class Point3Matrix
{
public:
    double matrix[9];

    Point3Matrix compose(const Point3Matrix& b)
    {
        Point3Matrix ret;
        for (int outx = 0; outx < 3; outx++)
        {
            for (int outy = 0; outy < 3; outy++)
            {
                ret.matrix[outy * 3 + outx] = 0;
                for (int in = 0; in < 3; in++)
                {
                    ret.matrix[outy * 3 + outx] += matrix[outy * 3 + in] * b.matrix[in * 3 + outx];
                }
            }
        }
        return ret;
    }
}

What does the compose function do? I can only deduct it is doing a row x col operation for two matrices.