vtlib Detail Texture

What's implemented in the VTP right now is a hardware application of the detail texture, as a second pass, with no masking, only one ground texture at a time. To allow the detail texture to co-exist smoothly with a lower-resolution geospecific texture (which contains the lighting, and ideally real-world imagery), the detail texture is blended based on distance from the viewpoint.

With the SMTerrain algorithm, distance to each vertex is already calculated by the CLOD algorithm, so that feeds directly into the alpha value of the detail texture.

With SRTerrain algorithm, we do not have a distance value, so the detail texture is drawn with a constant alpha value.

Development notes

• tried calling OpenGL like this:
  1. load the ground texture (glBindTexture)
  2. render the terrain primitives
  3. load the detail texture (glBindTexture), a texture with noise for RGB and a moderate value (around 40%) for alpha
  4. render the terrain again - same vertex locations, different uv mapping
• didn't look quite right
• Ivan-Assen Ivanov pointed out that glBindTexture is an expensive call, although twice per frame should be insignificant
• distance-based option
  • could only apply the detail texture to polygons which are close to the viewer, and even smoothly blend it based on distance, but that could require an expensive distance calculation
  • this could be tied in with the LOD algorithm, if it produced triangle distances as a by-product of the LOD calculation, which SMTerrain does
• current implementation:
  • LoadNormalTexture(...)
Render(...)
LoadDetailTexture(...)
// turn on alpha blending
glPolygonOffset(-1.0f, -1.0f);
glEnable(GL_POLYGON_OFFSET_FILL);
Render(...)    // uses alpha value per vertex, based on distance
glDisable(GL_POLYGON_OFFSET_FILL);
• definitely better than no detail texture, but still very "synthetic" looking because of the uniform, repeating nature of the texture
• exhibits some kinds of triangular patch artifacts - need to investigate, although the current approach is of limited usefulness in any case, since real-world terrain is rarely handled by a single detail texture
• support for multiple detail textures is a very high priority need