What is occlusion?

Question

Occlusion "is the effect of one object in a 3-D space blocking another object from view."

With this definiton in mind I cannot understand sentences like:

The interior of a tube is typically more occluded (and hence darker) than the exposed outer surfaces, and the deeper you go inside the tube, the more occluded (and darker) the lighting becomes.

Occlusion is for me a question of yes or no. Yes an object/point is in front of another, or not.

So how can some points be more or less occluded then others? I think I need another definiton for occlusion.

Answer 1

This quote sounds like it may relate to ambient occlusion, which is the shielding of a surface from the light approaching from all angles. Direct lighting requires that the light source have a straight line route to the surface being lit, whereas ambient lighting is the result of light approaching along many different paths.

Imagine the whole of a cloudy sky as a light source. Each point on a surface is lit according to the proportion of the sky that can directly light it. The floor next to a wall might only be able to be lit by half the sky, so will be half as bright. In this situation you could say that half of the sky is occluded, leaving only half to light the surface.

Similarly with a tube. At the entrance of the tube, 50% of the sky may be able to light it. Further inside the tube only the small patch of sky visible through the end of the tube can light it, and that patch becomes smaller the further you go into the tube. The lighting in the tube therefore diminishes the further you go in.