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IMP is the leader in this section, and it’s overflowing with plugins that affect either the whole image or selected regions. The downside is that many of the plugins are similar to one another, and it can take quite a lot of experimentation to sift through them all.
Krita features a lower number of image processing effects compared to GIMP, but we’d rate the average quality of what’s there to be higher. Both GIMP and Krita can work with the G’MIC image plugin system, so it’s unlikely that you would ever run into a genuine roadblock if you needed a particular image processing feature with either program.