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Document that histogram() uses 256 bins per channel

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Andrew Murray 2022-03-02 21:21:25 +11:00
parent 92c26a77ca
commit de968dd920
2 changed files with 16 additions and 5 deletions
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10
docs/reference/ImageStat.rst
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@ -14,6 +14,16 @@ for a region of an image.
statistics. You can also pass in a previously calculated histogram.
:param image: A PIL image, or a precalculated histogram.
.. note::
For a PIL image, calculations rely on the
:py:meth:`~PIL.Image.Image.histogram` method. The pixel counts are
grouped into 256 bins, even if the image has more than 8 bits per
channel. So ``I`` and ``F`` mode images have a maximum ``mean``,
``median`` and ``rms`` of 255, and cannot have an ``extrema`` maximum
of more than 255.
:param mask: An optional mask.
.. py:attribute:: extrema

11
src/PIL/Image.py
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@ -1492,11 +1492,12 @@ class Image:
def histogram(self, mask=None, extrema=None):
"""
Returns a histogram for the image. The histogram is returned as
a list of pixel counts, one for each pixel value in the source
image. If the image has more than one band, the histograms for
all bands are concatenated (for example, the histogram for an
"RGB" image contains 768 values).
Returns a histogram for the image. The histogram is returned as a
list of pixel counts, one for each pixel value in the source
image. Counts are grouped into 256 bins for each band, even if
the image has more than 8 bits per band. If the image has more
than one band, the histograms for all bands are concatenated (for
example, the histogram for an "RGB" image contains 768 values).
A bilevel image (mode "1") is treated as a greyscale ("L") image
by this method.