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.