Missing normalisation in _ehs for ISSM

[Mominno]

Hi,
I've noticed that function _ehs uses np.histogram2d to calculate H used in ISSM, which isn't normalised, its just a sum of values.
https://github.com/up42/image-similarity-measures/blob/8328230be0ab45cf3c01db301db4064564298a5c/image_similarity_measures/quality_metrics.py#L165C1-L171C50
In original paper on ISSM authors use normalized joint histogram, the normalisation being number of pixels in image (MxN for image of dimension MxN).

[seedlit]

@Mominno thanks for raising the issue.
We will take a look :)

Or, if you prefer to become a contributor as well, feel free to open a PR :)

[Mominno]

Sure, I'm currently playing around with ISSM trying to get it work. :)

[Mominno]

finally got around to creating PR. #63