GLSL gl_FragCoord.z Calculation and Setting gl_FragDepth

Question

So, I\'ve got an imposter (the real geometry is a cube, possibly clipped, and the imposter geometry is a Menger sponge) and I need to calculate its depth.

I can calc

Answer 1

For another future reference, this is the same formula as given by imallett, which was working for me in an OpenGL 4.0 application:

vec4 v_clip_coord = modelview_projection * vec4(v_position, 1.0);
float f_ndc_depth = v_clip_coord.z / v_clip_coord.w;
gl_FragDepth = (1.0 - 0.0) * 0.5 * f_ndc_depth + (1.0 + 0.0) * 0.5;

Here, modelview_projection is 4x4 modelview-projection matrix and v_position is object-space position of the pixel being rendered (in my case calculated by a raymarcher).

The equation comes from the window coordinates section of this manual. Note that in my code, near is 0.0 and far is 1.0, which are the default values of gl_DepthRange. Note that gl_DepthRange is not the same thing as the near/far distance in the formula for perspective projection matrix! The only trick is using the 0.0 and 1.0 (or gl_DepthRange in case you actually need to change it), I've been struggling for an hour with the other depth range - but that is already "baked" in my (perspective) projection matrix.

Note that this way, the equation really contains just a single multiply by a constant ((far - near) / 2) and a single addition of another constant ((far + near) / 2). Compare that to multiply, add and divide (possibly converted to a multiply by an optimizing compiler) that is required in the code of imallett.

Answer 2

For future reference, the key code is:

float far=gl_DepthRange.far; float near=gl_DepthRange.near;

vec4 eye_space_pos = gl_ModelViewMatrix * /*something*/
vec4 clip_space_pos = gl_ProjectionMatrix * eye_space_pos;

float ndc_depth = clip_space_pos.z / clip_space_pos.w;

float depth = (((far-near) * ndc_depth) + near + far) / 2.0;
gl_FragDepth = depth;