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Posted By: Dave

Eric,

I understand exactly what you mean.

The assumption (which I think is good) is that the black frame around a picture is a known distance. I don't know what this is but I do think it is a constant value.

By measuring this on a picture of a card, you could measure both the total width or height and the associated frame size. Since the frame is known, you can calculate the card size.

I've tried this a few times with limited results. The problem is that the resolution usually isn't good enough to accurately measure the frame and extend this to the card itself.

Another problem with this is if the scan isn't 100% flat. Often, especially with a camera (rather than a scanner) the card isn't really parallel, and the frame isn't the same width or height across the card. In theory, this is OK, as the ratio of card to frame is constant and can still be applied, but it is difficult.

If someone would like to try this as an experiment, I'll supply high res scans of cards, along with the accurate width and height measurements.