M. Langer, P. Breton, S.W. Zucker We present a massively parallel algorithm for computing radiosity in a scene containing an isotropic scattering medium of high albedo. The algorithm is based on a new formalism: a coordinate evolution on the set of light rays in the scene. The evolution reparameterizes rays from their points of origin to their points of termination. Local coordinate systems on the set of light rays are distributed over a two-dimensional lattice of parallel processors, and transformations between coordinate systems are computed using only local communication between processors. The algorithm is implemented on a MasPar-1 which is a SIMD computer having over 2000 processors.