Underwater Photogrammetry with GoPros - Alignment failures

[simon_29]

Hi everyone,

I am using some GoPros (Hero 4 Black) for underwater photogrammetry. In order to have enough overlap between pictures, I have extracted every 10 frames an image from the 4K video. As almost no exif data is available in the extracted frames, I copied the meta data from an usual GoPro picture to all of my frames.

During the acquisition, I also recorded the GNSS Positions ( I have a position every second). The Hero 4 records 30 frames per second, so when I extract 10 frames, I have three consecutive frames that are within the same second. For those frames, I give the same position.

When I try to align thoses images I met some failures. In some cases, PhotoScan can't find any matches between two overlaping pictures.

I cannot send in attachment the pictures as the zip file is too big. Here is a link to download it :
 
https://we.tl/WDPgM0brs8

In this  zip file  you can find two overlaping pictures, "GOPR0827 1232.jpg" and "GOPR0827 1233.jpg". Although the overlap is superior to 80%, I don't have any matches between them ("1232_Matches.png" and "1233_Matches.png").

For my whole project I have almost 1300 cameras. I have tested on a small set of cameras (9 cameras) and I cannot align them. I would like to know what I am doing wrong or what I should do.

Kind regards, Simon

[MeHoo]

Did you shoot a grid underwater to manually calibrate them?  the index of refraction of water is definitely going to mess with the camera models' defaults.. And make sure to check "fisheye" for the lens type.

technically.. even a change in temperature and humidity could affect it slightly.  Just an idea.. take those cameras back into that water and shoot the grid.

also.. with the caustics constantly changing the light on the rocks, I can see a huge problem with alignment there. 

Try the grid and picking a more overcast day.

- Justin

[simon_29]

Hi and thank you for your answer.

I haven't calibrated the GoPro beforehand and the lens type is checked as "fisheye".

The thing is, I made two acquisition of the same object (rocks of a seawall in my case) the same day. One in the morning around 9:30 am and the other one at 6pm. The aim was to see the impact of light for underwater photogrammetry. With the data set of the evening I get very good results ( I attached a picture of the evening session  to show you the difference between them). Therefore I know that, even without calibration, I should at least align cameras. But from what you said, the constant change in light would prevent PhotoScan from finding matches. What I don't understand is that, for somes pictures of the morning session, PhotoScan manage to align cameras although the light changes a lot between pictures as well.

I don't know if I am clear, but thank you very much Justin for your time and your answer. Have you some experience and feedback to share about underwater photogrammetry ?

[MeHoo]

No experience with underwater photogrametry, but I have experience with changing lighting conditions.  The evening photo is very diffuse and has very little pronounced highlight or shadow detail.  You can get nearly anything to align on a cloudy day.. but on a brightly lit day at high noon with trees blowing in the wind casting random shadows.. I've never had any luck.

Hope it helps.  Maybe Alexey can shed some light on this.