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Document the meaning of "premultiplied alpha" #6838

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Dec 29, 2022
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Document the meaning of "premultiplied alpha" #6838

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6 changes: 6 additions & 0 deletions docs/handbook/concepts.rst
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Expand Up @@ -64,6 +64,12 @@ Pillow also provides limited support for a few additional modes, including:
* ``BGR;24`` (24-bit reversed true colour)
* ``BGR;32`` (32-bit reversed true colour)

Premultiplied alpha is where the values for each other channel have been
multiplied by the alpha. For example, an RGBA pixel of ``(10, 20, 30, 127)``
would convert to an RGBa pixel of ``(5, 10, 15, 127)``. The values of the R,
G and B channels are halved as a result of the half transparency in the alpha
channel.

Apart from these additional modes, Pillow doesn't yet support multichannel
images with a depth of more than 8 bits per channel.

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