Transform an image into a pencil-sketch, using the <canvas> element.
The implemenation is not exactly efficient, so when you check out the demo page, try starting with a small image (a couple of hundred pixels across).
The principle is fairly simple. At each point in the image, the hue is converted into an angle of shading (so blue and yellow are at right-angles to each other). The saturation is converted into consistency of shading angles (so grey will be cross-hatched, grey-blue will be a bit rough-looking, and pure blue will be completely in line). Brightness is obviously represented by density of lines.
The actual method is a bit of a hack - we create image-size pencil textures (using randomly drawn pencil lines with a specified thickness, length and density) for a few different RGB values. The spacing of these values is determined by the "Level steps" control (so, 2 means that it will cover (0, 0, 0), (1, 0, 0) and so on - 3 means it will cover (0, 0, 0), (0.5, 0, 0) and so on).
Then, we simply composite these textures together (fading between them), and add some basic edge-detection to simulate outlines.
I hacked together a batch-processing system as a PHP script. It downloads one image at a time, processes it, and POSTs it back to the server.
The config is in batch-animation.json. Note the lineAlphaVariation and edgeAmountVariation properties, which randomise their corresponding parameters, resulting in a more or less coarse image.
The goal is to replicate the effect of tracing video frames using pencil sketches, where each frame will not turn out completely consistently:
The code is available as "public domain", meaning that it is completely free to use, without any restrictions at all (including re-licensing under your own terms). Read the full license here.
It's also available under an MIT license.