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:kvankoo...@virtualproteins.com] Sent: Wednesday, April 15, 2009 5:49 AM To: osg-users@lists.openscenegraph.org 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 _______________________________________________ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.or g _______________________________________________ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
