Graphics software in the classic style, i.e. visually.
1 Pinta
A tiny, simple one. Pinta: Painting Made Simple.
Pinta is a free, open-source program for drawing and image editing.
Its goal is to provide users with a simple yet powerful way to draw and manipulate images on Linux, Mac, Windows, and *BSD.
You can write extensions in C#.
2 Gimp
Gimp is a flagship open-source image editor. It does a lot of things passably, although it has had a hard time acquiring new features. . It has powerful plugins. Its user experience is variable. It can do a lot of image nerd things pretty well, but it also missing some stuff. It segfaults if I start a new taks while the progress bar from the previous task is visible.
Also, the latest version, GIMP 3 is a horrorshow on macos. I’m sure the sleek modernised interior is very lovely, but for sure it is slower, crashier, and buggier, and it adds no features. Steer clear. GIMP 2 is better.
There are such startling and unnecessary pain points as the fact that the plugin registry (which is a popular selling point of this dorknado) is broken in an undocumented way that probably has to do with it being an undermaintained security nightmare.
Paul Harrison’s resynthesizer is a texture synthesizer for GIMP. See also his rather amazing thesis, with a diversion into the Turing completeness of tiling. And yes, that G’MIC thing has a gimp plugin version.
3 Krita
Krita is a ground-up reimagining of fun open-source non-nerdy image editing. It has cute features like quasi-physical-modelling brush design and automatic tiling mode. Fairly pleasant to use. You can write your own plugins in Python, making this an art python app.
4 ImageJ
Classic “scientific” editor ImageJ has been useful for a decade or two. I think the most popular distribution is Fiji. ImageJ2 is the core:
ImageJ2 is a rewrite of ImageJ for multidimensional image data, with a focus on scientific imaging. Its central goal is to broaden the paradigm of ImageJ beyond the limitations of the original ImageJ application, to support the next generation of multidimensional scientific imaging.