eigenvalue & eigenvector

Hello at all! I must get from an image eigenvalues and eigenvectors to realize person detect with colors. How can I obtain eigenvalues and eigenvectors? The results are as the trace of matrix S where to each color band I use partial derivatives rx, ry, gx, gy, bx, by for the (r)ed, (g)reen and (b)lue bands?  thanks for your time!