Premultiplied (integral) alphas:
- pixel contains (α, αr, αg, αb)
- saves computation
αresultcresult = αfg cfg + αbgcbg - αfgαbgcbg
- alpha value constrains color magnitude
- alpha modulates image shape
- conceptually clean - multiple composites are well defined
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Non-premultiplied (independent) alphas:
- pixel contains (α, r, g, b)
- what Photoshop does
- color values are independent of alpha
- transparent pixels have a color
- divison required to get color component back
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