Did Compare behaviour change in v14.7.0 for normalized cross correlation?

[lewishazell]

Hi,

We've upgraded Magick.NET after a scan picked up the vulnerability in earlier versions. We jumped from 13.10.0 to 14.8.2.

As we're manipulating images, one of our automated tests uses the Compare method with the NormalizedCrossCorrelation error metric to check image integrity hasn't degraded too far. These tests failed upon upgrade, though manually checking the images showed they looked close to the original. This is just the following assertion,

output.Compare(input, ErrorMetric.NormalizedCrossCorrelation).Should().BeGreaterThan(threshold);

In our test, I noted that the value was essentially inversed (though not exactly) in the new version. Before the upgrade, Compare returned a value of 0.99291248113201691 where, after the upgrade, we get 0.00357649810057864. Similarly, if I make output and input load from the same file, Compare returns 1 before the upgrade but 0 after the upgrade. This appears to demonstrate inverse behaviour. It seems it's now returning distortion where before it was returning similarity?

I tried checking issues, discussions and release notes but I couldn't find anything specifically related to this. By bisecting the package versions, I found this seemed to change between v14.6.0 and v14.7.0. I can see that v14.7.0 includes some optimizations to the Compare function, which might be related but I'm not sure?

I've provided the input and output images for reference.

Input:

Output:

[dlemstra]

These changes are related to the following changes mentioned in the release notes:

• Accelerated Subimage-search Compare
• Request Enhancement To Magick Compare -metric MSE

With these change a lot of improvements and fixes where made to the compare logic.

[lewishazell]

Thanks Dirk, I had a hunch but wanted to make sure. We can fix the tests with confidence now.