How to use glDrawElementsBaseVertex to draw just select individual triangles out of an VBO mesh?

Question

So I have a simple cube "mesh":

the 6 cube faces are 12 triangles, hence 36 "vertices" organized as 36 indices reusing 8 unique vertex positions, 6 unique vertex normals and 4 unique vertex-texcoords.

This indexed mesh is stored in a VBO/IBO pair that may have other meshes before or after it, hence base-vertex is advisable here.

Now I can render the full cube completely and correctly in a single draw call like this:

gl.DrawElementsBaseVertex(
    gl.TRIANGLES, gl.Sizei(36), gl.UNSIGNED_INT,
    gl.PtrOffset(nil, uintptr(myMeshIndexBufOffset)),
    gl.Int(myMeshIndexBufBaseIndex))

However, now I just want to draw the "first 4 triangles" (aka the "first" 2 cube-faces) -- with 4 draw calls rather than 1 (as texture binds may happen in between) and I'm really at a loss as to how to properly specify the count and basevertex arguments.

Well for a single triangle, I'll just have to assume I need to specify a count of 3. For 12 triangles I had to use 36 to successfully render the full complete cube, so 3 for a single triangle makes sense. It then picks 3 "index entries" (here defined as three uint32s per entry) from the element array. Correct?

But I just can't figure out --neither through logical reasoning nor through experimentation-- how the base-vertex is now specified here. Specifically I'll need to add my own offset to the "cube-mesh base-index" which is known. If I were to glDrawElementsBaseVertex() all individual 12 triangles separately in a loop:

for i := 0; i < 12; i++ {
    gl.DrawElementsBaseVertex(
        gl.TRIANGLES, gl.Sizei(3), gl.UNSIGNED_INT,
        gl.PtrOffset(nil, uintptr(myMeshIndexBufOffset)),
        (i * fooBar) + gl.Int(myMeshIndexBufBaseIndex))
}

then... does anyone have any insight as to what exactly fooBar above should represent?

It's annoying: really, in theory I was pretty sure I did understand the glDrawElementsBaseVertex specification quite completely, yet in practice nothing yields properly separately rendered select individual triangles.

metaleap metaleap · Accepted Answer · 2013-02-18T16:03:38

Got it... it just dawned on me the base-vertex is added to the index value after being taken from the element-array-buffer, rather than added as an offset to the position from which to read from the element array buffer... ouch. So seems I have to offset the indices argument rather than the basevertex argument, and seems I'm getting the desired results now.

How to use glDrawElementsBaseVertex to draw just select individual triangles out of an VBO mesh?

1 Answers