Someone asked about how to get "smooth" clean up lines using an application like TVPaint . I answered with a little tutorial made from a quick rough sketch of a cartoony cat's head . This was done on the Cintiq tablet .
I usually start with a "rough" blue pencil from the Sketch Panel , then on a
new layer I ink the clean up lines in black . I make sure to take advantage of
the rotating "virtual animation disc" in TVPaint to rotate the drawing
to the best angle for laying down the clean up lines smoothly and I use
the zoom-in control to get closer in on the part of the drawing I am working on. (see the second image) .
Finish the black line clean up over top of the blue line underdrawing:
Then drop out the blue line to leave just the clean black lines.
You then have the option of coloring some of the lines a different color from black if you want to by using the Add function in the pen tool . Then add fill color. I've done this one in black & white because it felt sort of like a 30's character with the dot eyes and the pie-cut highlights in the eyes . Not the greatest drawing in the world , but it gives you the idea of what can be accomplished in terms of "smooth" line quality .