One of the simplest image-based pieces of generative art is creating a stippling effect. Random points are sampled, the colours picked up, and circles or any other shapes are drawn at the location with the same colour as a fill.
Variants include using circles, polygons or other shapes, varying the size and size range of the shape, playing with the transparency, etc.
The basic stippling effect :
Move mouse left-right to vary size, click to restart.
Adding transparency to the fill and a light outline to get a bubble’d effect.
Coding in a splatter pattern instead of just a basic circle. A range of smaller circles are generated around the larger one, with a certain amount of random variance.
Combining the splatter pattern, a transparency range, a size range, a distance-from-mouse factor, and some good ol’ trial and error results in this watercolour-like rain-on-the-glass effect !
Here is the original image from a Washington Post article.