Question

gl_PointCoord returns the 2D vector representing the UV mapping coordinates at that pixel. But it appears to be only available in the fragment shader. I know that you can send values from the vertex shader to the fragment shader with varying, but can you send it the other way? Can I get the UV coordinates for the current vertex?

Is it possible access gl_PointCoord in a vertex shader?

Answer 1

“UV mapping coordinates at that pixel” – Indeed, but in the vertex shader you don’t have any pixels. You’re missing how point sprites actually work. A single vertex shader invocation putting out a gl_PointSize larger than 1 pixel results in multiple fragment shader invocations for this point (because the point, well, covers multiple pixels). And those fragments get their gl_PointCoord based on their position inside this larger point sprite. But each point sprite covering multiple fragments always results from a single vertex. So conceptually this single per-vertex gl_PointCoord would just be (0.5, 0.5) for each vertex, but in the end even this doesn’t make sense because there really is no notion of any pixels let alone per-pixel UV-coordinates inside a vertex shader.

“but can you send it the other way” – And I’m sure this was meant rather informally, since of course you cannot send any data from a fragment shader to the vertex shader, as the graphics pipeline only works in a single direction.

EDIT: Regarding your comment, if not using point sprites, the gl_PointCoord variable doesn’t have any meaning at all. You cannot use it as a general UV-coordinate (how should the fragment shader come up with a reasonable value). For that you have to put out a valid texture coordinate youself from your vertex shader (as a varying) that then gets interpolated over the fragments to be accessed in the fragment shader, maybe ultimately passed into the vertex shader as an additional attribute, as is usual.