Help with mapping rectangular bitmap onto trapezoid with correct perspective in JavaScript using Canvas

I wish to map rectangular bitmaps onto trapezoid shapes. This is a part of 3d rendering. From the trapezoid shape it's possible to compute a vanishing point, and information about the plane. This is suitable for someone experienced in geometry and/or able to read and port C++ examples. See [login to view URL]~mikaelk/doc/perspectivetexture/ for more info on texture mapping. There is an example that does what I want but in a different way to how I want it at [login to view URL] I wish to be able to map each individual pixel, or use the optimization that Quake uses for its texture mapping.