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Document "transparency" info key

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Andrew Murray 2022-12-28 10:22:10 +11:00
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docs/handbook/concepts.rst
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@ -111,6 +111,18 @@ the file format handler (see the chapter on :ref:`image-file-formats`). Most
handlers add properties to the :py:attr:`~PIL.Image.Image.info` attribute when
loading an image, but ignore it when saving images.
Transparency
------------
If an image does not have an alpha band, transparency may be specified in the
:py:attr:`~PIL.Image.Image.info` attribute with a "transparency" key.
Most of the time, the "transparency" value is a single integer, describing
which pixel value is transparent in an "1", "L", "I" or "P" mode image.
However, PNG images may have three values, one for each channel in an "RGB"
mode image, or can have a byte string for a "P" mode image, to specify the
alpha value for each palette entry.
Orientation
-----------