Corrected JPEG subsampling documentation #6067
Merged
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Alternative to #4570
https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#jpeg
Discussion in that PR revealed that
-1
is not always equivalent tokeep
. The PR suggested fixing that discrepancy by changing Pillow to make-1
equivalent tokeep
.This PR instead suggests to leave Pillow behaviour unchanged, and just correct the documentation.
In Python, -1 is just the default value to pass to C.
In C, -1 is actually not a setting at all, it's just the default behaviour.
So I suggest removing
-1
from the documentation. If the user wants the default behaviour, they can just not specify the subsampling option.-1
is also the default for "quality", and that isn't mentioned in the documentation.I'm also explicitly mentioning that if subsampling is not specified, then it will be determined by the JPEG dependency.