I want to create a "tear" effect with regular triangular shapes to show I'm omitting part of the middle of a screenshot, something like:
When I search for tear effects all I find is realistic ripped paper edge effect. Anyone know terminology or Gimp instructions referring to this simpler geometric effect I'm looking for?
It's called a sawtooth pattern. You can make it by rotating a square 45 degrees then making a pattern out of that:
How to create a "zig zagged, jagged" effect with Photoshop?
The one in your example is made with several different sized squares, but the principle is the same. Good luck!
On GIMP 2.8 it was a simple matter of creating a new rectangular brush (on the brushes dialog, click on the new button) setting the rotation angle (on the tool options) and the spacing (on the brush) itself to suitable values, and draw a staright line (click on first dot, hold shift, click on destination dot).
My settings, and my result:
