Once I was searching for GIMP tutorials to convert a photograph to a pencil sketch. Almost all tutorials I found used same technique which I think can be roughly summarized as finding edges in the image and inverting the colors. Though the results were fine they seemed to lack something, and then I thought of adding the diagonal pencil strokes.
I used GIMP 2.6.2 and a 1024x768 sized image.
Following are the steps (including the ones I learned from other tutorials)
1] Open the image in GIMP
2] Duplicate the layer
3] Gaussian blur the top layer. I used the blur radius as 5 pixels.
5] Invert colors of the blurred layer, and set its transparency to 50%.
6] You should have something like this.
7] Merge down the top layer.
8] Open colors | Levels, set the minimum and maximum input levels as 100 and 155 as below.
9] Desaturate the layer
10] Open Colors | Levels, adjust the input levels so as to get something similar to the following
11] Now Duplicate this layer. Hide the top layer and select the second one.
12] Select Filters | Blur | Motion Blur. Select blur type as Linear, length as 10 angle as 135. Repeat motion blur with angle changed to 315. You should get something as:
13] Unhide the first layer set its transparency to 20%. Merge down the top layer with the blurred layer.
14] Select Colors | Contrast. Increase the contrast slowly till image looks more like a pencil sketch. I increased the contrast to a value of 15, to get the following result
15] Now we need to add some more random pencil strokes. Add a new transparent layer of size 512x384 i.e. of half width and height of the image.
16] Select Fileters | Noise | Hurl. Select a random seed. Set randomization% to 10 and repeat to 1. This should add some random dots to the layer.
17] Select Layer | Scale layer. Scale the layer to image size select interpolation as cubic. Move the layer if necessary to overlap the image correctly. You will have something like this
18] Select Colors | brightness and Contrast. Set Brightness to minimum and Contrast to maximum.
19] Now again apply motion blur on this layer as previously. But keep the length as 20.
20] Again merge down the blurred layer.
21] Now, Select Colors | Brightness-Contrast. Reduce the brightness and increase the contrast to get something like this:
22] Now you can apply canvas from Filters | Artistic | Apply Canvas, to give it a more realistic look. Here is the final image that I got after applying canvas: