On 10 Apr 2020, at 06:33, John Moore <bigfish@pacbell.net> wrote:regular key and linear key that allowed for a feathered key. I've always considered premultiplied vs. straight in the same way
Big mistake. The alpha is always 'feathered' (A bw gradient defining the opacity.)
The difference is in the foreground. A straight alpha file has all colors 'as they are' before applying the key. Hence they can look very strange if you have heavy gradients (fade to opaque.)
Premultiplied has the fg mixed to black based on the alpha, so if you open a file without applying the key it looks pretty, same as a non straight keyed to black.
Mathematical there is no difference between the two, but indeed the keying app (in this case Avid) must be able to undo the math, and hence it must know what to expect.
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