The Compound collision shape type lets you treat a group of objects as a single rigid body. 'When you select Compound, every mesh in the hierarchy under selected transform are considered sub-collision shapes of a compound.
Compound Examples: A tower made of 1000 bricks in a Rigid Body solve. The first tower is a compound object, while the second has each of its three tiers set as compound objects. The last two images show the individual brick set as Hull and Cube-type objcects.
Any collision shape attributes, other than Type, are not be used at this time. Dynamic & kinematic bodies: the sub-shapes are a type called Hull collision shape type.
Static bodies: the sub shapes are Mesh collision shape type.
Auto-Compound, on the other hand is used when you want to run simulations using complex (such as concave) objects. This is because Bullet has trouble with solving concave shapes. Auto-Compound automatically breaks down the object into many smaller convex shapes and solve them as an integral.
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