Pseudo-Sketching Effect

This example was using Gimp 1.2

I got this tip from the Gimp User Manual on the Gimp Homepage, in the section "Gimping Around In 80 Minutes".

This picture of James Morrison in Millenium is an example of the effect (reduced size):

I assume you already have a picture with your person-sans-background.

  1. Make yourself a new white layer below the person, and merge the layers.
  2. Desaturate the layer with Image -> Colors -> Desaturate.
  3. Reduce the number of colours with Image -> Colors -> Posterize. If you make it too few, then your person won't have any features left; if you make it too many, then it won't look like a sketch.
  4. Make yourself a new white layer above the person layer, and set it to Screen mode.
  5. Make this the active layer, and temporarily make it semi-transparent, so that you can see the image underneath.
  6. Because black is transparent in Screen mode (also Addition and Lighten Only) black pencil strokes drawn on a white layer will reveal the image underneath, just as if you had sketched all by hand. Of course, unless you have a tablet, it's a bit harder to control a mouse than a pencil, so it may end up looking like something drawn by a demented four-year-old. Practice, practice, practice.
  7. When you've scribbled to your satisfaction, make the Screen layer fully opaque, and do Layers -> Merge Visible Layers.

You could also get different effects by using different brushes, or maybe using the Airbrush tool. You can also apply effects afterwards, like Filter -> Artistic -> Apply Canvas.