjection0", decal_projection);
    if (!decal_material.SetPass(pass_idx))
    {
         Debug.Log("Decal splat material SetPass failed.");
         return;
    }
    Graphics.DrawMeshNow(m_Mesh, transform.position, transform.rotation);
    Graphics.SetRenderTarget(active_rt);
}

Producing the decal projection matrix is an interesting problem in and of itself, but I’m not going to get into detail here. In general, we want an orthographic projection matrix corresponding to the oriented bounding box of the decal in world space. (Again, in a production setting, applying decals in object space is likely to produce better results.)

Visual Artifacts

The primary visual artifact resulting from this method is visible UV seams:
decal_seam

The reason this occurs is that our decal rendering and final mesh rendering disagree slightly on what texels are affected by the UVs of a given triangle. This is due to the GPU’s rasterization fill rules: fragments along the edge of a triangle are deemed in or out of the triangle based on a consistent rule so that when two triangles share an edge the fragment is only shaded once. While absolutely the correct thing to do for general rendering, it bites us here.

Conservative rasterization extensions should fix this issue. However, Unity does not expose this extension to users yet, and in any case it is not widely available.

The fix I implemented is an image pass that grows the boundaries of UV islands by one texel.

First, we render a single-channel mask texture, using the same texture-space projection as above, and output a constant 1. The mask now contains 1 on every texel touched by UVs, and 0 everywhere else. This mask is rendered only once, at initialization. (Really, we could even bake this offline.)

Next, after rendering a decal, we run a pass over the entire texture, creating a second decal texture. This pass samples the mask texture at each texel: if the sample is 1, the texel is passed through unchanged into the output. However, if the sample is 0, we sample the neighboring texels. If any of those are 1, the corresponding color samples are averaged together and output.

The resulting fragment shader looks like this:

sampler2D _MainTex;
float4 _MainTex_TexelSize;
sampler2D _MaskTex;

fixed4 frag (v2f i) : SV_Target
{
    float mask_p11 = tex2D(_MaskTex, i.uv).r;
    if (mask_p11 > 0.5)
    {
        // This pixel is inside the UV borders, pass through the original color
        return tex2D(_MainTex, i.uv);
    }
    else
    {
        // This pixel is outside the UV border.  Check for neighbors inside the border and output their average color.
        //
        //   0 1 2
        // 0   -
        // 1 - X -
        // 2   -
        //
        float2 uv_p01 = i.uv + float2(-1, 0) * _MainTex_TexelSize.xy;
        float2 uv_p21 = i.uv + float2(1, 0) * _MainTex_TexelSize.xy;

        float2 uv_p10 = i.uv + float2(0, -1) * _MainTex_TexelSize.xy;
        float2 uv_p12 = i.uv + float2(0, 1) * _MainTex_TexelSize.xy;

        float mask_p01 = tex2D(_MaskTex, uv_p01);
        float mask_p21 = tex2D(_MaskTex, uv_p21);
        float mask_p10 = tex2D(_MaskTex, uv_p10);
        float mask_p12 = tex2D(_MaskTex, uv_p12);

        fixed4 col = fixed4(0, 0, 0, 0);
        float total_weight = 0.0;
        if (mask_p01 > 0.5) {
            col += tex2D(_MainTex, uv_p01);
            total_weight += 1.0;
        }
        if (mask_p21 > 0.5) {
            col += tex2D(_MainTex, uv_p21);
            total_weight += 1.0;
        }
        if (mask_p10 > 0.5) {
            col += tex2D(_MainTex, uv_p10);
            total_weight += 1.0;
        }
        if (mask_p12 > 0.5) {
            col += tex2D(_MainTex, uv_p12);
            total_weight += 1.0;
        }

        if (total_weight > 0.0) {
            return col / total_weight;
        }
        else {
            return col;
        }
    }
}

Note the mask comparisons are “greater than 0.5”, not “equal to 1”. Depending on the layout and packing of UVs, you may be able to get away with storing the mask texture in half-resolution and using bilinear filtering to reconstruct the edge. This produces artifacts where UV islands are within a few texels of each other (as they merge on the lower-resolution rendering).

Seams are filled:
decal_seam_fix

On the CPU side, this is a straightforward image pass:

public void DoImageSpacePasses()
{
    int pass_idx = DecalImageEffectMaterial.FindPass("DECALUVBORDEREXPAND");
    if (pass_idx == -1)
    {
        Debug.Log("Could not find decal UV border expansion shader pass.");
        return;
    }

    // We replace the existing decal texture after the blit.  Copying the result back into
    // the original texture takes time and causes flickering where the texture is used.
    RenderTexture new_rt = new RenderTexture(m_DecalTexture);

    DecalImageEffectMaterial.SetTexture("_MaskTex", m_UVMaskTexture);
    Graphics.Blit(m_DecalTexture, new_rt, DecalImageEffectMaterial, pass_idx);

    m_DecalTexture.Release();
    m_DecalTexture = new_rt;
    m_Material.SetTexture("_DecalTex", m_DecalTexture);
}

Of course, in doing this, we create a new render target each time we run this pass. Therefore, it’s not a great idea to run the image pass after every decal is rendered. Instead, render a batch of decals and run the image pass afterwards.

Advantages

Okay, so what does all of this *get* us, in comparison to existing decal techniques?

– The computation cost of a decal is low. There is a fixed per-frame overhead in using the decal texture in the main shader. Each decal rendered is a one-time cost as it renders to the texture. (The UV fixup can be amortized across a batch of decals, making it even cheaper.)

Contrast deferred decals, which require extra draw calls every frame. Permanent decals are therefore very expensive over time. (Forward+ decals encounter similar issues.)

The price paid, of course, is memory: the decal texture takes memory, the mask texture takes memory. The full decal texture must be kept around, even for distant objects, because we must render into the top mip at all times. (Otherwise, we’re stuck needing to upsample the rendered decal when the object gets closer.)

– Support for skinned meshes.

Deferred decals do not work for skinned meshes. While you can in theory attach them to bones and move the decal around with animation, the decal will not deform with the mesh, and so will appear to slide across the surface.

This is not a problem for the texture-space approach; the decal texture will stretch and squash around joints just like any other texture.

– Avoids UV distortion compared to simply “stamping” decals into the texture. By projecting the decal using the mesh vertex positions, we maintain size and shape, regardless of the topology of the UVs underneath. (As well as wrapping around UV seams.) This means texture-space decals still work well on organic shapes, where UVs may be stretched, skewed, etc. Do note this comes at the cost of inconsistent texel density over a decal in these cases; we can’t have everything.

Tradeoffs

This technique does come with some important caveats.

– No UV overlap.

Overlapping UVs will cause a decal in one area to show up elsewhere. (And since the overlap is almost always along a UV island boundary, one of these will have a sharp edge.)

– Sensitive to UV distortion.

While the texture projection can compensate for minor distortion in UVs, severe stretching will result in wildly varying texel density across the decal.

Both of these combine to make texture-space decals generally inadvisable for environments. (As environment art leans heavily on both UV overlap and stretching across low-detail regions.)

– Unique texture per object.

Each object using this decal method needs its own texture to store the decal layer. These can be combined into a texture atlas to reduce the impact on render batching, but the memory cost is still significant.

Conclusion

This is not a generally-applicable decal method. Deferred / Forward+ decal rendering still solves the general case much better.

However, this *is* a useful tool for decals on a relatively small number of meshes, in cases where general methods aren’t optimal: skinned meshes, permanent decals, large numbers of decals. As an example, a recent trend in FPS games is decal overlays for first-person weapon meshes (i.e, blood splatter on a recently-used melee weapon). Rather than pre-author them, this technique would allow those overlays to be generated dynamically.

A String Compare Bug

This is a bit of a departure from the current series. I encountered a bug last week that, while ultimately trivial to fix, was quite difficult to track down.

To start with, the root cause and how to fix it:

I rewrote my shader handling to work indirectly — instead of holding OpenGL handles directly, the renderer has an index into an array of shaders. The ultimate goal being that the asset system can recompile shaders at runtime, swap them out at the top of a frame, and the renderer is none the wiser.

In order to get that index in the first place, the renderer does a lookup using a name string. Asset names are stored in a NameString, which has a character pointer and size. However, the lookup query uses a raw C string. Herein lies the bug:

struct NameString {
    u32 len;
    char * str;

    bool operator==(char * other) {
        return (0 == strncmp(other, str, len));
    }
};

On first glance, this returns whether the strings match. However, it *actually* returns whether the first len characters of the string match. If str is a prefix of other, this will return true, even if the full strings don’t match.

The fix is to actually compute the length of the C string, and compare the lengths as well. As a precaution against bad data, we’d prefer strnlen over strlen. In order to make sure the check is correct, we use a maximum length of len + 1 for the strnlen — therefore, a longer string will still produce a longer length:

struct NameString {
    u32 len;
    char * str;

    bool operator==(char * other) {
        u32 other_len = strnlen(other, len + 1);
        return (other_len == len) && (0 == strncmp(other, str, len));
    }
};

(The *proper* fix, of course, is *don’t use strings for this*. That’s the direction I want to move towards.)

So that’s the error, but not quite the full story of the bug. I feel like it’s worth going into detail here, because it really underscored, to me, the way a subtle error can have wide, unexpected effects.

The problem originally showed itself as serious noise in the screen-space ambient occlusion (SSAO) buffer:
Screenshot 2017-12-15 01.52.21

My implementation of SSAO has two passes: the first takes the depth buffer, reconstructs normals, and then tests points in a hemisphere above the surface. The the percentage of the points that fall behind the depth buffer is used as the occlusion value. (The details get a bit messier but aren’t really relevant at the moment.) To get better coverage with fewer samples, the set of test points is rotated based on a noise texture tiled over the screen. The second pass takes this result and blurs it, using a blur exactly as big as our noise texture, which smooths out the result.

After determining I wasn’t getting NaNs or infinities anywhere, my initial thought was that the blur pass was simply ineffective. Noisy artifacts around depth discontinuities are expected when reconstructing normals from depth, and that’s similar to what we’re seeing here. The blurring *should* mitigate this.

In an attempt to fix it, I modified the blur shader to take the depth buffer as an input, and weight samples in the blur based on how close (depth-wise) they were to the current pixel. And it worked! I got a smooth(er) output:
Screenshot 2017-12-15 01.33.36

Of course, this wasn’t the actual error. (And this isn’t the correct output, either!)

I wanted to get a screenshot of the bug for this blog, so I commented out the changes to my blur shader… and nothing changed, the bug did not reappear.

Well, if I can’t *UN*fix a bug, I probably haven’t actually fixed it.

The one change I had not backed out was binding the depth buffer. Backing that out reintroduced the bug. What’s more, if I bound *another* texture (say, the color framebuffer), I got a glitchy-but-recognizable copy of that texture’s red channel in the SSAO map.

Exasperated, I went to bed.

The next morning, I finally sat down and stepped through in the debugger. And noticed, finally, that the bound shader was the same for both SSAO passes. Ah.

The main SSAO pass shader was named “ssao”, the SSAO blur shader was named “ssao_blur”. Due to the string comparison bug this post began with, both passes ran the main shader. The original visual artifact was due to having the depth input unbound while running the pass. (Obviously it was reading *something*, but I’m not sure what.) Once I bound the depth input for (what I thought was) the updated blur shader, the smoother output was just the main SSAO output.

Fixing the bug for real gives us the blurred version of that, and the correct output:
Screenshot 2017-12-19 21.28.16

There are a few things I could have done to catch this bug earlier and with less frustration.

– Warn or assert on invalid shader uniform names. I had expected the GL driver’s debug mode to catch this — obviously it didn’t. glGetUniformLocation returns -1 for uniform names not found in the shader, need to check that myself because the driver is perfectly happy binding to that location. (This would have caught the bug immediately, when “ssao_input” wasn’t found in the main SSAO shader.)

– First priority is to check the code I just changed, not code that is “obviously” related to the bug. The *only* issue with the SSAO pass was using the incorrect shader (the depth-sensitive blur helps, but wasn’t a big difference.) Even if I’m sure the code I just wrote only uncovered a bug, not caused it: verify that.

– I don’t understand a bug if I can’t force it to happen again.

– Use the debugger more. Use it!

hikari – Terrain Materials

Now that hikari’s material system can express ideas more complicated than “specific set of textures”, it’s time to implement the materials and shading for terrain.

Splat Maps:

The texture selection is going to be based off of splat maps, which means we need the vertex colors: something I conveniently ignored when doing mesh generation earlier. It’s a simple enough fix. I decided to go with layers of noise for testing again, both because it was easy (no need for tooling) and because it produces the worst-case scenario for performance (all pixels use all or most of the splats). If the splat / triplanar mapping is going to fall off the performance cliff I want to find out now, not later.

Splats are normalized (to sum to one) and packed into a 32 bit color per vertex:

inline u32
NormalizeSplat(float w0, float w1, float w2, float w3, float w4) {
    float sum = w0 + w1 + w2 + w3 + w4;
    Assert(sum != 0.0f, "Cannot have total weight of zero on terrain splat.");

    float r = w0 / sum;
    float g = w1 / sum;
    float b = w2 / sum;
    float a = w3 / sum;

    u32 ri = (((u32)(r * 255.0f) & 0xff) <<  0);
    u32 gi = (((u32)(g * 255.0f) & 0xff) <<  8);
    u32 bi = (((u32)(b * 255.0f) & 0xff) << 16);
    u32 ai = (((u32)(a * 255.0f) & 0xff) << 24);

    return ai | bi | gi | ri;
}

I wrote a basic shader to visualize the splat map (although, squeezing 5 values into 3 colors is not particularly effective):
splat_color.png

Textures:

By packing textures tightly, we can reduce the number of texture reads for each splat material to two:
– albedo color
– normal + AO + roughness

This requires encoding the normal map in two channels (which is completely fine for tangent space, just discard the Z component and reconstruct in the shader). It also assumes that none of the splat materials have a metal channel. This is not necessarily true, but the metal mask could be packed into the albedo’s alpha channel. For the textures I’m working with, the metal mask is a constant 0.

So, as a rough calculation:
2 textures per splat material
x 3 splat materials per splat (so that we can have different textures on each planar face)
x 5 splats
= 30 textures.

This immediately rules out the naive approach of binding each texture separately. GL_MAX_TEXTURE_IMAGE_UNITS, the maximum number of bound textures per draw, on my midrange GPU is 32. Since the lighting code uses several textures as well (shadow maps, environment probes, a lookup texture for the BRDF), we run out of texture units.

Fortunately, we don’t have to do things that way.

All of the terrain textures should be the same size, so that the resolution and variation is consistent between them. This in turn means we can pack all of the terrain textures into a set of texture arrays. Instead of binding different textures, each splat material provides an index into the texture arrays.

This does make some things, like streaming textures in and out, more complicated. I’ve ignored this for now, as hikari simply loads everything at startup anyway. (hikari’s handling of assets is not very good, and will need to be addressed sometime soon.)

Shader:

All of the shaders in hikari are plain GLSL, with some minor support for preprocessing (#includes). Material shaders share the same lighting code, via a function called AccumulateLighting() that takes a struct containing all of the surface parameters and returns an RGB color. The benefit of this is writing, debugging, and optimizing the lighting calculation and lookup *once*.

Writing a shader, then, is just a matter of filling out that structure.

For this terrain shader, we need to do two sets of blends: first, blending between splats; second, blending between the three planar projections.

The blending for splats is pretty much exactly what you’d expect:

vec3 SampleAlbedoSplat(vec2 uv, float spw0, float spw1, float spw2, float spw3, float spw4) {
    vec3 splat0 = texture(splat_albedo_array, vec3(uv, splat_id0)).rgb;
    vec3 splat1 = texture(splat_albedo_array, vec3(uv, splat_id1)).rgb;
    vec3 splat2 = texture(splat_albedo_array, vec3(uv, splat_id2)).rgb;
    vec3 splat3 = texture(splat_albedo_array, vec3(uv, splat_id3)).rgb;
    vec3 splat4 = texture(splat_albedo_array, vec3(uv, splat_id4)).rgb;

    vec3 splat_albedo = splat0 * spw0 +
                        splat1 * spw1 +
                        splat2 * spw2 +
                        splat3 * spw3 +
                        splat4 * spw4;

    return splat_albedo;
}

There is another, similar function for sampling and blending the surface properties (normal, roughness, AO).

The triplanar blending is more interesting. See these two blog posts for the fundamentals of what triplanar blending is and how it works:
Ben Golas – Normal Mapping for a Triplanar Shader
Martin Palko – Triplanar Mapping

The first step of triplanar blending is calculating the blend weights for each plane. I take the additional step of clamping weights below a threshold to zero. While this can create some visible artifacts if the threshold is too high, any plane we can avoid looking at is 10 fewer texture samples.

const float blend_sharpness = 4.0;
const float blend_threshold = 0.05;

// fs_in.normal is the interpolated vertex normal.
vec3 blend = pow(abs(fs_in.normal), vec3(blend_sharpness));

blend /= dot(blend, vec3(1.0));

if (blend.x < blend_threshold) blend.x = 0.0;
if (blend.y < blend_threshold) blend.y = 0.0;
if (blend.z < blend_threshold) blend.z = 0.0;

// Need to renormalize the blend
blend /= dot(blend, vec3(1.0));

By checking for a positive blend weight before sampling one of the projections, we can skip those that aren’t going to contribute much. This *does* seem to be a win (about a 0.25ms drop in render time in my test scene), but it’s pretty close to the noise level so I’m not certain.

Thresholding the splat weights may also be worthwhile; in the worst-case test I have set up it definitely isn’t, but actual artist-authored terrain is unlikely to use more than 2 or 3 channels per-pixel.

The actual blending is, mostly, exactly what you’d expect (multiply each planar projection with the corresponding weight and add them together.) The normal blending is slightly different, as described in Ben Golas’ blog post above:

// props_#.ts_normal is the tangent space normal for the plane facing that axis.
vec3 normal_x = vec3(0, props_x.ts_normal.yx);
vec3 normal_y = vec3(props_y.ts_normal.x, 0.0, props_y.ts_normal.y);
vec3 normal_z = vec3(props_z.ts_normal.xy, 0.0);
surf.N = normalize(fs_in.normal +
                   blend.x * normal_x +
                   blend.y * normal_y +
                   blend.z * normal_z);

The normal map for each plane is a linear blend of the splats. This is wrong, but looks okay in practice. I think the correct approach would be to swizzle *every* normal map sample out to world space and *then* blend? Not sure.

The result:
splat_texture.png

Per-plane Textures:

Stepping back a bit, let’s look at this render:
per_plane_color

The textures are placeholders, but each pixel is colorized based on the contribution by each planar mapping. For terrain rendering, this is really nice: it’s identified the slopes for us. In addition, we read a texture for each plane anyway — but there’s no need for it to be the *same* texture!

By exploiting this, we can use a different texture for slopes and level areas, “for free”:
grass_cliff

There’s more we can do here, too, like using different variants of a texture for X and Z planes to increase the amount of variation.

Performance:

I don’t have a very good feel on what makes shaders fast or slow. I was expecting 30+ texture reads to be a problem, but it doesn’t really appear to be. The depth prepass is possibly saving me a lot of pain here, as it means very little of the expensive shading goes to waste. I did notice some issues after first implementing the terrain shader, dropping to 30FPS on occasion, but after adding some GPU profiling code it turns out *shadow rendering* is slow, and just tipped over the threshold to cause problems. (Why does rendering the sun’s shadow cascade take upwards of 5-7ms? I dunno! Something to investigate.)

That said, the method I am using to time GPU operations (glQueryCounter) appears to be less than perfectly accurate (i.e., the UI pass seems to get billed the cost of waiting for vblank.) The GL specification, meanwhile, manages to be either extremely unclear or contradictory on the exact meaning of the timestamp recorded.

For now, I’m going to say this is fine and investigate more later. (ノ`Д´)ノ彡┻━┻

Continuing Work:

At this point, I have most of what I set out to implement, but there are a few things that still need to be done before I’d consider this terrain system complete:

– LOD mesh generation. In addition to simplifying the mesh itself, probably clamp low-weight splats to zero to make shading cheaper.
– Revisit hex tiling. I suspect it really is a more natural fit.
– Fix seams with normals and splats. These are still noticeable with real terrain textures.
– More detailed shading. The terrain materials I’m using all come with height maps; adding occlusion mapping would be interesting and would help sell the look up close.

Until next time.

hikari – New Material System

The next step in building hikari’s terrain system is proper material handling. In order to do so, I need to completely revamp the material system. This is going to be extensive enough to earn its own post.

Current Material System:

Here is the current state of hikari’s material “system”, a single struct:

struct Material {
    GLuint normal_map;
    GLuint albedo_map;
    GLuint metallic_map;
    GLuint roughness_map;
    GLuint ao_map;

    bool has_alpha;
    bool is_leaf;
    bool is_terrain;
};

And its usage, from the per-mesh main render loop:

main_pass.Begin();
main_pass.SetRenderTarget(GL_FRAMEBUFFER, &hdr_buffer_rt_ms);

RenderMesh * rmesh = meshes + i;
IndexedMesh * mesh = rmesh->mesh;
Material * material = rmesh->material;
if (material->has_alpha) {
    glDepthMask(GL_TRUE);
    glDepthFunc(GL_LESS);
}
else {
    glDepthMask(GL_FALSE);
    glDepthFunc(GL_EQUAL);
}
Assert(mesh && material, "Cannot render without both mesh and material!");

GLuint shader_for_mat = default_shader;
if (material->is_leaf) {
    shader_for_mat = leaf_shader;
}
if (material->is_terrain) {
    shader_for_mat = terrain_shader;
}
main_pass.SetShader(shader_for_mat);

main_pass.BindTexture(GL_TEXTURE_2D, "normal_map", material->normal_map);
main_pass.BindTexture(GL_TEXTURE_2D, "albedo_map", material->albedo_map);
main_pass.BindTexture(GL_TEXTURE_2D, "metallic_map", material->metallic_map);
main_pass.BindTexture(GL_TEXTURE_2D, "roughness_map", material->roughness_map);
main_pass.BindTexture(GL_TEXTURE_2D, "ao_map", material->ao_map);

// Material independant shader setup (lights list, shadow map binding,
// environment maps, etc.), elided for space.

// Draw the mesh.
mesh->SetAttribPointersForShader(main_pass.active_shader);
mesh->Draw();

main_pass.End();

The current notion of material is a set of textures and some flags used to determine the correct shader. Needless to say, this is neither flexible nor particularly efficient. Instead, what I’d like is a Material that consists of two bags of data: render states (shader program, blend mode, depth test) and shader uniforms (texture maps).

New Material System, First Pass:

struct Material {
    GLuint shader_program;
    GLenum depth_mask;
    GLenum depth_func;

    struct Uniform {
        const char * name;
        union {
            float value_f[4];
            Texture value_texture;
        };

        enum UniformType {
            Float1,
            Float2,
            Float3,
            Float4,

            TexHandle,
        } type;
    };

    std::vector<Uniform> uniforms;

    // Uniform setup methods elided

    inline void
    BindForRenderPass(RenderPass * pass) {
        pass->SetShader(shader_program);
        glDepthMask(depth_mask);
        glDepthFunc(depth_func);

        for (u32 i = 0; i < uniforms.size(); ++i) {
             Uniform u = uniforms[i];
             switch (u.type) {
                 case Uniform::UniformType::Float1: {
                     pass->SetUniform1f(u.name, u.value_f[0]);
                } break;
                case Uniform::UniformType::Float2: {
                    pass->SetUniform2f(u.name, u.value_f[0], u.value_f[1]);
                } break;
                case Uniform::UniformType::Float3: {
                    pass->SetUniform3f(u.name, u.value_f[0], u.value_f[1], u.value_f[2]);
                } break;
                case Uniform::UniformType::Float4: {
                    pass->SetUniform4f(u.name, u.value_f[0], u.value_f[1], u.value_f[2], u.value_f[3]);
                } break;
                case Uniform::UniformType::TexHandle: {
                    pass->BindTexture(u.name, u.value_texture);
                } break;
            }
        }
    }
};

A good first step. No more branching to select shaders inside the render loop, no hardcoded set of textures. Still doing some silly things, like re-binding the shader for every mesh. Also still looking up uniform locations every time, though there is enough information here to cache those at load time now.

Let’s look at the entire main pass for a bit:

for (u32 i = 0; i < mesh_count; ++i) {     main_pass.Begin();     main_pass.SetRenderTarget(GL_FRAMEBUFFER, &hdr_buffer_rt_ms);     if (!visible_meshes[i]) {         continue;     }     RenderMesh * rmesh = meshes + i;     IndexedMesh * mesh = rmesh->mesh;
    Material * material = rmesh->material;

    Assert(mesh && material, "Cannot render without both mesh and material!");

    material->BindForRenderPass(&main_pass);

    main_pass.SetUniformMatrix44("clip_from_world", clip_from_world);
    main_pass.SetUniformMatrix44("world_from_local", rmesh->world_from_local);
    main_pass.SetUniform3f("view_pos", cam->position);
    main_pass.SetUniform1f("time", current_time_sec);

    main_pass.BindUBO("PointLightDataBlock", point_light_data_ubo);
    main_pass.BindUBO("SpotLightDataBlock", spot_light_data_ubo);
    main_pass.BindUBO("LightList", light_list_ubo);

    main_pass.BindTexture(GL_TEXTURE_CUBE_MAP, "irradiance_map", skybox_cubemaps.irradiance_map);
    main_pass.BindTexture(GL_TEXTURE_CUBE_MAP, "prefilter_map", skybox_cubemaps.prefilter_map);
    main_pass.BindTexture(GL_TEXTURE_2D, "brdf_lut", brdf_lut_texture);

    main_pass.BindTexture("ssao_map", ssao_blur_texture);
    main_pass.SetUniform2f("viewport_dim", hdr_buffer_rt_ms.viewport.z, hdr_buffer_rt_ms.viewport.w);

    main_pass.SetUniform3f("sun_direction", world->sun_direction);
    main_pass.SetUniform3f("sun_color", world->sun_color);
    main_pass.BindTexture("sun_shadow_map", sun_light.cascade.shadow_map);
    for (u32 split = 0; split < NUM_SPLITS; ++split) {
        char buffer[256];
        _snprintf_s(buffer, sizeof(buffer) - 1, "sun_clip_from_world[%d]", split);
        main_pass.SetUniformMatrix44(buffer, sun_light.cascade.sun_clip_from_world[split]);
    }
    mesh->SetAttribPointersForShader(main_pass.active_shader);
    mesh->Draw();
    main_pass.End();
}

There is still a *lot* of uniform setup going on per-mesh, and almost all of it is unnecessary. But, since Material binds the shader each time, all of the other uniforms need to be rebound (because BindForRenderPass() may have bound a different shader).

Ideally, here’s the inner loop we’re aiming for:

for (u32 i = 0; i < mesh_count; ++i) {     RenderMesh * rmesh = meshes + i;     IndexedMesh * mesh = rmesh->mesh;
    Material * material = rmesh->material;
    Assert(mesh && material, "Cannot render without both mesh and material!");

    material->BindForRenderPass(&main_pass);
    main_pass.SetUniformMatrix44("world_from_local", rmesh->world_from_local);
    mesh->SetAttribPointersForShader(main_pass.active_shader);
    mesh->Draw();
}

Material Instances:

When rendering the Sponza test scene, there are a few dozen materials loaded. However, there are only 3 different sets of render states: leaves (a shader doing alpha test and subsurface scattering), alpha tested, and default PBR. Within each class of material the only difference is the texture set.

If we were to separate the set of render states and the set of uniforms into different entities, we’d be able to minimize modifications to the render state in this loop. So that’s what I’ve decided to do.

A Material is a bag of render states, while a MaterialInstance is a bag of uniforms associated with a particular Material. For example, the vases, walls, columns, etc. in Sponza would all be instances of the same Material. If we sort and bucket the mesh list according to Materials, we only need to bind the full render state for each material once. (This is also a convenient point to eliminate culled meshes, removing the visibility check in the main loop.)

At least for now, I’ve done this in the most naive way possible; the list of uniforms is removed from Material and becomes MaterialInstance. Each instance also contains a pointer to its parent material. Done!

This is not a great solution, there are a lot of ways for one to shoot themselves in the foot. For example, a MaterialInstance that doesn’t contain the full set of expected uniforms (will render with stale data), or containing extras (will assert when the uniform bind fails). The Material should probably have “base instance” that defines what set of uniforms are required, and defaults for them; each instance would validate calls against this base. I have not implemented this yet.

Here’s where we end up with MaterialInstance:

BucketedRenderList render_list = MakeBucketedRenderList(meshes, mesh_count, visible_meshes);

for (RenderBucket bucket : render_list.buckets) {
    main_pass.Begin();
    main_pass.SetRenderTarget(GL_FRAMEBUFFER, &hdr_buffer_rt_ms);

    main_pass.BindMaterial(bucket.material);

    // Additional global setup, as above.

    for (u32 i = bucket.start; i < bucket.end; ++i) {
        assert(i < render_list.mesh_count);
        RenderMesh * rmesh = &render_list.mesh_list[i];
        IndexedMesh * mesh = rmesh->mesh;
        MaterialInstance * material_instance = rmesh->material_instance;

        RenderPass main_subpass = main_pass.BeginSubPass();

        main_subpass.BindMaterialInstance(material_instance);
        main_subpass.SetUniformMatrix44("world_from_local", rmesh->world_from_local);
        mesh->SetAttribPointersForShader(main_subpass.active_shader);
        mesh->Draw();

        main_pass.EndSubPass(main_subpass);
    }

    main_pass.End();
}

(Material binding has been moved into RenderPass, which is in hindsight a more natural place for it to live.)

By carving the mesh list up into buckets by Material, we only need to do the global setup once for each bucket, and then only set the specific instance materials per-mesh. Even better, the check for culled objects disappears. And this list is reusable between the depth prepass and main render pass.

Still a lot of room for performance improvement: the RenderMesh struct causes a lot of unnecessary pointer chasing, meshes using the same instance of a material could be batched as well, there are *sprintf* calls in the outer loop. It’s pretty clear I need to spend a lot more time here.

However, this is progress! More importantly, Materials are now general enough I can implement the terrain materials. So that’s next.

hikari – Terrain Mesh Generation

Continuing on from the last post, I have implemented the basic mesh generation for heightmapped terrain. This was probably the easiest part, but did require some cleanup of existing code first.

Mesh Handling Cleanup:

Up to this point, meshes have not been dealt with in a systematic manner in hikari. The Mesh struct contained buffer handles and an index count and that was it. All meshes were assumed to be triangle lists. This was workable, because all object meshes had a common vertex layout, and rendered with variants of a single shader with fixed attribute indices. Rendering a mesh was easy enough:

glBindVertexArray(mesh->vao);
glBindBuffer(GL_ARRAY_BUFFER, mesh->vertex_buffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, mesh->index_buffer);

glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);
glEnableVertexAttribArray(2);
glEnableVertexAttribArray(3);

glDrawElements(GL_TRIANGLES, mesh->index_count, GL_UNSIGNED_INT, NULL);

glDisableVertexAttribArray(3);
glDisableVertexAttribArray(2);
glDisableVertexAttribArray(1);
glDisableVertexAttribArray(0);

There were already other meshes with different layouts, but they were special cases: a quad used for full-screen render passes (with only position and uv), a cube used to render the skybox (position only), etc.

Rather than hack around the limitations here (such as using separate render pass for terrain), I decided a mesh should know its vertex layout and be able to do the attribute setup itself.

Enter VertexBufferLayout:

struct VertexBufferLayout {
    struct Property {
        char * name;
        u32 size;
        GLenum type;
        bool normalized;
        u32 offset;
    };
    
    u32 stride;
    std::vector<Property> props;

    u32 GetPropertySizeBytes(Property p) {
        static const u32 TypeToByteTable[] {
            GL_BYTE, 1,
            GL_UNSIGNED_BYTE, 1,
            GL_SHORT, 2,
            GL_UNSIGNED_SHORT, 2,
            GL_INT, 4,
            GL_UNSIGNED_INT, 4,
            GL_HALF_FLOAT, 2,
            GL_FLOAT, 4,
            GL_DOUBLE, 8,
        };

        for (u32 i = 0; i < array_count(TypeToByteTable); i += 2) {
            if (TypeToByteTable[i] == p.type) {
                u32 byte_size = TypeToByteTable[i + 1];
                return byte_size * p.size;
            }
        }
        return 0;
    }
    
    void AddProperty(char * name, u32 size, GLenum type, bool normalized) {
        Property p;
        p.name = name;
        p.size = size;
        p.type = type;
        p.normalized = normalized;
        p.offset = stride;
        u32 byte_size = GetPropertySizeBytes(p);
        if (byte_size > 0) {
            stride += byte_size;
            props.push_back(p);
        }
        else {
            Assert(false, "Invalid vertex buffer property type.");
        }
    }
};

No real surprises here, just stuffing everything into an array, with a bit of extra bookkeeping to prevent errors in calculating stride and offset pointers. This isn’t a general solution, it makes the assumption that vertex attributes are always stored interleaved (not necessarily true), and does not permit a base offset (for storing multiple meshes in one vertex buffer). But it works, and there’s little point buying more complexity than needed.

Meshes each store a pointer to a layout. Prior to rendering, the renderer passes the shader handle to the mesh, which then uses the handle plus its layout data to determine attribute indices in the shader and bind the pointer to the buffer:

void
SetAttribPointersForShader(GLuint shader_program) {
    Assert(layout != NULL && attrib_indices.size() == layout->props.size(),
            "Invalid vertex buffer layout.");

    glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer);

    for (u32 i = 0; i < attrib_indices.size(); ++i) {
        VertexBufferLayout::Property p = layout->props[i];
        attrib_indices[i] = glGetAttribLocation(shader_program, p.name);
        if (attrib_indices[i] != -1) {
            glEnableVertexAttribArray(attrib_indices[i]);
            glVertexAttribPointer(attrib_indices[i], p.size, p.type, p.normalized, layout->stride, (void *)p.offset);
            glDisableVertexAttribArray(attrib_indices[i]);
        }
    }
    
    glBindBuffer(GL_ARRAY_BUFFER, 0);
}

(Ideally we’d like to cache this — the index lookup should only need to be run once per layout/shader combination, not per-mesh.)

Now we have the list of attribute indices to enable, so we can render very similarly to how we did before.

This still leaves a lot to be desired, but it is enough to unblock the terrain work, so let’s get back on track.

Terrain Mesh Generation:

After all that, generating the terrain mesh itself seems almost trivial. Here is the entire loop for building the vertex positions:

u32 vertex_count = x_count*y_count;
TerrainVertex * verts = (TerrainVertex *)calloc(vertex_count, sizeof(TerrainVertex));

float xo = x_count * 0.5f;
float yo = y_count * 0.5f;\
Vector3 bounds_min(FLT_MAX, FLT_MAX, FLT_MAX);
Vector3 bounds_max = -bounds_min;

for (u32 y = 0; y < y_count; ++y) {
    for (u32 x = 0; x < x_count; ++x) {
        float map_val = map->ReadPixel(x_off + x, y_off + y);

        Vector3 pos = Vector3(
            x - xo,
            map_val, 
            y - yo);

        // Scale up from pixel to world space.
        pos *= chunk_meters_per_pixel;

        verts[y * x_count + x].position = pos;

        bounds_min = MinV(bounds_min, pos);
        bounds_max = MaxV(bounds_max, pos);
    }
}

This is a regular grid of squares in the XZ plane, with the Y coordinate taken directly from the heightmap. Next we build an index buffer (in the most naive manner), and calculate surface normals from the resulting triangles.

All of this is pretty straightforward, but there are a few subtle issues that come up:

– Triangulation. Unlike triangles, the vertices of a quad do not necessarily all fall in one plane. This is in fact *usually* the case with terrain. So how we decide to split that quad into triangles has an effect on the final shape:

quad_triangulation_illust

Currently, I split all quads uniformly. Another option I explored is switching the split direction in a checkerboard fashion. With the noise-based heightmap I’m testing with, the difference is negligible. This may prove more of an issue when producing sparser, low-LOD meshes for distant terrain. It also would *absolutely* matter if the terrain was flat-shaded (i.e., constant normal for each face), but that isn’t the case here.

– Calculating vertex normals. I calculate normals for this terrain mesh the same as any other mesh: loop through the faces and add face normals to each vertex, then normalize in another pass:

for (u32 i = 0; i < index_buffer_count; i += 3) {
    u32 idx_a = indices[i + 0];
    u32 idx_b = indices[i + 1];
    u32 idx_c = indices[i + 2];

    Vector3 a = verts[idx_a].position;
    Vector3 b = verts[idx_b].position;
    Vector3 c = verts[idx_c].position;

    Vector3 normal = Cross(a - b, a - c);

    verts[idx_a].normal += normal;
    verts[idx_b].normal += normal;
    verts[idx_c].normal += normal;
}

// We sum the unnormalized normals for each triangle, and then normalize
// the sum.  Since the cross product is scaled by the area of the triangle,
// this means that larger triangles contribute more heavily to the resulting
// vertex normal.
for (u32 i = 0; i < vertex_count; ++i) {
    verts[i].normal = Normalize(verts[i].normal);
}

So far so good, but there is a crucial problem here.

We iterate the faces of the mesh. But some of the points in question actually belong to several meshes! When we’re at the edge of the terrain chunk, we only account for the face normals of the triangles inside the current chunk. Which means the same point, on two different chunks, may produce a different normal.

And so, we get seams:

normal_seams

This is not necessarily easy to fix. The simplest option would be to iterate over neighboring chunks, find the shared vertices, and average the normals — not 100% correct, but it eliminates the seam. It’s unclear, though, how this would interact with streaming: a new chunk being loaded might change the normals on a mesh that’s been loaded for a while; should we read that mesh data back from the GPU? Keep a CPU copy and update? In addition, one of the design goals is for the terrain system to be agnostic about the source of meshes. For a well-structured grid finding the adjacent vertex is simple, but how does that work with an arbitrary hand-produced mesh?

Ultimately, there’s also the question of whether this seam in the normals actually matters. Obviously it *is* a bug, but it is only so glaringly visible because of the shiny plastic material I’m using to debug. I’m not sure how many of these artifacts will remain visible when applying actual terrain materials.

I’m counting this as a bug but I’m going to leave it alone for now.

– Alternate tiling strategies. I started with laying a square grid over the world. Why squares? There are other good tiling arrangements. In fact, we can tile the plane with *just* triangles, and avoid the question of how to triangulate our tiles altogether.

It turns out this is *really* easy to do:

pos.x += (y & 1) * 0.5f;  // Offset odd rows by half a unit
pos.z *= sqrtf(3.0f) / 2.0f; // Scale vertically by sin(pi/3)

Make sure the triangles cut across the short diagonal of that skewed grid, and now the plane is tiled with equilateral triangles.

There isn’t a huge amount of difference with my noise terrain (just a scaling on the Z axis):

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However, tiling square chunks with equilateral triangles has a sticking point when it comes to downsampling the terrain for distant LOD meshes. The sawtooth edge can’t be merged into larger triangles without either breaking the tiling pattern or leaving gaps or overlap between chunks.

With that said, I feel like the triangle tiling has some really nice properties. No triangulation needed, and no ambiguity on the final mesh’s shape. It can still be represented easily in an image map (although, we’re pushing the limits on that). Grids make it easy to create 90 degree angles, which stick out, while a triangle tiling makes 60 degree angles which are less obviously objectionable. (This one is decidedly a matter of taste, but: my engine, my taste.)

Things get *really* interesting when we decide chunks don’t need to be rectangular. A hexagonal chunk tiles perfectly with equilateral triangles, which means the LOD reduction works fine as well. Using hexes for chunks also fits into the half-offset tiling described by Alan Wolfe here. (In addition to the memory savings he mentions in the post, the hex tiling also means a constant streaming cost as you move about the world, which is a very nice property to have.)

I haven’t implemented hex tiling, and this post is getting quite long as it is. I do think it is an avenue worth exploring, and will probably return to this after fixing up the material system.

Until next time!

hikari – Designing a Terrain System

The next big addition to hikari is going to be a system for rendering terrain. I’ve done a few weeks of research and wanted to write up my plans before implementing. Then, once it’s complete, I can review what did and did not work.

That’s the idea, anyway.

Source Format:

I decided to use simple heightmap-based terrain. Starting with a regular grid of points, offset each vertically based on a grayscale value stored in an image. This is not the only (or best) choice, but it is the simplest to implement as a programmer with no art team at hand. Other options include marching-cubes voxels, constructive solid geometry, and hand-authored meshes.

However, I’m consciously trying to avoid depending on the specific source of the mesh in the rest of the pipeline. This would allow any of (or even a mix of) these sources to work interchangeably. We’ll see how realistic that expectation is in practice.

Chunking:

Terrain meshes can be quite large, especially in the case of an open-world game where the map may cover tens of kilometers in any direction, while still maintaining detail up close. Working with such a large mesh is unwieldy, so let’s not.

Instead, we cut the terrain up into chunks. For example, if our world map is 4km x 4km, we could cut it up into 1600 separate 100m x 100m chunks. Now we don’t need to keep the entire map in memory, only the chunks that we might need to render in the near future (e.g, chunks around the camera.)

There are additional benefits to chunking the map; for example, the ability to frustum cull the chunks. Chunks provide a convenient granularity for applying level-of-detail optimizations as well.

Materials:

The mesh is only part of our terrain, of course. It needs texture and color from materials: grass, rock, snow, etc. Importantly, we don’t necessarily want to texture every part of the terrain with the same material. One solution would be to assign a single material to each chunk, and have an artist paint a unique texture.

This seems to be a non-starter to me — potentially thousands of textures (of high resolution) would need to be created and stored, not to mention streamed into and out of memory alongside their mesh chunks. In addition, producing a distortion-free UV-mapping for terrain is difficult. Finally, while we don’t want to texture everything the same, there are only a limited set of materials we want to use, and it’d be nice if we could reuse the same textures.

Enter splat maps. The basic idea is straightforward: assign a color to the terrain. The components of that color determine the blend weights of a set of materials. If the red channel weights a rock texture and the green channel weights a grass texture, ‘yellow’ terrain is a blend between rock and grass. This allows us to smoothly blend between 5 different materials.

Wait, 5? How?

A color has four components, red, green, blue, and alpha. However, if we assume the material weights must add to equal 1.0, we can construct a new weight that is (1.0 – (r + g + b + a)). This becomes the weight of our 5th material. (Of course, we need to actually enforce these constraints in art production, but that is a separate issue.)

Furthermore, since chunks of the terrain get rendered in their own draw call, there’s no reason we can’t swap the materials used in each chunk. So while the number of materials in a specific chunk is limited, the total number of materials is unlimited (well, close enough).

The name “splat map” may imply using a texture to assign this color (and, for a heightmap terrain, that’s the easiest way to store it on disk), but I actually want to use vertex colors. This puts heightmaps and authored meshes on the same footing, and keeps decisions about mesh source in the mesh loading where it belongs.

In order to do texturing, we need texture coordinates. The obvious way to do this is assign a UV grid to our heightmap mesh. This will look fine for smooth, subtle terrain features, but the more drastic the slope the more distorted the texturing will be.

Instead, we can use so-called triplanar mapping to generate texture coordinates. Project the world position to each of the XY, YZ, and ZX planes, and use that value as the texture coordinate for three different lookups. Then, blend between them based on the vertex normal.

As an added bonus, there’s no requirement that the various axes in triplanar mapping use the same material. By mixing materials, we can create effects such as smooth grass that fades to patches of dirt on slopes.

Of course, there is a cost to all of this. Triplanar mapping requires doing 3x the material samples. Splat mapping requires doing up to 5x the samples. Together, it’s up to 15x the material samples, and we haven’t even begun to talk about the number of individual textures in each material. Needless to say, rendering terrain in this fashion is going to read a *lot* of texture samples per-pixel.

That said, hikari already uses a depth prepass to avoid wasted shading on opaque geometry, so I suspect that would mitigate the worst of the cost. In addition, it’s easy enough to write another shader for more distant chunks that only uses the 2-3 highest weights out of the splat where that’s likely to be an unnoticeable difference.

Plumbing:

There are a bunch of assumptions that hikari makes regarding meshes and materials that are a bit at odds with what I’ve mentioned above. The renderer is simple enough that I can work around these assumptions for special cases. However, I will need to be on the look out for opportunities to make some of the rendering systems (or, at least, the exposed API) more general.

Some examples: at the moment, meshes are assumed to use a specific common vertex format (position, UV, normal, tangent) — terrain meshes will not (no need for UV and tangent, but we need the splat color.) Meshes are expected to use only one material, and materials do not pack any texture maps together (so, for example, baked AO and roughness are separate maps). Neither of these will play nicely with terrain, but should be simple enough to fix. “Materials” are currently just a bag of textures and a handful of uniform parameters, since at the moment there are only two mesh shaders in the engine. A more generic material model that ties directly into its associated shader would be a useful thing to fix.

So, there’s the plan of attack. We’ll see how well it stacks up. I’ll be documenting the implementation of each part of the system in a series of blog posts, capped off by a final report on what worked and where I got it totally wrong.