Hi Thomas,
it is indeed a matter of the actual argument names. As the function
interface just duplicates glOrtho(), it probably won't confuse anyone
but me. My personal pick would be nearDistance, farDistance or something
similar.
Regards,
Kees
Jolley, Thomas P wrote:
Hi Kees,
I would expect (0,0,-zNear,1) to transform to (0,0,-1,1).
zFar and zNear are positive in the direction you're looking and negative
behind you. I didn't see anything in the osg code that defines zFar and
zNear as being the actual Z values. Perhaps having the variables begin
with "z" is misleading you. They really don't have anything to do with
the Z axis. Dropping the "z" is a problem since some compilers use near
and far as keywords. Less confusing names might be nearClip and
farClip.
-----Original Message-----
From: Kees van Kooten [mailto:[email protected]]
Sent: Wednesday, April 15, 2009 5:49 AM
To: [email protected]
Subject: [osg-users] The orthographic projection near/far plane
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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