Nice and smooth.

I've been experimenting with vectoring artwork, but not yet at this scale. It is kind of fun once you get the hang of moving those lines about. Ever fool around with Inkscape?

Can't say as I have, no!

It pretty much amounts to:
--Import the pencils or original art
--create a layer over them
--draw new lines using the pen tool:
---set fill color to "none".
---start on any curved line.
---pull out a straight line to the end of that curve or point along that line where the curve reverses direction.
---end the line segment.
---switch to the point edit tool and pull/push the line till it matches the one underneath.

---wash/rinse/repeat. When done, hide the original and export a bitmap.

---If vectoring thick lines (like brush strokes or similar thin-thick-thin lines), set fill color to black, zoom in and trace the outline of each line rather than just putting one down the middle. Then push/pull to make the shape match the stroke.

(slightly oversimplified)