by Hello,
I'm using OpenGL for my perspective projection, but there's something I can't
understand. Here is the perspective matrix, as it is computed after a call to
glFrustum().
So if you call glFrustum(l, r, b, t, n, f), the perspective matrix is :
_ _
| 2n r+l |
| ----- 0 --- 0 |
| r-l r-l |
| |
| 2n t+b |
R = | 0 ---- --- 0 |
| t-b t-b |
| |
| -(f+n) -2fn |
| 0 0 ------ ---- |
| f-n f-n |
| |
| 0 0 -1 0 |
|_ _|
(taken from the OpenGL programming guide, and the Mesa source code)
Okay, so now, let's take a concrete example.
If I call glFrustum(-3, 3, -2, 2, 1, 10), then the following perspective matrix
is generated :
[ 0.3333 0 0 0 ]
[ 0 0 0 0 ]
R = [ 0 0 -1.222 2.222 ]
[ 0 0 -1 0 ]
Now, let's take the point (-3, 2, 10, 1), and let's apply the perspective matrix
on it. The result is (-1, 1, -10, -10). Until now, all is fine.
But if now I take another point (-3, 2, 50, 1), then I obtain the same x and y
coordinates as the previous example ! (i.e. x = -1 and y = 1).
Normally, since the second point is farther than the first one, the x and y
coordinates should not be the same !! They should be smaller !
But as we can see in the perspective matrix, all the matrix elements that could
be influenced by the value of the z coordinate are set to 0, so you can set the
z coordinate to any value, it won't change anything !! (which is the opposite of
the perspective transformation method, where the z coordinate is very
important).
So could someone tell me what's going wrong, please ?
It's common to have r == -l and b == -t, no ? But in this case, many matrix
elements become equal to 0, generating the problem I described above...
Thank you very much for your precious help ! (and please send a copy of your
replies to my personal email address, since my news server discards a lot of
messages).
Best regards,
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