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Update ImageDraw.rst to avoid transparency issue

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Per the conversation https://github.com/python-pillow/Pillow/issues/2496#issuecomment-3268255530 I have added a FAR TOO LONG explanation of the situation and the resolution. Please for the love of Guido, help edit this down.
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Ross Olson 2025-09-08 18:38:07 -07:00 committed by GitHub
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docs/reference/ImageDraw.rst
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@ -57,6 +57,31 @@ Color names
See :ref:`color-names` for the color names supported by Pillow.
Transparency and Alpha Channel
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
When generating an image in memory and adding transparent elements (for
instance a red rectangle placed on a blue background) to be composited
against the background (creating a purple rectangle), you may mistakenly
assume that the base image (generated from `Image.new()`) must be use RGBA
color space. This is incorrect. For a semi-transparent element to be
composited against the base image, the base image object must be in the
RGB color-space. If the base image is kept in the RGBA color space then an
alpha channel mask is created and all added elements will have their
shapes completely added to the alpha channel of the base image, causing
the element to show as composited against 100% white.
::
# generate a blank image with a blue background
custom_image = Image.new('RGB', (100, 100), color=(128, 128, 255))
# generate a draw object with the RGBA colorspace
d = ImageDraw.Draw(custom_image, 'RGBA')
# generate a red rectangle with transparency
d.rectangle([25, 25, 50, 50], fill=(255, 128, 128, 128))
Fonts
^^^^^