Abstract: An algorithm for simulating diffuse interreflection in complex three dimensional scenes is described. It combines techniques from hierarchical radiosity and multiresolution modeling. First, the radiosity method is reformulated in terms of vector irradiance and power. Also, a new face clustering technique for automatically partitioning polygonal models is described. The face clusters group adjacent triangles with similar normal vectors. They are used during radiosity solution to represent the light reflected by a complex object at multiple levels of detail. Together, face clustering and the vector formulation of radiosity permit large savings. Excessively fine levels of detail are not accessed by the algorithm during the bulk of the solution phase, greatly reducing its memory requirements relative to previous methods. Consequently, the costliest steps in the simulation can be made sub-linear in scene complexity. Using this algorithm, radiosity simulations on scenes of one million input polygons can be computed on a standard workstation.