Question

I was looking at examples of being able to represent transformations using a matrix instead of the built in functions. I want to learn more about the low level functions WITHOUT using any of opengl’s matrix or transform functions

Say I have a matrix class:
class Matrix
{
public:
float data [ 3 ] [ 3 ];

Matrix ( void )
{
    int i, j;

    for ( i = 0; i < 3; i++ )
    {
        for ( j = 0; j < 3; j++ )
        {
            data [ i ] [ j ] = 0;
        }sa
    }
}
};

Example function:

Matrix scale ( Pt p, float alpha )
{
    Matrix rvalue;


        rvalue.data[0][0] = alpha;
        rvalue.data[0][1] = 0;
        rvalue.data[0][2] = (1-alpha)*p.x;

        rvalue.data[1][0] = 0;
        rvalue.data[1][1] = alpha;
        rvalue.data[1][2] = (1-alpha)*p.y;


        rvalue.data[2][0] = 0;
        rvalue.data[2][1] = 0;
        rvalue.data[2][2] = 1;


    return rvalue;
}

How would I apply those transformations using opengl? I’m not where where to start. Is there a function that cane take a 3×3 matrix and load it in?

Answer 1

You probably want one of glLoadMatrix or glMultMatrix. They take column-major 4×4 matrices so you’ll want to write a shim like:

GLfloat matrix[16];

matrix[0] = data[0][0];
matrix[1] = data[0][1];
matrix[2] = data[0][2];
matrix[3] = 0.0f;

matrix[4] = data[1][0];
matrix[5] = data[1][1];
matrix[6] = data[1][2];
matrix[7] = 0.0f;

matrix[8] = data[2][0];
matrix[9] = data[2][1];
matrix[10] = data[2][2];
matrix[11] = 0.0f;

matrix[12] = 0.0f;
matrix[13] = 0.0f;
matrix[14] = 0.0f;
matrix[15] = 1.0f;

glLoadMatrixf(matrix);

So I’ve just padded out your data with the relevant parts of the identity matrix.