Original
JPG
Difference + 128
Some claim the JPG compression reduces saturation and in particular dulls reds.
I conducted an experiment using Paint Shop Pro Version 7, using the first image I could find from my files that was heavy in reds. The first image is the original. The second image is a JPG with a quality setting of 1 in PSP. The third image is the difference of the first, with 128 added to avoid negative color values. Thus if the images were identical, the difference image should be a uniform gray.
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Original
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JPG
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Difference + 128
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Color shifts and intensity shifts should show up dramatically in the diffference image. Instead, around 95% of color values are with 2 of each other in the two images, and the few larger values are due to the artifacts introduced by color subsampling around sharp transitions. (Note, for example, the reflection of the sunset.)
Do you see a color shift?
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