How to find length of a rectangular tile when viewing at some angle I have a question on angles. 
I have a rectangular tile. when looking straight I can find the width of the tile, but how do I find the apparent width when I see the same rectangular tile at some angle. Below I have attached an image for more clarity. So how do I find y in the image below?


It depends on your projection. If you assume orthogonal projection, so that the apparent length of line segments is independent of their distance the way your images suggest, then you cannot solve this, since a rectangle of any aspect ratio might appear as a rectangle of any other aspect ratio by simply aligning it with the image plane and then rotating it around one of its axes of symmetry. So you can't deduce the original aspect ratio from the apparent one, much less the original lengths.
