I most often see this with compressed video, but I’ve also seen it often enough with JPEG images. Many times blacks, reds, and other generally dark colors seem to be especially prone to compression artifacts, even if you’re just opening a file and saving a copy of it. What causes this, and what countermeasures can be taken to prevent it?
Answer
The problem with most image compression methods is that computers see brightness linear. your eyes and brain see light logarithmic. What this means is that a small difference in brightness in dark colours is very noticeable but a small change in brightness in bright colours is hardly noticeable at all. A solution would be to use a format that compresses images based on logarithmic brightness. I am however not aware of any such compression method. This will of course come with its own downsides. but should be a significant improvement for darker images.
Attribution
Source : Link , Question Author : Kaji , Answer Author : RecoverForest