# Clip Space Approach – Implementation Details

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The following function, from class FrustumG, performs the plane extraction as described above, assuming that the matriz A=M*P is given as a parameter:

#define m(row,col) m[col*4+row-5] void FrustumG::setFrustum(float *m) { pl[NEARP].setCoefficients( m(3,1) + m(4,1), m(3,2) + m(4,2), m(3,3) + m(4,3), m(3,4) + m(4,4)); pl[FARP].setCoefficients( -m(3,1) + m(4,1), -m(3,2) + m(4,2), -m(3,3) + m(4,3), -m(3,4) + m(4,4)); pl[BOTTOM].setCoefficients( m(2,1) + m(4,1), m(2,2) + m(4,2), m(2,3) + m(4,3), m(2,4) + m(4,4)); pl[TOP].setCoefficients( -m(2,1) + m(4,1), -m(2,2) + m(4,2), -m(2,3) + m(4,3), -m(2,4) + m(4,4)); pl[LEFT].setCoefficients( m(1,1) + m(4,1), m(1,2) + m(4,2), m(1,3) + m(4,3), m(1,4) + m(4,4)); pl[RIGHT].setCoefficients( -m(1,1) + m(4,1), -m(1,2) + m(4,2), -m(1,3) + m(4,3), -m(1,4) + m(4,4)); } #undef M

The function `setCoefficients`

from the class Plane is as follows:

void Plane::setCoefficients(float a, float b, float c, float d) { // set the normal vector normal.set(a,b,c); //compute the lenght of the vector float l = normal.length(); // normalize the vector normal.set(a/l,b/l,c/l); // and divide d by th length as well this->d = d/l; }

To extract the matrices M and P from OpenGL state the function glGetFloatv can be used:

float m[16],p[16]; glGetFloatv(GL_PROJECTION_MATRIX,p); glGetFloatv(GL_MODELVIEW_MATRIX,m);

Matrix multiplication is then performed to compute A = M*P. A simple matrix multiplication such as the one below will do:

void multMat(float *res,float *a, float *b) { for (int i=0;i<4;i++) { for (int j = 0;j < 4;j++) { res[i*4+j] = 0.0; for (int k = 0; k < 4; k++) { res[i*4+j] += a[i*4+k] * b[k*4+j]; } } } }

The following is a solution for lazy people, using OpenGL to perform the multiplication for you (in my laptop it is actually slightly faster!) :

void multMat2(float *res, float *a, float *b) { glPushMatrix(); glLoadMatrixf(b); glMultMatrixf(a); glGetFloatv(GL_MODELVIEW_MATRIX, res); glPopMatrix(); }

Once the planes are extracted, testing points, spheres or boxes, works exactly as in the geometric approach.

Prev: Clip Space Approach - Extracting the Planes | Next: Radar Approach - Testing Points |

### 2 Responses to “Clip Space Approach – Implementation Details”

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I am attempting to implement this approach (based on the Hartmaan and Gribb method found here; http://www.cs.otago.ac.nz/postgrads/alexis/planeExtraction.pdf) but I am having little success.

I’m not using the fixed-function pipeline, so I have access to the model, view and projecton matrices seperately. Am I correct in thinking that, in this example, the Model matrix is always an identity?

When I move my camera, the frustum plane normals always seem to point toward the origin, regardless of camera position – I’m not sure where I’m going wrong.

I might be wrong here, in that case please excuse my ignorance, but when transforming a vertex (v) from object space to view space (v’), the order of multiplications is:

`v' = P * V * M * v`

Where P is the projection matrix, V is the view matrix, and M is the model matrix (of course, in OpenGL model and view are represented as a single model-view-matrix). And the order is important, since matrix multiplication is not commutative:

`A * B != B * A`

So is it really correct that

`A = M * P`

since that is exactly the opposite order used when transposing a vertex.