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I tried to look in the internet, but every question and topic was about improving quality of animated GIF, however it is not in my case.

What I have is static image with 256 colors, it is saved in PNG format, but it seems like it originally was saved in GIF, because every gradient is suffered from heavy dithering.

I want to improve quality of image and get rid of dithered gradients.

Do you have something in mind what I can use to make gradients look like they supposed to do, in other words revert dithering and make smooth gradients again?

I have many images. There is 3 example areas which was affected with dithering:

flower

gradient1

gradient2

EDIT: Forget to mention beforehand, that other part of image are detailed, so I provide more examples both with dithering and details:

dithered gradient in detailed area:

dithered gradient in detailed area

thin lines:

thin lines

more thin lines

another example of thin lines

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1 Answer 1

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You can get rid of the 'green dots' issue using Gaussian Blur.

A setting of 3px will kill the dots, but keep the darker 'swash'.

enter image description here

8px will also almost kill the swash.

enter image description here

On anything with detail you want to retain, then 1px is getting as far as you can take it

enter image description here

You might be able to regenerate some edges with a high-pass filter

single pass

enter image description here

two-pass

High-pass used injudiciously will give odd glowing edges like a poor HDR photo, so you need to keep your pixel radius down to try avoid it. I should have gone finer in this example.

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  • Hm, I also have some text and thin lines, I can't combine blur for more than 1px because other part of the image will be destroyed. If, say, I use mask, then on the edge of different blurs filters border will appear. It is such pain because like 2nd and 3rd image will require a lot of blur radius, but other part of same image is very detailed. I posted there only high affected by dither areas, but the other problem, what on other part of image with a lot of details it not that visible and blur here will destroy this detailed area instead.
    – XCanG
    Commented Mar 7, 2021 at 19:39
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    The best way to get a very specific answer to a very specific problem is to ask that in the first place, not add it as clarification underneath an answer someone already spent time to work through, with examples.
    – Tetsujin
    Commented Mar 7, 2021 at 19:42
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    Try smart sharpen it deconcolves so theoretically it can undo the gaussian blur but with a statistical mean. Also try surface blur.
    – joojaa
    Commented Mar 7, 2021 at 20:29
  • @Tetsujin sorry for that, I added 4 more examples with problematic details where gaussan blur will create issue. For thin lines they either become less visible, either become bolder even it I try to retain edges.
    – XCanG
    Commented Mar 7, 2021 at 21:00
  • there are also "fourier-transform" based filters that may be suitable. In the context of photoshopish tasks, they are sometimes used to remove paper grain and other forms of semi-uniform noise.
    – Yorik
    Commented Dec 2, 2021 at 22:14

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