Transforming photos in pencil sketches

I have just finished the first version of the sketch effect. Now you can transform a photo in a pencil sketch.

I intend to make this kind of effect in other flavors, like representing other drawing styles. I'm still researching about that.

Concerning the example above, for best results, I made a simple preprocessing of original photo before applying the effect:

- made it gray;

- improved its contrast.

As you can see, the result gives a nice sketch with no falsehood. The pencil strokes are really there.