c++ OpenGL rotations and calculations

Question

In an OpenGL project, I have a function that rotates the model:

glRotatef(M_PI/4, 0, 0, 1); //rotate by PI/4 radians i.e 45 degrees

Now when I

Answer 1

Your question is a little confusing, as I don't understand what you mean by this stopping criteria. But if you want to get the transformed values for points, then you can simply pass them through the appropriate transform matrices to get them into the space you want. If you want the values in eye space, then pass your vertex coordinates through the modelview matrix, for example. See here for details of how that works.

Answer 2

if you use glRotatef from OpenGL do not forget that rotation angle is in Degrees not in radians !!!

for the transformation and confusion part user1118321 is absolutely right.

I thing for your purposes you need only to apply GL_MODELVIEW_MATRIX

if you need coordinates in model space you must multiply inverse modelview matrix by global coordinates vector.

If you need coordinates in global space than you must multiply modelview matrix by model coordinates vector.

coordinates send by vertex are in model space you can obtain actual modelview matrix with gl functions glGetFlotav/glGetDoublev

double m[16];
glGetDoublev(GL_MODELVIEW_MATRIX,m);

inverse matrix and matrix x vector multiplication is not present in OpenGL so you must code it yourself or use some lib. Do not forget that matrices in OpenGL are column oriented not row oriented and coordinate vectors are homogenuous so x,y,z,w where w=1 for your purposes. Here is code I use for my OpenGL sub-calculations, all vectors are double[4] and matrices are double[16]

void  matrix_mul_vector(double *c,double *a,double *b)
        {
        double q[3];
        q[0]=(a[ 0]*b[0])+(a[ 4]*b[1])+(a[ 8]*b[2])+(a[12]);
        q[1]=(a[ 1]*b[0])+(a[ 5]*b[1])+(a[ 9]*b[2])+(a[13]);
        q[2]=(a[ 2]*b[0])+(a[ 6]*b[1])+(a[10]*b[2])+(a[14]);
        for(int i=0;i<3;i++) c[i]=q[i];
        }
void  matrix_subdet    (double *c,double *a)
        {
        double   q[16];
        int     i,j;
        for (i=0;i<4;i++)
         for (j=0;j<4;j++)
          q[j+(i<<2)]=matrix_subdet(a,i,j);
        for (i=0;i<16;i++) c[i]=q[i];
        }
double matrix_subdet    (         double *a,int r,int s)
        {
        double   c,q[9];
        int     i,j,k;
        k=0;                            // q = sub matrix
        for (j=0;j<4;j++)
         if (j!=s)
          for (i=0;i<4;i++)
           if (i!=r)
                {
                q[k]=a[i+(j<<2)];
                k++;
                }
        c=0;
        c+=q[0]*q[4]*q[8];
        c+=q[1]*q[5]*q[6];
        c+=q[2]*q[3]*q[7];
        c-=q[0]*q[5]*q[7];
        c-=q[1]*q[3]*q[8];
        c-=q[2]*q[4]*q[6];
        if (int((r+s)&1)) c=-c;       // add signum
        return c;
        }
double matrix_det       (         double *a)
        {
        double c=0;
        c+=a[ 0]*matrix_subdet(a,0,0);
        c+=a[ 4]*matrix_subdet(a,0,1);
        c+=a[ 8]*matrix_subdet(a,0,2);
        c+=a[12]*matrix_subdet(a,0,3);
        return c;
        }
double matrix_det       (         double *a,double *b)
        {
        double c=0;
        c+=a[ 0]*b[ 0];
        c+=a[ 4]*b[ 1];
        c+=a[ 8]*b[ 2];
        c+=a[12]*b[ 3];
        return c;
        }
void  matrix_inv       (double *c,double *a)
        {
        double   d[16],D;
        matrix_subdet(d,a);
        D=matrix_det(a,d);
        if (D) D=1.0/D;
        for (int i=0;i<16;i++) c[i]=d[i]*D;
        }

For more info see:

• Understanding 4x4 homogenous transform matrices