You can get better results by using the Path Editor to "tweak" the positions of the nodes on the two paths so they align better which will give better results in "edge" cases where you are tempting the definition of "convex."
In the case of a complicated outline, you can assure another path will have an identical number of segments if you duplicate the path and scale it (using Shift-T on it, and clicking on the Path icon in the "Transform" line of the Scale tool's options. Then use the Path Editor to re-arrange the nodes of the larger path into the outline you want.
"It would be nice if..." there was some way to see the nodes of BOTH paths while editing one.
"Does anyone know if..."
...it's possible to display both at the same time?
I've developed this trick to see both:
- export the inside and outside paths as CSV files [appar. these have to exist before-
- concatenate the two files into a single CSV
- edit it to change the path names to a single new name
- change the Stroke# ",1," for all the nodes in the second path to ",2,"
- import the new CSV into GIMP and double-click it to see the nodes (now both sets)
- rearrange nodes on both paths
- export the path as a CSV, and separate it into two again
- import those and you're ready to run Filters->Render->PathBlend
(If this is the only way to see both sets of nodes at the same time, "someone should
make a script"--someone who knows how, that is.)
EDIT: Btw, I edited the script and changed the output of the gimp-message handler
to ERROR-CONSOLE because I noticed in the Toolbox that it's possible to add a tab to
see the Error Console.
I also added some gimp-messages to (try to) dump some progress information: I added this:
(gimp-message (string-append "Segment#" (number->string counter "%") "of Stroke#" (number->string strokecount "%") "of TotalStrokes:" (number->string (car strokes1) "%")))
(while (< (+ counter 1 inLap) L1)