I need to determine the aspect ratio of the rectangular object (such as mobile phone or credit card) on the image. The problem is that the object might be photographed from an angle or perspective. So on the image borders will have different length. Knowing the fact that it is a rectangle, is it possible to calculate actual aspect ratio? I have tried to search for this, but the amount of information is huge, and I don't really want to study all perspective related math. I assume there should be some relatively easy formula for that? I would be grateful if you kind point me out the direction where I should look for an answer.