What is the transformation math for creating a 45 deg angle drop shadow?

I read somewhere in Google's reference (and I can no开发者_如何学Pythont find it again) that the shadow for the map marker should be at a 45 degree angle. What is the transformation to apply to the original image to achieve this? By eyeball it looks like it could be a horizontal shear of about 50% followed by a vertical compression to 50%, but I suspect it could be more complicated.

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Not exactly the answer, but this website painlessly eliminated the problem.

http://cycloloco.com/shadowmaker/