I'm trying to use the unscaled (true distance from the front clipping plane) distance to objects in my scene in a GLSL fragment shader. The gl_FragCoord.z value is smaller than I expect. In my vertex shader, I just use ftransform() to set gl_Position. I'm seeing values between 2 and 3 when I expect them to be between 15 and 20.
How can I get the real eye-space depth? Thanks!
Pass whatever you want down as a varying from the vertex shader.
the Z value available in the fragment shader has gone through normalization based on your z-near/z-far (from the projection matrix), and DepthRange. So it is not directly what you're after. Technically, you could try to reconstruct it by reverting the various OpenGL operations on Z that happen after the vertex shader, but it's probably more trouble (starting with the fact that reverting the projection matrix is non-linear) than just passing down what you want, exactly.
As a side note, the Z you would compute with gl_ModelViewMatrix * gl_Vertex is the Z from the point of view, not the near-Z plane.