I have a geometric wallpaper that I downloaded for my phone, but it has these horizontal lines in it, from what a presume is an artifact of JPG compression. Any thoughts on how to fix?
I guess the unwanted horizontal lines had been there already when the image was purchased or downloaded.
Fixing the already done download or paid purchase is beyond my possiblities, but the image can be fixed to something you can probably accept. ´
At first blur it so that there's no triangle shapes visible and adjust levels to restore the brightest and darkest point. I adjusted the brightest point to white and the darkest point to black. The fixing reduces contrast, so this is a partial compensation.
Then apply in GIMP Mosaic filter. The filter has plenty of settings which affect much the result. Here's one attempt to find good enough settings:
The triangles are not strictly the same as in the original, but otherwise I see this quite plausible. Adjust it darker or to have more contrast, if needed. Here's one adjustment example:
One could say: "It's not a fix, you made a new image!". I bet the creator of the original would see it totally differently.
It's hard to say exactly what it is without having the source file. I do believe, however, that it is being caused by gradients in the image. Where they are so subtle that it results in what is called "banding". This does not look like jpg artifacts due to compression. Increasing the value contrast between the two colors in the gradient could solve for this if you had the original source file.
I'm assuming you don't have the original source file and only the JPG. In that case, the only option would be to redraw the shapes that are showing the banding as was suggested.
I suspect the best and simplest method will simply be to redraw this - I can think of several multi-step procedures which might help, but the more I think about it and look at the angles of intersection, the more I think it'll be faster and more effective to just redraw it.
There are a range of issues embedded in the banding in gradients - it can be the compression algorithm, but it's most often to do with the bit-depth of the image (8-bit vs 16-bit vs 32-bit floating point) as lower bit-depth images (8-bit) just don't have enough available tones to get s smooth gradient (especially where the two graduated tones are very close to one another) and so you can end up really stuck very quickly.
If the original image was a 16-bit
.png for example, you might have seen super-smooth looking gradients, whereas if then downsaved as 8-bit
.jpg you see this strong banding.
The reason my first response was "oh dear, best to redraw" is that you may well have to go through a lot of steps to remediate this, and the results may not in the end be that great - the best bet is to get hold of the original file if you can. Next will probably be, as I've recommended, bringing this image into Adobe Photoshop or Affinity Photo as a background, on a new layer drawing polygon selection areas based on the shapes in the background, and using the gradient fill tool in each of those selection areas in sequence to re-create the image as you'd prefer.
Once done, save it as a 16-bit
.png, and you should be fine.