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Document the meaning of "premultiplied alpha"

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Andrew Murray 2022-12-29 13:18:45 +11:00
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docs/handbook/concepts.rst
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@ -64,6 +64,12 @@ Pillow also provides limited support for a few additional modes, including:
* ``BGR;24`` (24-bit reversed true colour) * ``BGR;24`` (24-bit reversed true colour)
* ``BGR;32`` (32-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 Apart from these additional modes, Pillow doesn't yet support multichannel
images with a depth of more than 8 bits per channel. images with a depth of more than 8 bits per channel.