I wish I'd drawn this diagram a bit differently. Imagine the Mona Lisa not once, but as an infinite plane of that face repeating over and over like endless tiling wallpaper. Now picture putting the A, B, and C points anywhere you like on that plane. This will, of course, form a triangle. The image within that triangle will be mapped to the shape of the 3D triangle we're drawing on screen. (Even if they're wildly different proportions.)
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