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I find the "Line Contraints" feature especially interesting.

OLD topic , but I thought I would mention for anyone interested that the app Hej Stylus! has many of the same features as Lazy Nezumi for line smoothing and various types of drawing guides (perspective guides, ellipse drawing guides ,etc.) which will work in TVPaint. (of course , TVPaint has a line smoothing function built-in and also various drawing guides , but I must say that for me the Hej Stylus! guides work better and the line smoothing can be fine-tuned to a much higher degree in Hej Stylus! than in TVPaint's own line smoothing function)