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What do the tag-specific hex codes in the c files represent? #282

@joellyons

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@joellyons

What do the tag-specific hex codes in the c files represent? For example, in tag25h9.c, tag 0's code is represented as 0x156f1f4. I assumed that each bit in that number (1010101101111000111110100) represented a block in tag 0's visual representation starting at top left and moving left to right, top to bottom, where 1 is a black square and 0 is white.

But, if I render it that way, I end up with this:
image

instead of what I see in apriltag-imgs, which is this:
image

Are the graphics in apriltag-imgs outdated? Or am I misinterpreting the numeric values in tag25h9.c?

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otim

otim commented on Aug 19, 2025

@otim

I've noticed the same discrepancy for tag family 36h11... Interestingly, the hex codes in the java tag generation code seem to be matching the "old convention". See https://github.com/AprilRobotics/apriltag-generation/blob/master/src/april/tag/Tag36h11.java#L86
I wonder if the c++ code has a different convention for the bit order?

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          What do the tag-specific hex codes in the c files represent? · Issue #282 · AprilRobotics/apriltag