GLSL Geometry Shaders and projection matrices

3
votes

So from playing around with it so far, I gather that GLSL geometry shaders work after the input vertices are transformed by the projection/modelview matrices. In other words, the geometry shaders processes things on clip coordinate.

What if I was to use the geometry shader to transform GL_POINTS into, say, cubes made out of GL_TRIANGLES? When calculating things on clip coordinates, the resulting shape always seem to face you / ignore rotations/scaling etc.

Also, it seems that GL_TRIANGLES is not supported as one of the possible geometry output types. But I tried anyways, and it seems to work. I suppose this is video card dependent? Is it possible to make cubes if GL_TRIANGLES is not supported? Make zero width triangle strips in between spaces maybe??

1
openglglslgeometry-shader

1 Answers

4
votes

You are using shaders: geometry shaders work on whatever the vertex shader passed them. If you want that to be clip-space values, then the geometry shader works on clip-space values. If your vertex shader passes them eye-space values, then the geometry shader must work on eye-space values.

What matters is what the final pre-rasterization shader stage outputs to gl_Position. That is what needs to be in homogeneous clip-space. A vertex shader that has a geometry shader behind it doesn't even need to write to gl_Position.

Also, it seems that GL_TRIANGLES is not supported as one of the possible geometry output types.

You must be using ARB_geometry_shader4, not the actual core geometry shader functionality. You probably should avoid that extension if you are able. Any hardware that has geometry shaders can run OpenGL 3.2.

In any case, the core feature doesn't support triangles as output. It supports points, line strips, and triangle strips.

Is it possible to make cubes if GL_TRIANGLES is not supported?

That's what EndPrimitive() is for. You call it when you are finished with a primitive; there's nothing that stops you from emitting a second primitive. Or third.

Also, you should be advised that this will probably be slow. Geometry shaders are not known for fast rendering performance.