How to remove image artifacts from a foggy background?
December 28, 2009 1:43 PM   Subscribe

I use this image (source image here) as my desktop background - it's one of Ubuntu's default wallpapers. Can I do something about the artifacts?

The artifacts in question are only visible when I view the image from an angle on my LCD, and appear in the brightest part of the background. The effect looks like a noisy posterization. I opened the image up in GIMP and looked at the histogram, and I think that this part of the background has been overexposed, but I don't understand why this would make it look like it does. So, why is this, and can I fix the image in GIMP so that it doesn't show these artifacts?
posted by topynate to Media & Arts (4 answers total)
My $.02:

Duplicate layer> filter> noise> median (2 pixels)

adjust layer to 50% opacity

Duplicate first layer > Gaussian blur (1 pixel)

adjust layer to 50% opacity, flatten layers with adjustments
posted by limited slip at 1:54 PM on December 28, 2009

Best answer: My guess is that it's JPG overcompression. That's a 2.5 megapixel image but it's only 215 kB. That can cause blocks to show up in solid-color areas, which should normally be invisible since they will only be a small step apart from the adjacent ones. But sometimes LCDs get funky at an extreme angle, like you can get effects that are similar to solarization.

My suggestion is to take the original image from Flickr, resize it yourself in GIMP, and then save it as JPG at a higher quality setting, like 95%. Or PNG, which would avoid the issue entirely.
posted by smackfu at 2:30 PM on December 28, 2009

Are you absolutely sure it isn't your video card and/or display creating the artifacts? I ask because I'm looking at it on my desktop and I'm not getting any of the artifacts you describe. The tones in that image are incredibly delicate and I can easily see some displays having real problems with it.
posted by Thorzdad at 2:54 PM on December 28, 2009

Response by poster: smackfu, I think you're right. I managed to remove the effect in two different ways - by slightly lowering the white point, and with selective gaussian blur - but when I tried to save as JPG, the artifacts just came back! I think it has something to do with an interaction between the compression, very bright, blurred areas, and the way LCDs work.

I took the original image, applied a selective gaussian blur (radius 30, maximum delta 1), but only to the area with artifacts, and saved the result as a png (wow, that takes a long time). The resulting file is 6.2 MB. Resizing to 1920x1285 before saving gives a 2.3 MB file, which is fine.

Thorzdad, I think that this probably wouldn't happen on a CRT or a new iMac display, as the colours in question should look almost indistinguishable, but I can see with the colour picker that many of the patches of constant colour are 8 pixels wide, which I believe is the width of a JPG block.
posted by topynate at 3:27 PM on December 28, 2009

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