Histogram Comparison: Sensitive to Illumination?

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Hi there!

I'm trying to compare the color of two images of an object in two different lightning conditions. I'm using the code from the documentation.

These are the two images I try to compare:

How come I get really bad correlation for these to images? The code just uses the H and S channels of the image so the value should be ignored, shouldn't it? Using the CV_COMP_CORREL (ranges from -1 to 1) method I get around 0.017. If I take a completely different color image, like making everything red and compare it I get -0.001.

I tried the other methods and combinations of them with equally bad results.

How can I get better results if I just want to compare the color correlation of two images under different lighting conditions?

Thank you!