Projection matrix question

Graphics and GPU Programming Programming OpenGL

Started by schupf December 07, 2011 05:01 PM

3 comments, last by Brother Bob 12 years, 4 months ago

221

Author

December 07, 2011 05:01 PM

Hello,

I am using OpenSceneGraph, which internally uses OpenGL. I transform a vector from view space to projection space with this code:



Vec4 vecVS(0, 0, -10, 1);

Vec4 vecProj = vecVS * projMat;

The view space vector has coordinates of (0, 0, -10, 1), so I expected the point to have x,y coordinates of 0,0, since the point is exactly in the middle
of the near plane. But when I print vecProj, I surprisingly get these values:

(0.000, -4.512, 8.018, 10.000)
[/quote]
x seems to be correct, but how can y be -4.512? Shouldn't y be 0 too? Has anyone an idea what could be wrong?

alvaro

21,604

December 07, 2011 05:05 PM

How did you make your projection matrix? And what is its value?

schupf

221

Author

December 07, 2011 05:41 PM

The matrix is automatically computed by OpenSceneGraph and looks like this:



 (1.732, 0.000, 0.000 0.000)                                   

 (0.000, 3.112, 0.000 0.000)                                   

 (0.000, 0.451, -1.002 -1.000)                                 

 (0.000, 0.000, -2.002 0.000)

knighty

313

December 08, 2011 09:50 AM

What about:

Vec4 vecVS(0, 0, -10, 1);

Vec4 vecProj = projMat * vecVS ; //Or someting like : vecProj = vecVS * transpose(projMat)

Brother Bob

10,350

December 08, 2011 10:47 AM

Given the vector and the projection matrix, the result you get is correct. The question is; why is there a 0.451 in the second element on the third row? That element is what is commonly used as the translation part of the matrix, so it means that you have a non-symmetric projection matrix (it is in the Y-direction, so your projection matrix has, for example, non-symmetric bottom and top values). If that is not what you want, then you have to go back to the source of that matrix and see why it is non-symmetric.

Projection matrix question

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