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Improve SSIM documentation and warn about data range. #6595

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merged 3 commits into from Oct 31, 2022
Merged

Improve SSIM documentation and warn about data range. #6595

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d2b40c9
Improve SSIM documentation and warn about data range.
mcourteaux Oct 26, 2022
e97b19d
Fix whitespace requirements.
mcourteaux Oct 26, 2022
fe5f1ae
Apply suggestions from code review
mcourteaux Oct 29, 2022
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21 changes: 18 additions & 3 deletions skimage/metrics/_structural_similarity.py
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Expand Up @@ -20,6 +20,7 @@ def structural_similarity(im1, im2,
gaussian_weights=False, full=False, **kwargs):
"""
Compute the mean structural similarity index between two images.
Please pay attention to the `data_range` parameter with floating-point images.

Parameters
----------
Expand All @@ -34,7 +35,9 @@ def structural_similarity(im1, im2,
data_range : float, optional
The data range of the input image (distance between minimum and
maximum possible values). By default, this is estimated from the image
data-type.
data type. This estimate may be wrong for floating-point image data.
Therefore it is recommended to always pass this value explicitly
(see note below).
channel_axis : int or None, optional
If None, the image is assumed to be a grayscale (single channel) image.
Otherwise, this parameter indicates which axis of the array corresponds
Expand Down Expand Up @@ -76,8 +79,20 @@ def structural_similarity(im1, im2,

Notes
-----
To match the implementation of Wang et. al. [1]_, set `gaussian_weights`
to True, `sigma` to 1.5, and `use_sample_covariance` to False.
If `data_range` is not specified, the range is automatically guessed
based on the image data type. However for floating-point image data, this
estimate yields a result double the value of the desired range, as the
`dtype_range` in `skimage.util.dtype.py` has defined intervals from -1 to
+1. This yields an estimate of 2, instead of 1, which is most often
Comment on lines +85 to +86
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... for signed integers. Then the question is that of type inference.

Maybe link to https://scikit-image.org/docs/stable/user_guide/data_types.html as well.

required when working with image data (as negative light intentsities are
nonsensical). In case of working with YCbCr-like color data, note that
these ranges are different per channel (Cb and Cr have double the range
of Y), so one cannot calculate a channel-averaged SSIM with a single call
to this function, as identical ranges are assumed for each channel.

To match the implementation of Wang et al. [1]_, set `gaussian_weights`
to True, `sigma` to 1.5, `use_sample_covariance` to False, and
specify the `data_range` argument.

.. versionchanged:: 0.16
This function was renamed from ``skimage.measure.compare_ssim`` to
Expand Down