File:Spawning science.jpg

Summary
Example of a mob pack spawning. The 41x1x41 spawning area is shaded blue. The yellow figures represent the actual positions that mobs could spawn in after checking the environment. Note that the mobs can spawn inside torch and ladder blocks. But they can't spawn on top of glass because it's not opaque. The red cube is the center of the pack, which must be an air block, but the blocks above and below it can be anything.