Freely transforming an image to correct perspective

C# newbie here so please forgive me if my terminology isn't quite correct.

As part of this project, I have a user hold up a piece of paper to a webcam so I can capture, isolate and then eventually display back what they've drawn on it. I've put some restrictions on where the corners of the paper have to be in order for the program to accept it, but there's still the cha开发者_如何转开发nce that it's distorted by perspective.

Here's an example image that I've captured and isolated the paper out of: image

What I want to be able to do is to distort this image so that the corners of the piece of paper are turned back into a 8.5x11-proportioned rectangle (as if the user had scanned it rather than held it up to the webcam). Rotation and skewing can only get me so far, ideally I would be able to freely transform the image, like in Photoshop. I found this example, I am basically trying to do the opposite. Curious if anyone's had to do this, before I start trying to reverse that four-point image distortion example.

This is sometimes called a Quadrilateral warp.

Disclaimer: I work for Atalasoft.

Our DotImage Photo SDK can do this and it's free. Look at QuadrilateralWarpCommand. You need to know the source and destination quadrilateral.