View Full Version : compute the depth value explicitly

01-15-2004, 07:29 PM
Hi all,
For some reason, I need to compute the depth values in the main memory and upload it into the Z buffer. Could anybody tell me how openGL computes the depth value given the z coordinate (world coordinate). I am using orthogonal projection.

I know this question might have been asked before, but I really couldn't find what I need.It would be greatly appreciated if anyone could help me with this.

01-15-2004, 08:32 PM
Compute the depth value of what?

It matters because if you want to transform a vertex, then you can use something like gluProject.

If you need a the depth of a certain fragment, then that involves a rasterizer's algorithm, in which case it depends on primitive, depends on the hw.

01-16-2004, 12:05 AM

The formula for Z-buffer depth based on the z-value of a vertex is :

Z_buffer = A * (B - C / Z_vertex)

Where :
A = 2^(N-1), N being the number of bits in your z-buffer (probably 24).
B = F/(F-N),
C = F*N/(F-N),
F = your far clipping plane Z-value
N = your near clipping plane Z-value

It considers that you're in modelview projection, and I don't know if it changes anything that you're in Ortho projection, so you'll have to figure that one out (or get another reply !)

Hope it helps,


01-16-2004, 08:54 AM
Thanks for help.

It seems that the above formula will give depth values greater than 1. However, the values read back from Z buffer are all clamped between 0 and 1. What else do I need to know?

Actually, I want to calculate the new depth value of a pixel from its old one assuming that the z-coordinate of the pixel is changed. More specifically, let d be the depth value of a pixel with z = z1. what would be its depth value if z = 2*z1-R where R is a constant?