How Sketching Softwares Work?

Now-a-days there are lot of sketching softwares available in market. Question is how these sketching softwares work? As we know that any picture is comprised of small grainy particles known as pixels. These pixels are very much small but you can identify them if you see them very closely or with the help of magnifying glass. These pixels are spread all over the screen.

Any image is composed of three primary colours Red, Blue and Green. These colours are mixed in diffrent proportions to make all other colours. In digital imagery devices (Almost all of common devices are digital now-a-days) every pixel is stored as a entry of a matrix. Every entry is further composed of three more sub-entries known as frames. These frames are the corresponding entities for primary colour generators. Each frame is capable to produce one of the primary colour at any intensity. 

Now to produce the sketch of any desired photo, the image editor crawls all the pixels of the photo one by one, of each row. It then calculates the trend with which the colour varies. As we know that sketch can be made by checking the area of colour variation by boundaries. Same is the case here. The software checks the pixels where there is a considerable change in colour, and marks it with the proportional intensity of black colour. This is indicated as the boundary. This is how a sketch is extracted from any image.