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bubudumbdumb t1_j4n08pi wrote

Reply to comment by hundley10 in [D] Model for detecting rectangle corners? by hundley10

So basically you are printing the cards? Or you have a jpg of the cards or you can scan them?

If yes then what you can do is apply SIFT or even faster ORB to the pictures of the cards to detect and describe the salient points. Build a nearest neighbors index of the key point feature space.

(Optionally) Then you can scale the coordinates of the key points to match the intended dimensions in centimeters (or inches of that's your favorite)

Then you can perform the same with the images from your camera. Get run the key points you detect from the camera through the nn index to match each to the most similar key point from the cards. You are going to get a lot of false positives but don't worry : you can use a ransac approach to filter the matches that don't result in a consistent geometry.

The ransac procedure will return a calibrated fundamental matrix that you can use to project the rectangle of the card to the image space captured by the camera.

All the algorithms I mentioned are available in opencv (also the nn index but I dislike that since there are more modern alternatives). Also there are tutorials on how to use and visualize this stuff.

If this is geometrical gibberish to you check out the ORB paper. Figure 1, 9 and 12 should confirm whether this is the kind of matching you are looking for.

https://scholar.google.com/scholar?q=ORB:+An+efficient+alternative+to+SIFT+or+SURF&hl=en&as_sdt=0&as_vis=1&oi=scholart#d=gs_qabs&t=1673904812693&u=%23p%3DWG1iNbDq0boJ

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hundley10 OP t1_j4n3tz2 wrote

Thanks for the suggestions. I edited my post to give some examples of the detection that needs to be performed... notice how sometimes corners can be obscured, and the background can make "simple" rectangle detection a poor fit. I will check out ORB though.

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bubudumbdumb t1_j4n54nk wrote

The Key here is that by detecting key points you don't need to detect the corners per se : you detect at least a dozen points from the pattern on the card then assuming the card is a rectangle on a plane you can identify the corners.

In other words this can be very robust to occlusions, like you might not see more than half of the card and still be able to identify where the corners are

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