math - Extracting View Frustum Planes (Gribb & Hartmann method)

Question

I have been grappling with the Gribb/Hartmann method of extracting the Frustum planes for some time now, with little success. I want to build a camera view-frustum to cull my scene.

I am working with column-major matrices in a right-handed coordinate system. (OpenGL style - I'm using C# and Playstation Mobile, but the math should be the same)

I want to get my planes in World-Space, so I build my frustum from the View-Projection Matrix (that's projectionMatrix * viewMatrix). The view Matrix is the inverse of the camera's World-Transform.

The problem is; regardless of what I tweak, I can't seem to get a correct frustum. I think that I may be missing something obvious.

If I "strafe" my camera left or right while still looking down the z-axis, the normals of my planes change so that they are always pointing at the origin of the scene - which makes me think that they are not in world-space...

Answer 1

The planes from a projection matrix can be extracted using the Gribb/Hartmann method as follows, (column major):

void extract_planes_from_projmat(
        const float mat[4][4],
        float left[4], float right[4], float top[4], float bottom[4],
        float near[4], float far[4])
{   
    for (int i = 4; i--; ) left[i]      = mat[i][3] + mat[i][0];
    for (int i = 4; i--; ) right[i]     = mat[i][3] - mat[i][0]; 
    for (int i = 4; i--; ) bottom[i]    = mat[i][3] + mat[i][1];
    for (int i = 4; i--; ) top[i]       = mat[i][3] - mat[i][1];
    for (int i = 4; i--; ) near[i]      = mat[i][3] + mat[i][2];
    for (int i = 4; i--; ) far[i]       = mat[i][3] - mat[i][2];
}

Where mat4 is the product of the projection matrix and the model-view matrix.

See:

• https://fgiesen.wordpress.com/2012/08/31/frustum-planes-from-the-projection-matrix/
• http://www8.cs.umu.se/kurser/5DV051/HT12/lab/plane_extraction.pdf

Note: if the matrix components aren't normalized and you require a Hessian Normal Form plane, then you will need to normalize the resulting planes.