Hi,
I don't know whether this has been brought up before (I couldn't find
any posts regarding this matter in the user forum), but when using the
orthographic projection matrix's setProjectionMatrixAsOrtho(), the
near/far plane z values supplied as arguments are not set as according
to the OpenGL spec. Specifically, the third column of the projection
matrix as applied by osg is:
0.0,
0.0,
-2.0/(zFar-zNear),
-(zFar+zNear)/(zFar-zNear)
with zFar and zNear being the actual Z values of the far and near plane
- as opposed to the distance to the far and near plane mentioned in the
spec, which in practice turns out to be the negated z value of the far
and near plane.
If this has been a conscious choice, it seems highly confusing to me.
For example, the vector (0,0,zNear,1) now transforms to
(0,0,-(3zN+zF)/(zF-zN),1) in clip space, while the expected
transformation would yield (0,0,-1,1). In practice, it gave me a few
problems integrating an orthographic projection with depth to a
partially rendered scene.
Of course, the workaround is easy, as one should just supply the negated
zNear and zFar to setProjectionMatrixAsOrtho(), and in a certain way
this is all just a matter of interpretation of the function arguments.
In my opinion, supplying the actual z values for the near and far plane
is the more intuitive choice, as this is what the names of the function
arguments imply.
Best regards,
Kees van Kooten
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