Detail Textures

• the basic idea is that, since geospecific textures are rarely high-resolution enough, you can render additional high-frequency detail with geotypical textures, to avoid having a big blurry surface

Distance-based blending

• Jason Booth of Turbine Games said:
  • "Why not use a single texture, essentially noise, over your terrain and then color the vertices of your polygons. Then, when close, multipass that texture with your actual texture (using the alpha values at the vertices to blend in the texture smoothly). Over a set number of frames blend the two from 100% vertex/noise based to 100% texture. Then use another pass to blend in a smaller, detail noise texture to add noise at the pixel level and avoid the big blur syndrome."
• John Ratcliff of Verant put it this way:
  • "Synthesize a high resolution detail map for the polygons immediately around the camera position, to show a great deal of perceived local detail. For everything else, mountains and such off in the distance, the base texture map works fine."
• the vtlib implementation re-uses the distance from camera to each vertex, already computed by the CLOD algorithm, to do an alpha-fade of the detail texture

Masking

• you need some way to specify what areas are covered with which ground type(s)
• this is commonly done with a bitmap mask, giving an 'amount' 0-255 for each type, for the entire terrain surface
• another way would be to use a polygonal representation for the coverage, which is a better fit with how data is actually obtained from GIS layers, and allows very sharp transitions without the cost of a large mask bitmap
• however, so far i have only heard of people (game developers) implementing the bitmap mask approach

Blending: How to Render

• Once you've got geotypical texture and a mask for each one, the decision is whether to do the blending with the CPU or the graphics card
• With the CPU, you can blend each of the ground textures together into a single texture, which simplifies the rendering down to a single texture pass. This rapidly exceeds texture memory, unless texture LOD is well-managed such that textures are not computed until necessary, which probably works best for applications with a near field of view, and slow-moving viewpoints. It becomes complicated to handle distant terrain, which must be synthesized from lower-resolution downsamples of the ground type masks to produce lower-resolution textures.
• The alternative is to the use the graphics card to do your blending. The entire terrain mesh repeatedly drawn per frame, for each potential ground type, using each type's mask. Ideally, these passes are combined as much as possible using the card's multi-texture capabilities.
• The performance tradeoffs are complex, involving CPU consumption vs. bus bandwidth vs. assumption of multi-texture hardware. These have been discussed in great depth on the 3DGameDev Algorithms mailing list:
• some game developers refer to this subject as "Splatting"
  • Charles Bloom's Terrain Texture Compositing by Blending in the Frame-Buffer (aka "Splatting" Textures) (text)
  • screenshots here
• Multi-pass issues
  • you should consider the Z-buffer problem of drawing the surfaces over each other
  • Seumas says how he does it is:
    • he alters the front & back clipping planes slightly before drawing the second pass, which solves the overdraw-Z buffer problem
    • his detail texture is dark and monochromatic, using alpha to vary the amount of darkening
  • with the vtlib implementation (see below) i haven't seen any Z-buffer problems, so there's been no need to play with clipping planes or other tricks
  • there is API support for multitexturing in both Direct3D, OpenGL 1.1 extensions, and OpenGL 1.2, but there are still some older cards around that won't have it
• if blending with a low-resolution texture which already contains color information, should the detail texture be monochromatic or not?
• e.g. for grass, does it make sense to use a detail texture containing an RGB image of grass, alpha-blended with the base color texture, or just supply a grey detail texture and rely on the base texture for the 'green' color?