Calculating Depth value in Fragment Shaders.

[Vaticanfox]

Is it possible to compute a fragment's depth value in a fragment shader, and if so, does anyone have an equation or algorithm that does this? Or is the equation used to generate depth values platform specific? I want to use it to compare a fragment's depth value with those stored in a depth texture representing a terrain mess' depth values, sort of like doing depth testing in the fragment shader but with more control.

[zed]

in glsl the fixed function fragments depth can be optained with gl_FragCoord.z (or is this not what u mean)

[Vaticanfox]

I'm using Cg, so I don't think I have access to that value.

[Vaticanfox]

Actually, does that represent the value that would be written to the depth buffer?

[Relic]

You have access to that in Cg. Search the Cg manual for the WPOS semantic.

And for the other question read the GLSL spec.
"The variable gl_FragCoord is available as a read-only variable from within fragment shaders and it holds the window relative coordinates x, y, z, and 1/w values for the fragment. This value is the result of the fixed functionality that interpolates primitives after vertex processing to generate fragments. The z component is the depth value that would be used for the fragment’s depth if a shader contained no writes to gl_FragDepth. This is useful for invariance if a shader conditionally computes gl_FragDepth but otherwise wants the fixed functionality fragment depth."

[Vaticanfox]

With respect to distance from eye, what kind of function is used to calculated depth? I know it's not linear.

[Relic]

Depends on the space. E.g. distance from eye is linear in eye-space.
Check the transformation matrices in the OpenGL Programming Guide (RedBook, search for the PDF) appendix about homogenous transformations or read the OpenGL 2.0 spec chapter 2.11 on transformations.