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/