  We provide a qualitative and quantitative evaluation of 8 clear sky models
used in Computer Graphics. We compare the models with each other as well as
with measurements and with a reference model from the physics community. After
a short summary of the physics of the problem, we present the measurements and
the reference model, and how we "invert" it to get the model parameters. We
then give an overview of each CG model, and detail its scope, its algorithmic
complexity, and its results using the same parameters as in the reference
model. We also compare the models with a perceptual study. Our quantitative
results confirm that the less simplifications and approximations are used to
solve the physical equations, the more accurate are the results. We conclude
with a discussion of the advantages and drawbacks of each model, and how to
further improve their accuracy.
