Rendering Volume Filling Triangles in OpenGL (with no buffers)


Randall Rauwendaal


August 9, 2014

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);

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);
glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 3, depth);

Which would look sort of like the following:

Volume Filling Triangles

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.


BibTeX citation:
  author = {Randall Rauwendaal},
  title = {Rendering {Volume} {Filling} {Triangles} in {OpenGL} (with
  date = {2014-08-09},
  url = { - Rendering Volume Filling Triangles in OpenGL (with no buffers)},
  langid = {en}
For attribution, please cite this work as:
Randall Rauwendaal. 2014. “Rendering Volume Filling Triangles in OpenGL (with No Buffers).” August 9, 2014. - Rendering Volume Filling Triangles in OpenGL (with no buffers).