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Color correcting a premultiplied foreground incorrectly alters the background (left). Color correcting a nonpremultiplied foreground works correctly (right).
Double Premultiplied RGBA Means Double Trouble
A common mistake made by many artists is to over-compensate for premultiplication. As important as it is to premultiply the alpha before compositing in a Merge node, it’s just as important not to double premultiply the alpha. Performing a premultiply operation two times in a row can create a darken halo effect around your images. You are effectively multiplying by the gray semi-transparent pixels twice; this is not optimal.
Premultiplied Alpha Channels and Filtering
When dealing with filtering, the state of the RGBA channels shouldn’t matter for most composites. However, an exception might be if the filter algorithm you choose includes color modification. For instance, if a filter attempts to simulate a defocus by blooming highlights like a real light source, that filter might over-brighten pixels near a transparent edge, which will result in some manner of artifact when that image is composited.
Premultiplied Alpha Channels and Filtering
When dealing with filtering, the state of the RGBA channels shouldn’t matter for most composites. However, an exception might be if the filter algorithm you choose includes color modification. For instance, if a filter attempts to simulate a defocus by blooming highlights like a real light source, that filter might over-brighten pixels near a transparent edge, which will result in some manner of artifact when that image is composited.
Premultiplied Alpha Channels and Filtering
When dealing with filtering, the state of the RGBA channels shouldn’t matter for most composites. However, an exception might be if the filter algorithm you choose includes color modification. For instance, if a filter attempts to simulate a defocus by blooming highlights like a real light source, that filter might over-brighten pixels near a transparent edge, which will result in some manner of artifact when that image is composited.
Double premultiplied image displays dark edges (left); premultiplied image with correct edges (right)
Alpha Channel Status in MediaIn and Loader Nodes
When using a Loader node to add a clip to the composite, the Import tab in the Inspector includes a group of checkboxes that let you determine how an embedded alpha channel is handled. There are checkboxes to make the alpha channel solid (ignore transparency), to Invert the alpha channel, and to Post-Multiply the RGB channels with the alpha channel, should that be necessary.