i often need to convert artwork and pictures to line drawings for my router projects, so i'm gonna share the procedure I use.
needed software is Gimp and the demo of Img2CAD.
first locate a picture you want to use, recently I have been making Service Branch seals so im gonna use the Air Force seal from wikpedia.
then load your pic into gimp and desaturate it and change the type to grayscale. no sense using extra memory with color if your not using it as these paths we are making can get large.
This particular emblem isnt the best to work with, and should have had the colors reduced with threshold before converting to desaturating. but you will get the idea.
Step 1: make into a path
select all and pick the edge detect - Sobel
i leave everything checked, and use the preview to check if it looks correct. then apply the filter.
using the threshold adjust the black & white till you have what you want to cut and crisp lines
adjust the canvas size to about 50% larger and center the artwork
add a new layer with white so you can see the remaining lines and paths easier.
now color select the transparent area, invert the selection and use selection to path
this may take a while if the art is complicated.
make sure you save the pic often as your working on it as an .xcf (native gimp) file to save the paths.