--estimate-focal-length always returns -1.00. about apriltools

[thegoodhen]

Hello!
It's hard to tell without looking at the footage.
The software uses a vanishing points method in order to determine the camera focal length. This means it extends the sides of the square tag till they intersect and then calculates the focal length based on the position of the intersection. Generally, this works. However, in order to combat numerical instability, it discards the estimate when the opposing sides of the tag are too close to being parallel.

This might happen if you are viewing the tag from far, from angles close to "dead on" and/or when the tag is small.

You can take a different footage, estimate the focal length from it and then use the estimate to track the footage you wanted to track.

Alternatively, you can also provide the focal length in mm and the sensor width also in mm to the tracker, if you know those values (for example, if you googled them!).

Which camera do you have?

Also, can you show me the footage?

[jm02394]

Okay, this helped me get an understanding of what's going on. First of all, I was filming from an angle almost perpendicular to the ground, so there weren't many lines it could have used to determine the vanishing points. Second of all, I was using my terrible phone camera which had a ton of motion blur, which would make it even harder to pick out lines. Also, I did try googling the focal length and sensor width, but something seemed off with the tracking. I'm pretty sure that it was the motion blur, though. I'll try filming something with an actual camera tommorow and get back to you. EDIT: I was using the camera on my Samsung Galaxy S7.

[thegoodhen]

Hm... The camera perpendicular to the ground surely is the bigger problem there. I am pretty certain that Galaxy S7 should be good enough for the job. It's a 2016 phone and I tried shooting on iPhone 6 (2014) in bad lighting conditions without major problems.

Please .zip the file sequence and upload it somewhere (such as google drive), then post the link here. I'll try it myself.

[thegoodhen]

Actually, you should be able to tell whether the application sees the tags or not. It tells you how many images were unusable.

[thegoodhen]

@Poipt Any updates?

[jm02394]

It totally slipped my mind yesterday, sorry. I'll have time today so I'll do it soon.

[thegoodhen]

No worries. I was just asking to know if I should close the issue or not.

[jm02394]

Alright, I tried it with footage with actual lines in it and it worked fine. Thanks for all of your help.

[thegoodhen]

No problem! You're welcome. What do you mean by "actual lines" ? @Poipt

[jm02394]

I was filming on my floor, and I think the lines weren't thick enough to be detected.