Overview:
Terrain maps have been used in video games since the 1990s. They are 2d textures, traditionally 8bit grayscale that represent height values in a terrain mesh at a position along its surface. They are an effective way to store topographic data, and an easy way to generate and present realistic terrain in real time. They are small, easy to use and intuitive to make. Artists can hand draw them in a graphics editor like photoshop, or a noise generating program can be written to output noise data to a height map image.
The earliest well known games to use the technology were the comanche series. Although the games used a voxel renderer to draw the image, the data used was a simple heightmap.
As shader technology has improved, more complex terrain mapping techniques have become possible. Texture lookups in the vertex shader allowed height maps to be read during render time. This allows for highly interactive terrain. An example of this can be seen in the game startopia. Startopia is a great example of interactivity between game logic and a shader program. In order to interact with the terrain geometry, its a matter of converting player inputs into uv positions within the heightmap texture and then modifying the color channels to change the amount of displacement when the texture is processed by the shader program.
The new shader types that have come with Shader Model 5.0 have breathed new life into terrain map techniques. With tesselator, it is possible to adjust mesh resolution based on camera distance. This means we can greatly increase mesh complexity up close and decrease it far away where the player can't appreciate it. In fact, Nvidia released a demo to siggraph demonstrating this. It can be found here.
The geometry shader allows us to add entirely new mesh detail to the input mesh. Unlike the tesselator it isn’t limited to simply modifying the input, although it certainly can do that as well. The geometry shader is therefore appropriate for creating detail like grass, foliage, etc. Unfortunately, due to the extremely restrictive primitive emission limits, more complicated meshes aren’t presently viable for generation through the geometry shader. Complex mesh generation is however possible through instancing. Instancing allows the programmer to specify a number of instances of a geometry shader to operate on the same set of primitives. Using the instance ID as an offset, its possible for a set of geometry shaders to wind pieces of a complex mesh together and then pass this mesh as normal to the fragment shader for coloring. However instancing is expensive.
Accompanying implementation:
The accompanying terrain generator will demonstrate the use of a geometry pass with terrain maps. The shader program will consist of 3 shaders: vertex, geometry, fragment. The vertex shader is responsible for height displacement and normal recalculation, the geometry shader will add foliage to the terrain and the fragment shader will calculate uv offsets and apply diffuse color, lighting, snow cover, water and fog coloring. The demo will also take advantage of the dynamic nature of the program. Since the terrain map is being processed and rendered in real time, theres no excuse to make the demo dynamic. The shader will demonstrate a terrain rendering system that places the player at the center of a fixed plane that is used as a canvas to render the terrain around the player as the player moves throughout the world.
For input, the shader will use a 100x100 vertex plane mesh, various diffuse color textures for coloring and a texture map. Unlike a traditional height map, the terrain map represents more than height data. Height data is stored in the red channel as a 0 - 1 value, an accompanying floating point uniform serves as a multiplier to produce the final height. Foliage types are stored in the green channel. The shader only demonstrates 1 foliage type, tree at 255, but could easily be extended to display a variety of foliage types.
The red channel is used by the vertex shader to displace the vertex height. The vertex shader then recalculates the vertex normal before releasing the vertex for primitive assembly. The geometry shader pulls in primitives as triangles. The first thing it does is pass through the incoming triangle to the primitive output stream. Then it uses the primitive’s mesh coordinates x and z components (its position within the input plane mesh), converts those to uvs (mesh size is 100x100, so divide the coord by this, giving you the equivalent position in uv space) and look at the value of the green channel in the texel at that position. Depending on the value in the green channel, a different kind of foliage will be generated, which means additional geometry has to be added to the output stream. Two intersecting, orthogonal quads are generated and placed at the height of the current triangle. Depending on the primitive’s height, the uvs for these quads are offset, so that during coloring in the fragment shader, the demo will render snow covered trees at high elevations, green trees at low elevations. A slight time variant horizontal offset is added to the top of the quads too to represent the breeze.
Part of the fragment format is an int that stores type. These types are Terrain, Foliage1, Foliage2, etc. Type is used to determine which diffuse texture to use. Diffuse texture used for the terrain holds coloring for grass, which is used when the normal is pointing up, and rocks, when the normal is pointing to the side. As the frag’s height increases, the color is increasingly blended white to represent mountain peak snow covering. If the height is sufficiently low, there is it is blended blue to represent water. The demo camera is setup at a perspective where the water coloring trick works quite well.
Closing thoughts:
Terrain mapping has always been a great solution for rendering topographies. Using tessellators, its possible to dynamically adjust the mesh’s level of detail and using geometry shaders, its possible to encode geometry data such as foliage directly into the terrain map. Dynamic tesselation offers an alternative to clipmaps. Geometry shaders offer a GPU based alternative to geometry instancing to quickly render surface detail. The new types of shaders has opened up a new GPU based environment for improvement on the initial concept.