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This plugin displays a correlation diagram for two images (8bits, same size). Drawing a rectangular selection on this diagram allows you to highlight corresponding pixels on a RGB overlap of the original images, and if selected, on a 3rd image. Analysis can be restricted to pixels having values with a minimum ratio. Selection settings are logged to a results window.
Parts of this code were taken from plugins by W. Rasband, P. Bourdoncle, and G. Chinga. Example results:
The results table with computer Pearson's correlation, Overlap coefficient, contribution of both channels to OC, Slope and intercept of the linear regression,% pixels selected, min and max in both channels.