Paint Shop Pro version 5 introduced layers known from Adobe Photoshop. This is similar to what GIMP version 1 did. Of course, both programs have their own native file formats and at the moment (to my knowledge) layered images can only be transferred from Paint Shop Pro to GIMP via the Adobe Photoshop format, but not the other way around. The question arose »Why not have a plugin for GIMP« to handle this - it would be much harder for me to write a plugin for Paint Shop Pro (assuming this is possible) than for GIMP. Furthermore I do not like Microsoft Windows that much I would want to write some program especially for it.
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