Calculating 2D angles for 3D objects in perspective

Posted by Will on Stack Overflow See other posts from Stack Overflow or by Will
Published on 2010-04-26T17:37:52Z Indexed on 2010/04/26 17:43 UTC
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geometry

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perspective

Imagine a photo, with the face of a building marked out.

Its given that the face of the building is a rectangle, with 90 degree corners. However, because its a photo, perspective will be involved and the parallel edges of the face will converge on the horizon.

With such a rectangle, is it possible to calculate the angle in 2D of the edges of a face that is 90 degrees to it?

In the image below, the blue is the face marked on the photo, and I'm wondering how to calculate the 2D vector of the red lines of the other face:

example

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