Graphics Reference
In-Depth Information
glEnableVertexAttribArray ( COLOR_LOC );
// Set one color per instance
glVertexAttribDivisor ( COLOR_LOC, 1 );
// code skipped ...
// Bind the index buffer
glBindBuffer ( GL_ELEMENT_ARRAY_BUFFER, userData->indicesIBO );
// Draw the cubes
glDrawElementsInstanced ( GL_TRIANGLES, userData->numIndices,
GL_UNSIGNED_INT,
(const void *) NULL, NUM_INSTANCES );
Applications should make sure that
glDrawElements
and
glDrawElementsInstanced
are called with as large a primitive size
as possible. This is very easy to do if we are drawing
GL_TRIANGLES
.
However, if we have meshes of triangle strips or fans, instead of making
individual calls to
glDrawElements*
for each triangle strip mesh, these
meshes could be connected together by using primitive restart (see the
earlier section discussing this feature).
If you cannot use the primitive restart mechanism to connect meshes
together (to maintain compatibility with an older OpenGL ES version),
you can add element indices that result in degenerate triangles at the
expense of using more indices and some caveats that we will discuss
here. A degenerate triangle is a triangle where two or more vertices of the
triangle are coincident. GPUs can detect and reject degenerate triangles
very easily, so this is a good performance enhancement that allows us to
queue a big primitive to be rendered by the GPU.
The number of element indices (or degenerate triangles) we need to
add to connect distinct meshes will depend on whether each mesh is a
triangle fan or a triangle strip and the number of indices defined in each
strip. The number of indices in a mesh that is a triangle strip matters,
as we need to preserve the winding order as we go from one triangle to
the next triangle of the strip across the distinct meshes that are being
connected.
When connecting separate triangle strips, we need to check the order of
the last triangle and the first triangle of the two strips being connected. As
seen in Figure 7-5, the ordering of vertices that describe even-numbered
