  In this paper we show that the introduction of an attenuation factor in the
%image irradiance brightness equations relative to various perspective Shape
from Shading models allows to make the corresponding differential problems
well-posed. We propose a unified approach based on the theory of viscosity
solution and we show that the brightness equations with the attenuation term
admit a unique viscosity solution. We also discuss in detail the possible
boundary conditions that we can use for the Hamilton-Jacobi equations
associated to these models.
