Texture Coordinates

To utilize our texture clamp settings we now need to calculate the appropriate texture coordinates for our decal mesh. If our decal is not aligned to the top left corner of our mesh, then the decal texture offset will be greater than zero.

[Figure 4] The decal has a texture offset of x:5 by y:5

We use this offset, along with the decal's size, to calculate the mesh's texture coordinates. Here is the code to generate the array of texture coordinates. Note that the texture array is a bottom-left to top-right column incremented data structure i.e. it starts with the bottom left grid vertex and ends with the top-right vertex.

// These values have been derived from Figure 4
Vector2 decalOffset    = { 5.0f,  5.0f  };
Vector2 decalPosition  = { 15.0f, 20.0f };
Vector2 meshPosition   = { 15.0f, 20.0f };

float fDecalWidth      = 20.0f;
float fDecalHeight     = 20.0f;
float fMeshWidth       = 30.0f;
float fMeshHeight      = 30.0f;
float fHalfDecalWidth  = fDecalWidth  / 2.0f;
float fHalfDecalHeight = fDecalHeight / 2.0f;
float fHalfMeshWidth   = fMeshWidth   / 2.0f;
float fHalfMeshHeight  = fMeshHeight  / 2.0f;
float fCellWidth       = 10.0f;
float fCellHeight      = 10.0f;
int   verticesX        = 4;
int   verticesY        = 4;

// how far from the top-left corner of the mesh does the decal start
decalOffset.x = ( decalPosition.x - fHalfDecalWidth  ) - ( meshPosition.x - fHalfMeshWidth );
decalOffset.y = ( fMeshHeight - fDecalHeight ) –
               (( decalPosition.y - fHalfDecalHeight ) - ( meshPosition.y - fHalfMeshHeight ));

float invDecal.x  = 1.0f / decalWidth;
float invDecal.y  = 1.0f / decalHeight;

int x;
int y;
int index;
int t;

for( x = 0; x < verticesX; x++ )
{
  index = verticesY - 1;

  for( y = 0; y < verticesY; y++ )
  {
    // get the index into the texture array
    t = ( x * verticesY ) + y;

    // calculate the uv coordinates for the vertex at t
    vertex[t].u = ( ( x * fCellWidth  )     * invDecal.x ) - ( decalOffset.x * invDecal.x );
    vertex[t].v = ( ( index * fCellHeight ) * invDecal.y ) - ( decalOffset.y * invDecal.y );

    --index;
  }
}

And the results:

[Figure 5] The resulting texture coordinates

As we can see in Figure 5, there are UV values below zero and above one. Any part of the mesh that falls under those regions will be painted with a transparent texel.

Ripping Vertices and Pre-Calculating Indices

We want the decal to be the right shape so that is can match the slopes of the terrain perfectly. To accomplish this we must grab the existing vertices from the terrain. This process will vary depending on how you store your terrain data, as pulling vertices out of a tree hierarchy will evidently require a different technique to that of a simple vertex array. But once you have the vertices copied into the mesh, it only takes a translation or two to get them into decal space. As for indices, when using a uniform grid and creating many decals of the same size, it makes sense to pre-compute the indices and store them in a shared index buffer. Once the mesh is complete it's time to insert it into the scene graph. Make sure the mesh is raised slightly above the terrain to avoid z-buffer sorting issues.

Conclusion

Well that just about wraps up the essentials for terrain decals. Thanks to my AIE tutors (http://www.aie.act.edu.au/about/staff.php) for all of their help and guidance. I hope this article can assist anyone looking for a simple terrain decal solution, if you have any questions/abuse/job offers feel free to email me at gtowse@yahoo.com. Thanks for reading.