I believe that the similarity image from compare is not working in the current release.
mandril3.jpg
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mandril3_156_22.jpg
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In a previous release, I got the following images:
compare -metric rmse mandril3.jpg mandril3_156_22.jpg mandril3_similarity.png
mandril3_similarity-0.png
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mandril3_similarity-1.png
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In the current release, I get the following images:
compare -metric rmse mandril3.jpg mandril3_156_22.jpg mandril3_sim.png
118.467 (0.00180768) @ 156,22
mandril3_sim-0.png
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mandril3_sim-1.png
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The second images is totally white.