# gl_FragDepth computation

Hello,

I’ve got two seemingly stupid problems regarding depth writes in shader (the first one could very well go to the beginner’s section, but anyway)

# 1st (sillier):

From steve baker’s many-times referenced article about z-buffer, we have the following :

a = zFar / ( zFar - zNear )
b = zFar * zNear / ( zNear - zFar )
gl_FragDepth = a + (b / eyeZ)

But, since ‘a’ is positive & usually around 1, and ‘b’ negative, shouldn’t eyeZ be positive, in order to get a value less than 1.0? But eyeZ is always negative. So, in the above formula eyeZ should be positive, or my logic is dead wrong??

# 2nd

I have some quads for which I want to specify depth. I have some linear depth values in a texture, from 0.0 to 1.0, and I want to map them in order to recreate a virtual volume (cube), say with an extent cubeExt.

Now, when I render the quads, in the fragment shader I add cubeExt*sampledDepth to the eyeZ, and using the above formula, I compute & write gl_FragDepth. But the results are … wrong. The code is this :

``````
float new_eyez = - (eyez - cubeExtent*sampledDepth);
a = zFar / ( zFar - zNear )
b = zFar * zNear / ( zNear - zFar )
gl_FragDepth = a + (b / new_eyez)

``````

Is there anything wrong with the above?? (should be I guess)

Many thanks,
babis

1. Yes, if written this way eyeZ should be positive.
2. Could you describe the results in more detail?

Distance and Z buffer can be converted with help of two values from the projection matrix:
Z = pm.z/(Zbuffer * -2.0 + 1.0 - pm.z)

Zbuffer = (pm.z/ Z + pm.z -1.0)*-0.5

Thanks guys,

oc2k1 : shouldn’t the formula, if we replace the pm.z & pm.z with their variable equivalents (-2/(far - near), -(far + near)/(far - near)), result in the same formula I wrote initially? (Either it doesn’t, or I’ve forgotten maths)

Xmas : Actually I found and solved the problem, but another one (which I expected) appeared. I’m trying to pack the depth values along with transparency (its either opaque or not), but the mipmap generation unleashes hell to the values as I lose my 0-or-1 transparency. Any chance to pack 1-bit transparency with 7-bit depth values, in a mipmap friendly way?? (probably asking too much)

The ‘wanted’ look is like this :
http://www.fileden.com/files/2008/1/8/1687460/good.PNG

The problem looks like this :
http://www.fileden.com/files/2008/1/8/1687460/problem.PNG
(in the outlines of objects the depth values are completely wrong, so they pop up front)

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