We have a series of coordinates as someone drags a mouse across an image, and the goal is to modify the image in a way that respects the artistic intent of the mouse stroke. For example, what algorithm would Photoshop use for this? What is even the technical name for such an algorithm?

One could rasterize a filled circle centered around each mouse position using the midpoint circle algorithm, but that is a crude approach that would behave terribly if we try to do any artistic effects like translucency or soft edges in the stroke. We are looking for more advanced algorithms that produce high quality results for artistic purposes.

The source code for GIMP is available, and surely such algorithms must be somewhere in there, but the source code is vast and it is not clear where one should look to find the relevant algorithm.