Pinta actually uses an older version of the paint.net image manipulation functionality from when Paint.net was open source, with a rewritten UI (in gtk# if I recall correctly, compared to WPF for Paint.net). However, that new UI is significantly inferior to Paint.net's. One example - last time I tried it, the "marching arts" to highlight selections was sized in terms of pixels in the source image. It was always 1 source image pixel wide, rather than 1 screen pixel. This significantly hurt its usefulness for pixel art type situations.
