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JPEG bug - not just in Paint.Net


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I have just noticed that several programs including Paint.Net are exhibiting what appears to be a bug in how jpegs are created. It exhibits as follows:

 

Create a pure red image with rgb: 255,0,0. Save it to a jpeg file with quality of 100% (or any other value). Then load that image and check the colour using the Color Picker tool. The values returned will be 254,0,0. Trying with green, 0,255,0 and it gets converted to 0,255,1. Blue, 0,0,255 gets converted to 0,0,254. Grey, 136,136,136 stays the same.

 

I assume it is due to some rounding errors. I realize jpeg is lossy but I can't understand why an image of a single colour, saved at a high quality setting (100%), would result in such changes. The exact same changes occur with other Windows and Android programs, so the error must be within a commonly used jpeg library.

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