How can I get view direction from the OpenGL ModelView Matrix?

Question

I am writing a volume render program that constantly adjusts some plane geometry so it always faces the camera. The plane geometry rotates whenever the camera rotates in ord

Answer 1

Instead of trying to follow the modelview matrix, to adjust your volume rasterizer's fragment impostor, you should just adjust the modelview matrix to your needs. OpenGL is not a scene graph, it's a drawing system and you can, and should change things however they suit you best.

Of course if you must embedd the volume rasterization into a larger scene, it may be neccessary to extract certain info from the modelview matrix. The upper left 3×3 submatrix contains the composite rotation of models and view. The 3rd column contains the view rotated Z vector.