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POextracting a quadrilateral image to a rectangle
 

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    <p><img src="https://2.bp.blogspot.com/_cP3Pd1BRVXc/STmOuCiGLWI/AAAAAAAADrQ/YH548niXLhQ/s400/french+fries+in+Europe.JPG" alt="A photo"></p> <p><strong><em>BOUNTY UPDATE</em></strong></p> <p>Following <a href="https://stackoverflow.com/questions/2992264/extracting-a-quadrilateral-image-to-a-rectangle/2999689#2999689">Denis's</a> link, this is how to use the <a href="http://threeblindmiceandamonkey.com/?p=16" rel="nofollow noreferrer">threeblindmiceandamonkey</a> code:</p> <pre><code>// the destination rect is our 'in' quad int dw = 300, dh = 250; double in[4][4] = {{0,0},{dw,0},{dw,dh},{0,dh}}; // the quad in the source image is our 'out' double out[4][5] = {{171,72},{331,93},{333,188},{177,210}}; double homo[3][6]; const int ret = mapQuadToQuad(in,out,homo); // homo can be used for calculating the x,y of any destination point // in the source, e.g. for(int i=0; i&lt;4; i++) { double p1[3] = {out[i][0],out[i][7],1}; double p2[3]; transformMatrix(p1,p2,homo); p2[0] /= p2[2]; // x p2[1] /= p2[2]; // y printf("\t%2.2f\t%2.2f\n",p2[0],p2[1]); } </code></pre> <p>This provides a transform for converting points in destination to the source - you can of course do it the other way around, but it's tidy to be able to do this for the mixing:</p> <pre><code>for(int y=0; y&lt;dh; y++) { for(int x=0; x&lt;dw; x++) { // calc the four corners in source for this // destination pixel, and mix </code></pre> <p>For the mixing, I'm using <a href="http://en.wikipedia.org/wiki/Supersampling" rel="nofollow noreferrer">super-sampling</a> with random points; it works very well, even when there is a big disparity in the source and destination area</p> <hr> <p><strong><em>BACKGROUND QUESTION</em></strong></p> <p>In the image at the top, the sign on the side of the van is not face-on to the camera. I want to calculate, as best I can with the pixels I have, what it'd look like face on.</p> <p>I know the corner coordinates of the quad in the image, and the size of the destination rectangle.</p> <p>I imagine that this is some kind of loop through the x and y axis doing a Bresenham's line on both dimensions at once with some kind of mixing as pixels in the source and destination images overlap - some sub-pixel mixing of some sort?</p> <p>What approaches are there, and how do you mix the pixels?</p> <p>Is there a standard approach for this?</p>
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