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As previewsly seen, Mypaint-0.6.0 add multilayer feature and rotating canvas between other good tools in its big toolbox of good numerical painting application.
As seen on the Calcyum web site, Gimp paint studio is a set of brushes tools and presets to add to Gimp to enhance Gimp powerfull capabilities in this area.
But another good news is last beta of Krita-2, giving lot of new features like nice brushes (notably chinese/watercolor brush and chalk) for numerical painting and canvas rotation. Katelier sources (subset of Koffice with only dependencies to run Krita 2 and Karbon 14 (vector illustration tool from KDE).