We have seen a digital editing tool called opensource FotoSketcher : that turns your photos into drawings.
Let's see how you can modify its parameters to obtain a clean sketch from a photograph.
Start from a homogeneous color image such as the sailboat that can be considered monochromatic (blue range) and apply the pencil filter values to the default..
If we agree to the parameters that adjust the resulting image, We can change the color intensity decreasing it to its minimum value in order to get a black and white image.
We may also increase the maximum contrast level, in this case to distinguish between the edges and filling forms. The edge threshold may be at a low level to remove noise in the image, but high intensity so that clearly mark.
The last setting is the level of darkness / lightness, indicating the amount of global pencil has image. I will place a high value (clarity) to leave just the basic strokes of the image.
While we modify the image, in the preview window, we're getting an idea of the result that we will get. The experience will guide us in changing tuning parameters that allow a significant range of possibilities.