How to reduce number of colors to 1024 with PIL python

Question

I am trying to reduce number of colors to 1024, but got error ValueError: bad number of colors

img = img.convert('P', palette=Image.ADAPTIVE, colors=1024)

or

img = img.quantize(256, 0)

What is wrong?

Answer 1

As you can see here from the PIL.Image module source code https://pillow.readthedocs.io/en/stable/_modules/PIL/Image.html the documenting comment for the convert method states that:

 When converting from "RGBA" to "P" without a ``matrix`` argument,
 this passes the operation to :py:meth:`~PIL.Image.Image.quantize`,
 and ``dither`` and ``palette`` are ignored.

so the quantize method is used for the operation because you didn't provide a matrix argument to the convert method and now inside of the quantize method the documenting states that:

Convert the image to 'P' mode with the specified number
of colors.

:param colors: The desired number of colors, <= 256

so the number of colors should be less than or equal to 256 instead a ValueError exception will be raised.