  The discrete cosine transform is a valuable tool in analysis of data on
undirected rectangular grids, like images. In this paper it is shown how one
can define an analogue of the discrete cosine transform on triangles. This is
done by combining algebraic signal processing theory with a specific kind of
multivariate Chebyshev polynomials. Using a multivariate Christoffel-Darboux
formula it is shown how to derive an orthogonal version of the transform.
