GIMP can do a line drawing of a photograph just the way a human artist would: by looking at the edges of objects in the picture and picking them out in shades of gray, ignoring the colors. Fortunately, GIMP features many edge-detecting algorithms, and one of them, called "Difference of Gaussians," can detect salient details of a photographed figure in much the way that a human eye might see them. By tweaking the results of the edge detection, you can create a very naturalistic line drawing.
Load the photograph into GIMP by selecting the "Open" option under the "Files" menu. Navigate to your photo file and double-click on it.
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Drop down the "Filters" menu in the GIMP editing window and select "Edge-Detect," and then "Difference of Gaussians."
Check "Normalize" and "Invert" at the bottom of the EoG dialog box. Your image should now appear in the preview screen with the edges highlighted and most of the color dropped out.
Adjust the Smoothing Parameters numbers next to "Radius 1" up or down until you've dropped out almost all the color but have preserved the dark edges you want to see in your picture. Set the "Radius 2" parameter down to 0. Then click "OK.:
Drop down the "Colors" menu and select "Desaturate," and then click "OK." The image converts to grey scale.
Drop down the "Colors" menu again and select "Brightness-Contrast." Using the sliders in the Brightness-Contrast dialog box, slide brightness down to about -100 and contrast up to about 100. Adjust these settings until you get nice dark lines and the amount of detail you want in your drawing. Then click "OK."
Drop down the "Colors" menu again and select "Threshold." Slide the threshold slider to the right until you reduce the image to dark lines without too much texture detail. Then click "OK."