### Rendering Volume Filling Triangles in OpenGL (with no buffers)

This is the promised follow-up to Rendering a Screen Covering Triangle in OpenGL (with no buffers), except this time the goal is to write a shader that accesses every location in a 3d texture (volume). We use the same screen covering trick as before to draw a triangle to cover a viewport match to the X and Y dimensions of the volume, and we use instanced rendering to draw repeated triangles for each layer in the Z-dimension.

The vertex shader looks the same as before with the addition of the `instanceID`

.

flat out int instanceID; void main() { float x = -1.0 + float((gl_VertexID & 1) << 2); float y = -1.0 + float((gl_VertexID & 2) << 1); instanceID = gl_InstanceID; gl_Position = vec4(x, y, 0, 1); }

The fragment shader can then recover the voxel coordinates from `gl_FragCoord`

and the `instanceID`

.

flat in int instanceID; void main() { ivec3 voxelCoord = ivec3(gl_FragCoord.xy, instanceID); voxelOperation(voxelCoord); }

Very similar to drawing the single screen covering triangle, we set our viewport to the XY-dimensions of the volume, bind a junk VAO to make certain graphics drivers happy, and call `glDrawArraysInstanced`

with the Z-dimension of the volume, so that we draw a triangle per-slice of the volume.

glViewport(0, 0, width, height); glBindVertexArray(junkVAO); glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 3, depth);

Which would look sort of like the following:

This can be useful for quickly processing a volume. Initially, I used this as an OpenGL 4.2 fallback (instead of compute shaders) so that I could still use the NSight debugger, until I realized this approach was actually outperforming the compute shader. Of course, when to use compute shaders, and how to use them effectively deserves a post of its own.

November 21, 2014 at 9:07 pm

If you switch from a vertex shader to a geometry shader, here you could use the gl_InvocationID to pick a viewport, then tell the fragment shader which slice it is in with a flat in the same manner.

The benefit there is you’d be able to write out a volume texture in the fragment shader.

It’d have to be comparatively thin in Z, though.