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

You'll want to look at this picture @ http://db-in.com/images/local_vectors.jpg

The Direction-of-Flight ( DOF) is the 3rd row.

GLfloat matrix[16]; 
glGetFloatv( GL_MODELVIEW_MATRIX, matrix );

float DOF[3];
DOF[0] = matrix[  2 ]; // x
DOF[1] = matrix[  6 ]; // y
DOF[2] = matrix[ 10 ]; // z

Reference:

• http://blog.db-in.com/cameras-on-opengl-es-2-x/

Answer 2

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.