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I am learning how to use python's pillow library to produce images, and for a test I made it produce a 256*256 RGB image with each pixels green value equal to its x position and its blue value equal to its y position (relative to the top left corner) Everything worked fine, I got an image with green and blue gradients, just as expected, however their seems to be a solid-ish blue line from the upper left to the lower middle. This color artifact made me intrigued. I checked that my program worked correctly, which it did. I don't know if It's my computer monitor, or some visual effect, but I'm puzzled as to why I am seeing this? Could you shed some light on this? The image is attached.

EDIT: When I uploaded the image, the artifact disappeared, I took a screen shot of the image in a viewer, then cropped it and posted it. My image viewer might be partially responsible.Here's the image

This is the original

  • What software do you use to view these images with? "Uploading" may or may not have anything to do with it (it's possible an online image store compresses or downsamples your image); but if you can upload it, you also can view it directly in your local browser. The top is your screenshot? Upload the original data somewhere so we can take a look. – usr2564301 Dec 12 '17 at 22:34
  • The original image is a 256*256 image with blue = to y coord and green = to x coord (Cartesian origin upper left corner) I'm guessing the problem is with my image viewer, which is GNOME Image Viewer 3.14.4 – CatOnAKeyboard Dec 13 '17 at 22:38
  • This CSS Tricks post may provide some useful information – Zach Saucier Apr 12 '18 at 1:40
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Banding effects like this can occur when the image viewer tries to compensate for differences in gamut between two color spaces or color profiles.

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--"When I uploaded the image, the artifact disappeared" --

That would indicate a limitation of the video display you are using and an inability to distinguish the subtle blue variations due to the video card, the monitor, color profiles, etc.

  • Surely OP did not change either video card or monitor between first viewing and uploading? Color profile is a contender, but I'd also regard the software used to view the PNG with as suspect. – usr2564301 Dec 12 '17 at 22:32
  • No uploading would even out the subtle tones. All web sites processes images upon upload. Therefore the difference could be associated with profiles, or compression, or some other "generalization" of the image when uploaded. Never meant to suggest video card/monitor changed after upload, in fact just the opposite. That on the viewing system, there's a limitation to displaying the subtle blues... those subtle blues are toned down after the upload. – Scott Dec 12 '17 at 22:52
  • Its probably not a monitor problem, if I scroll up I can see the artifact in the top one but not the bottom one. – CatOnAKeyboard Dec 13 '17 at 22:37
  • "When I uploaded the image, the artifact disappeared" it's kind of expected that you'll see the difference in this question. That's a far cry from what software may be displaying using your monitor directly. – Scott Dec 13 '17 at 23:05
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    I think scott's comment is a likely contender (not so much the answer, though). It could be when uploaded it was compressed into a JPG (or even just compressed in general) which would add noise to the gradient...which also happens to be one of the 'tricks' to get smooth grades in print...always add a tiny bit of noise to smooth it all out. – DA01 Apr 12 '18 at 1:24

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