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I have a quad defined by four arbitrary points, A, B, C and D all on the same plane. I then have a known point on that quad, P.

The quad ABCD

I want to find the value of 's' as shown in the diagram above, where t and s are parametric values in the range (0, 1) that interpolate along the edges.

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    The line containing $P$ isn't parallel to anything?2011-09-13
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    See http://stackoverflow.com/questions/808441/inverse-bilinear-interpolation and http://www.iquilezles.org/www/articles/ibilinear/ibilinear.htm2011-09-13
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    @J.M., it seems the OP is assuming [bilinear interpolation](http://en.wikipedia.org/wiki/Bilinear_interpolation) on the quad.2011-09-13
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    Ah yes, inverse bilinear interpolation is what I'm talking about - guess that makes this a duplicate question2011-09-13
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    Possibly interestingly related: _["What is this technique, if not bilinear interpolation?"](http://stackoverflow.com/q/24019709/405017)_2014-06-03

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