opengl es2 - How can I make a shader effect that looks like a lightly shaded pencil drawing?

I want to make a shader effect using OpenGL ES 2.0 that looks like this image:

I'm not sure if this image was painted or is the result of some filter, but I want to create a shader that produces images looking similar. I want to know a Shader algoritm to produce this kind of output!

Answer

I can't tell if that specific image you linked was painted that way originally or not, but the resulting effect looks similar to an edge detection filter.

Edge detection post-processing is often done using a Sobel filter implementation. For example, as seen here (a CPU-side implementation). The effect can be achieved in shaders as well (here is an HLSL example that is trivially adaptable).