local/global coordinate system problem

OpenGL OpenGL: Basic Coding
1

When I translate and rotate to a local coordinate system, how do i find the local coordinates of a point in global coordinates. In other words, I know the global coordinates for a vertex, then I move to a local coordinate system, how would I find the coordinates of the vertex with respect to the new local coordinate system?

2

Get the matrix that does the translations and multiply your vertex by it.

Chris

3

so, you mean use

glGetFloatv(GL_MODELVIEW_MATRIX,Minv);

to store the system into Minv and then do something like this?:

void VMatMult(GLmatrix16f M, GLvector4f v)
{
    GLfloat res[4];
    res[0]=M[ 0]*v[0]+M[ 4]*v[1]+M[ 8]*v[2]+M[12]*v[3];
    res[1]=M[ 1]*v[0]+M[ 5]*v[1]+M[ 9]*v[2]+M[13]*v[3];
    res[2]=M[ 2]*v[0]+M[ 6]*v[1]+M[10]*v[2]+M[14]*v[3];
    res[3]=M[ 3]*v[0]+M[ 7]*v[1]+M[11]*v[2]+M[15]*v[3];
    v[0]=res[0];
    v[1]=res[1];
    v[2]=res[2];
    v[3]=res[3];		
}
4

I think that’s right - I’m never quite sure what order GL’s matrices are in but I think you’ve got it.

Chris

5

Look at the other topic on matrix, I think we cover it pretty well now, it is over 70 replies now.

6

I stopped following when it hit 50

Chris