What does the 1/w coordinate stand for in gl_FragCoord?

fragment-shader, opengl

Solution

The GLSL and OpenGL specifications are needlessly confusing in this regard. The OpenGL spec is easier to understand: `gl_FragCoord` stores the X, Y, and Z components of the window-space vertex position. The X, Y, and Z values are calculated as described for computing the window-space position (though the pixel-center and upper-left origin can modify the X and Y values). This is described in the `Coordinate Transforms` section of the spec.

The W component of `gl_FragCoord` is (1 / Wc), where Wc is the clip-space vertex position. It's `gl_Position.w` from your vertex shader.

The only useful purpose for keeping Wc around is to reverse-transform `gl_FragCoord` to get the clip-space position back. Which, as that page shows, requires multiplying by Wc. But since `gl_FragCoord` only stores the inverse of this value, it now requires dividing by `gl_FragCoord.w`.

Therefore, we can assume that OpenGL stores it this way because OpenGL isn't allowed to make too much sense ;) See, it's a rule that every part of the OpenGL specification must have something that's a bit nonsensical. The XYZ components made too much sense, so they decided to have it stores the inverse of the value you actually want.

OK, technically this is a historical artifact from the days when 3D Labs created GLSL. I'm sure they did it for purely selfish hardware reasons, but I have no real proof of that.

Problem

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