A premultiplied image is one where the RGB channels have been multiplied by the alpha channel. In other words, it is a four-channel image in which the red, green and blue channels have already been multiplied by the integrated matte channel.
Typically, images from a 3D renderer are premultiplied, which means that the transparent areas are black in both the RGB channels and the alpha channel. As such, in premultiplied images, the RGB channels never have a higher value than the alpha channel.
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