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Please correct me if I'm wrong. When using vertex and pixel shaders, we usually provide the code to compute the output gl_position of the vertex shader. Then, we find ouselves with the input gl_FragCoord in the pixel shader. What are the name of the operations performed by OpenGL to compute gl_FragCoord from gl_position ? Is it correct that those are "projection" and "clip coordinates transform" ? Then, what exactly are the transformations performs during those operations ? In other terms, what is the mathematical relation between gl_FragCoord and gl_position, that I could use to replace the openGL function ?
Thank you very much for any contribution.
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Vertex shaders manipulate coordinates in a 3D space and are called once per vertex. The purpose of the vertex shader is to set up the gl_Position variable — this is a special, global, and built-in GLSL variable. gl_Position is used to store the position of the current vertex.
Available only in the fragment language, gl_FragCoord is an input variable that contains the window relative coordinate (x, y, z, 1/w) values for the fragment. If multi-sampling, this value can be for any location within the pixel, or one of the fragment samples.
A fragment shader processes… fragments. A fragment is basically a position on the window (X,Y), a depth value (Z), plus all the interpolated data from previous stages.
varying variables contain data shared from a vertex shader to a fragment shader. The variable must be written in the vertex shader and the read-only value in the fragment shader is then interpolated from the vertices which make up the fragment.
gl_Position is in post-projection homogeneous coordinates.
It's worth noting that gl_Position is the (4d) position of a vertex, while gl_FragCoord is the (2d) position of a fragment.
The operations that happen in between are
So, while you can find the formula to transform the arbitrary point that is represented from the vertex position in the 2d space that is the viewport, it's not in and of itself that useful, as the generated fragments do not align directly to the vertex position. the formula to get the 2d coordinate of the vertex is
2d_coord_vertex = viewport.xy + viewport.wh * (1 + gl_Position.xy / gl_Position.w)/2
Again, this is not gl_FragCoord. Check the details on rasterization in the GL specification if you want more in-depth knowledge.
I'm not sure exactly what you mean by "replace the openGL function", but rasterizing is non-trivial, and way beyond the scope of an SO answer.
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