So, in my vertex shader I'm doing something like this:
void main() {
gl_Position = vec4((u_matrix * vec3(a_position, 1)).xy, 0, 1);
}
But now I'm in my fragment shader and I would like to access that location. That is, I'm looking for a variable to access the current location of the pixel being drawn, but in clip-space coordinates. The reason for this is that I am looking to blend that pixel with a pixel at the same location in a texture.
It looks like gl_FragCoord is similar to what I want, but it's in window-space. I think I would be able to convert it like so:
vec2 what_i_want = vec2(gl_FragCoord.x / 1920., gl_FragCoord.y / 1080.);
But then I need to know my canvas size. I could pass in the canvas size as a uniform, but that seems like a bit much. Is there no way to just access this value somehow in the fragment shader?
You are on the right track. You must set the size of the viewport (canvas) in a uniform variable. There is no other option. That's what everyone does in WebGL as in OpenGL.
uniform vec2 u_resolution;
void main()
{
vec2 uv = gl_FragCoord.xy / u_resolution;
// [...]
}