Here’s a study from life of a giant albino bullfrog from an aquarium. The creature was the size of a plucked chicken, and about the same color. He held still for twenty minutes while I did this study.
I drew him in pencil on gray mat board, and then laid a milky wash of opaque watercolor over his whole body, saving the brightest whites for the accents and highlights. When the overall light wash was dry, I added the dark accents in pencil. These include the pupil of the eye and the places where forms push together in the folds and wrinkles.
Lighting specialists in the 3-D CG animation field call these dark places “occlusion shadows.”
Wherever two forms touch each other, or a form touches a floor, a dark line or accent results. You can see the effect by pressing your fingers together and looking at the little dark line where they touch. Not much light makes it to that point of contact. You’ll also notice it gets darker in the inside corner of a room where the walls meet.
Computer lighting programs don’t create this dark accent automatically. Until recently it had to be added by hand. But software pioneers have recently made lighting tools that can anticipate when the light will be occluded and such an accent will appear.
As a traditional oil painter, I'm fascinated by such new terminology and visual analysis developed my brother artists in the CG arena. I wonder if one of you who is familiar with 3-D CG lighting might be willing to comment on the challenges presented by occlusion shadows.