[nilknarf]

The math for working out the transform is actually the same (not an inverse!) since you are still just trying to map 4 points to 4 points.

I had a demo for the 'inverse' transform also: http://codepen.io/fta/pen/LHonf

Here is the same thing except using the video from this post: http://codepen.io/fta/pen/JoGybG