Calculating the aspect ratio of the rotated rectangle on the image

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.

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doesn't seem possible mathematically-- not unless you have a reference object. this is supposed to be a comment-- not allowing me to write. – ashley Dec 11 '12 at 7:58
Why would I need a reference object? I don't want to calculate the real dimensions. Just aspect ratio: Length/Width. You can calculate aspect ration without any reference object as far as I understand – Tofig Hasanov Dec 12 '12 at 2:35
a reference that you know of the aspect ratio. eg. that of font printed on the card. – ashley Dec 12 '12 at 8:22
the credit card itself can be a reference. those cards come in standard size and thus ratio. – ashley Dec 13 '12 at 1:18