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What is a bounding box?


Jacki Silverfall
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A bounding box is the physical shape (that you run into or that other physical shapes encounter) for a given object. It defines the physical shape of an object, though it may be much different in size and shape than the objects visual appearance. All prim types have them (prim, sculpty, mesh).

• With regular primitives it is defined by a shape almost identical to the size and shape of the prim

• With sculpted prims, it is a box shape but the visual appearance of the item may be much smaller within the edges of that box that the physical shape. You can scale sculpts to fit closer to the limits of the bounding box edges, however, to limit this, though with odd shapes this becomes much more difficult. In some apps this function is called "maximize."

• With mesh prims, however, you have many more choices in how you define the physical shape, instead of just a box. Generally a very simple, rough outline of the shape will do, and it can be based on the lowest level of detail shape you import. This can be specified on import of your mesh item into SL, though you can get much more reliable results by defining a specific shape in your modelling program, as with the different LOD's. Once specified, your physical shape is what you will bump into, stand or sit on, etc.

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Actually, what Maxwell was describing was the physics shape (also sometimes referred to as the collision proxy).  The "bounding box" is a box with sides parallel to the X, Y, and Z axes just large enough for the entire model to fit inside.  I will have to check, but I believe the XYZ dimensions shown on the Edit window are for the bounding box of the model. 

Besides limiting how large you can scale the model once in SL (64x64x64m), the center of the bounding box is what determines the XYZ position of the model in the region, and the diagonal is used to calculate the Prim Equivalent (PE) streaming cost. 

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