Limelight Apriltag detection inaccurate with multiple tags

Technical Programming
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I was working with limelight trying to get the MegaTag botpose with April Tags.

When only one tag is in view, the botpose is unstable, but it is acceptably accurate. (can’t embed an image because I’m a new user, but here is a link to the image: IMG_2764.jpg - Google Drive)

When two tags are in view (like in front of the speaker), the botpose becomes completely wrong, especially in the side-to-side directions:

IMG_2765
IMG_27651920×1440 233 KB

The pictures are taken with the bot in the exact same position (we just covered up tag 3).

Any idea as to what would be causing this?

[Edit] It should be straight ahead from between tag 4 and 3

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Which tag field location file are you using?

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I’m using this field map: Crescendo AprilTag Map - #7 by KH987

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Thanks. Just in case - did you re-verify coordinates of the tags vs their field location? Or at least did you verify if only one of the two tags in question are visible, you get the right coordinates?

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Yeah, if only one of the tags are visible we are getting the right coordinates (its like three or four inches off, but for our use case it is acceptable)

We haven’t assembled the full field we only have the two april tags on the speaker. We verified that they are positioned correctly

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I meant - did you look inside the map file and verified the actual x and y coordinates of these tags?

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@NiranSchneller you mentioned you had the tag map working. Did you happen to test with the same tags mentioned here? I don’t have access to a full field so I can’t verify.

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Haven’t got around to verifying the field position, but it looks like it’s been verified by other people (see Is there a 2024 Limelight Release? - #11 by NiranSchneller)

When we plot the pose on the SmartDashboard Field2d for 2024 given by Limelight (for one tag), it appears to be correct.

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Hm… We will check it out as well. We also want to use LL (LL2+ in our case) to deal with the pose estimation. Thanks for the heads-up. I will let you know what our testing will reveal.

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Specifically on our speaker, we are using apriltags 7,8. They have been accurate for pose estimation, including MegaTag.

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Which LimeLight firmware are you using? Something similar was in an earlier version.
What was the recalibration error value when calibrating?

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Can you post screenshots of each of your limelight settings pages?

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https://drive.google.com/drive/folders/13aTaIYqMcnEuBHZssp1e8Dny5NXavhsm?usp=sharing

^ Settings is here

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We are using 2023.6 which iirc is the most up-to-date version. Not sure how to find recalibration error, right now we just have default calibration.

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You really need to do ChArUco Camera Calibration | Limelight Documentation (limelightvision.io)

Reprojection (not recalibration) errors come up from a calibration. Check for other posts on this.

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The issue was solved by making sure that all tags were in the correct place. Issue was caused because the tags were a few inches off of where they actually should have been (relative to each other)… we didn’t take into account the whitespace when calculating April Tag positions.

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