Use the sliders.
https://en.wikipedia.org/wiki/Shear_mapping for more detail This is for shearing an ellipse, by making a rotated ellipse via shearing, and there's a simulation of it at g6auc.me.uk/shear.html Basically, a shear sort of "stretches' or distorts an object in some way, where it becomes slanted for one direction of shear, either X or Y. It can turn a square into a non-square rhombus. If you combine both shear directions things become more complicated. For a shear at X, (x, y) becomes (x + my, y), and for shearing in y, (x, y) becomes (x, y + mx), where m is the shear factor. Combining both shears give from (x, y) to (x + my, y + mx)
