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copied!<p>Try the following: split the original rectangle and this figure with 2 diagonals. Their crossing is (k, l). You have 4 distorted triangles (ab-cd-kl, cd-ef-kl, ef-gh-kl, gh-ab-kl) and the point xy is in one of them.</p> <p>(4 triangles are better than 2, since the distortion doesn't depend on the diagonal chosen)</p> <p>You need to find in which triangle point XY is. To do that you need only 2 checks:</p> <ol> <li>Check if it's in ab-cd-ef. If true, go on with ab-cd-ef, (in your case it's not, so we proceed with cd-ef-gh).</li> <li>We don't check cd-ef-gh, but already check a half of it: cd-gh-kl. The point is there. (Otherwise it would have been ef-gh-kl)</li> </ol> <p>Here's <a href="http://alienryderflex.com/polygon/" rel="nofollow noreferrer">an excellent algorythm</a> to check if a point is in a polygon, using only it's points.</p> <p>Now you need only to map the point to the original triangle cd-gh-kl. The point xy is a linear combination of the 3 points:</p> <pre><code>x = c * a1 + g * a2 + k * (1 - a1 - a2) y = d * a1 + h * a2 + l * (1 - a1 - a2) a1 + a2 <= 1 </code></pre> <p>2 variables (a1, a2) with 2 equations. I guess you can derive the solution formulae on your own.</p> <p>Then you just make a linear combinations of a1&a2 with the corresponding points' co-ordinates in the original rectangle. In this case with W (width) and H (height) it's</p> <pre><code>X = width * a1 + width * a2 + width / 2 * (1 - a1 - a2) Y = 0 * a1 + height * a2 + height / 2 * (1 - a1 - a2) </code></pre>

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