I'm looking to draw things like natural cave walls and such, with a mouse, but with a quasi low fractal effect between mouse points. i.e. the ability to draw something like this:
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without spending a huge amount of time (and yet controlling the general shape of it, no purely random generator that I do know how).

I'm sure it should be fast and easy, and I'm pretty sure I saw that somewhere, but can't find it now nor can I find the right wording to do a meaningful Google search.

Any ideas?