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From Odwiki

Premultiplication

In order to composite images correctly with an "Over" CompositingOperation, we multiply an images Color by its Alpha. This gives the impression that the object is rendered in a black environment (as opposed to nothing environment). In the case of semi-transparent objects, we do this premultiplication in the shader code so that mantra can composite all the overlapping samples in depth together with an Over operation (internally in mantra). Mantra will also premultiply geometric silhouette edges of objects in the scene. This behaviour cannot be changed even if you attempt to set SuperSampling to 1x1. The only option in this case is to UnPreMultipy the image afterwards. You can do this in COPs with the PremultiplyCOP, or simply in the shader with a simple VEX operations or a couple of VOPs.

See Also

• UnPreMultiply