4-point transform images

Check out the Perspective warping examples from ImageMagick. It is available for most mainstream platforms.

The keyword here is homography. Manolis Lourakis has written a GPL'ed homography implementation in C that is available here; however, this will not be able to be ported very easily because it relies on some external libraries such as LAPACK.

Disclaimer: I work at Atalasoft

If you are willing to go commercial, DotImage Photo can do this with the QuadrilateralWarpCommand. Sample C# Code

// Load an image.
AtalaImage image = new AtalaImage("test-image.jpg");

// Prepare the warp positions.
Point bottomLeft = new Point(100, image.Height - 80);
Point topLeft = new Point(130, 45);
Point topRight = new Point(image.Width - 60, 140);
Point bottomRight = new Point(image.Width - 20, image.Height);

// Warp the image.
QuadrilateralWarpCommand cmd = new QuadrilateralWarpCommand(bottomLeft,
   topLeft, topRight, bottomRight, InterpolationMode.BiLinear, Color.White);
AtalaImage result = cmd.Apply(image).Image;

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