Doing a little image editing tonight with the Gimp.
Two things I wanted to do tonight, with similar techniques: turn a picture into a sketch
and take some handwriting and blow it up but transform the inevitable pixelation to something like a sharpee.
The link above with the picture and the sketch got me into a back and forth with Gaussian Blur and Levels (like shown) - playing those back and forth allows you to first smudge out the pixels and then tighten up the external lines while still keeping the general feel of the original drawing.
First, duplicate the layer.
Filters=>Blur=>Gaussian Blur the top layer with an appropriate radius. (The larger the radius, the “fatter” the lines in the final sketch).
Then, to blend the two, we adjust the Layer's Opacity slider to 50%, and our high pass filtered image appears;
Right-click on the top layer and pick Merge Down.
Layer->Colors->Levels to increase the contrast a bit, so that it's easier to inspect visually
when just working with text:
(no duping layer or inverting colors)