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I saw this paper/algorithm some time ago. It was about detecting markers for augmented reality applications. I think you could adapt this algorithm in order to detect cards, because detecting a card is more ore less like detecting a marker in an image.

So I recommend you to read this paper:

Marker Detection for Augmented Reality Applications

Blog and source code

Algorithm:

1. Divide image in regions
2. Detect edgels in regions
3. Find segments in regions
4. Merge segments to lines
5. Extend lines along edges
6. Keep lines with corners
7. Find markers (cards)

Hope this helps