pixel-based image manipulation program

 Krita is a creative application for raster images. Whether you want to create
 from scratch or work with existing images, Krita is for you. You can work with
 photos or scanned images, or start with a blank slate. Krita supports most
 graphics tablets out of the box.
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 Krita is different from other graphics design programs in that it has
 pluggable brush engines, some supporting brush resources like Gimp brush files,
 others offering sophisticated simulation of real brushes, and others again
 offering color mixing and image deformations. Moreover, Krita has full
 support for graphics tablets, including such features as pressure, tilt and
 rate, making it a great choice for artists. There are easy to use tools for
 drawing lines, ellipses and rectangles, and the freehand tool is supported by
 pluggable "drawing assistants" that help you draw shapes that still have a
 freehand feeling to them.