I can understand why sharpness is seen as poor. This:
Sharpness is poor to start with. Water doesn't not have the clarity of air, or as much light available
And this:
This is why my Manta Point dive site scanning is done at an altitude of just 4-5m!
At this distance a housed DSLR would comfortably produce fuzzy, soft and unsharp underwater images. With a tiny sensor (compared to a DSLR) a GoPro has the odds stacked against it.
The greatest challenge is not suspended matter - backscatter - but how water robs light. When I first picked up an UW camera I was given a piece of advice:
Get close. Then get closer still.
The single reason we all shoot with ultra wide angle lenses is typically their ability to focus very close to the front element, maybe just 20cm from the subject. Working close to the subject removes the maximum amount of water.
This example was shot at a distance of 1~2m in the English Channel. Vis was maybe 3~4m max:
https://construkted.com/asset/a4wijcuy6yw/
This one at the same distance, in better vis - 6~8m - but almost pitch black conditions thanks to a layer of plankton:
https://construkted.com/asset/art1ve7lisi/
And this one was shot in much better conditions, but again the 1~2m distance to subject was used:
https://construkted.com/asset/a3q7vacu0cq/
A GoPro will deliver better results when working much closer than 4~5m. It's not a question of kit (Kind of...read on...) but a question of method and technique that will go some way to really improve sharpness.
The goal for my scanning was to map the dive site. So my 4-5 altitude allowed me to scan the whole site in 15 dives... This was with me learning from near scratch as i went along, swimming all over the show like a busy bee, but i could probably do the whole site in 10 dives with better quality, and MUCH, MUCH shorter alignment times, if i stuck to a 'mowing the lawn' pattern... Though, this is not easy as i'm swimming 400m with very few cues to help, only my compass, before turning to do the next swath. Area scanning is a LOT harder to get constant overlap, compared to scanning a wreck, or rock formation, etc... I think my mapped area is about 350m wide, 200m tall... So 2.5 Thistlegorm by 1.5 Thistlegorms.
If i'd scanned it at 1-2m altitude, i'd still be down there today! And probably owing some deco time!
With my 3 cameras, on a 4m pole, @2m pitch, this gives me a swath of 6m (4m with 1m either side)...
The second thing that will dramatically improve sharpness is artificial lighting. We cant control the shutter speed on a GoPro...but throw more light at the sensor and the camera will do that automatically. Lighting can be fixed and constant, which is better than no light, but strobes are better still as they deliver a short burst of intense light that helps freeze the action and improve sharpness.
Strobes really help improve efficiency as you can work much faster - less risk of motion blur.
Backscatter - the suspended particles - are always present. But photogrammetry software ignores them as they are moving relative to the primary subject. Any clumps of tie points from backscatter can be reduced using recursive optimisation, or manually excluded.
Why does sharpness matter?
Photogrammetry software will cope with blurred, soft or noisy images. They will align and things might look OK...the model always looks OK and without any constraints, who can really say if there are accuracy issues?
But when faced with specific issues - like the OP with a bent model - then the first look at what has gone wrong starts with the source images.
Look at the reprojection errors for each image. Anything >1.0 pixels is not good. Look at the number of tie points. Anything <100 is not good. Together they may (stress may, not seen the data) be causing curvature, as discussed in the paper I linked to.
There are few shortcuts in UW photography. We have supplied some technical services to design a rig to work at 3~4m and deliver a GSD of <1mm, but each camera sensor is 64mp and there is a huge amount of artificial light available. Even with this, the ROV speed is/was absolutely critical...too fast and it's all blurred.
I would urge anyone working UW to work really close to the subject, and to include artificial light. Quality jumps and with it the chances of greater, repeatable accuracy.
I would also hesitate to suggest that adding more cameras into a method that is creating images that lack inherent quality is the correct way to fix what appears to be fundamental image quality issues?
I would absolutely love to get some lights to aid my scanning, but again, i'm limited by being absolutely skint.
But again, i think you're missing the point... IF you're stuck or not with gopro cameras, and with no lights, it doesn't matter what you're scanning, if you add a 2nd camera, it'll be twice as fast! It's no different than swimming twice... With the benefit of having perfectly parallel paths... Add the 3rd camera, and it's as if you're swimming 3 times, again, perfectly parallel. 4 cameras... etc. etc. You just set the pitch of the cameras to give you the good side-to-side overlap, when you consider the altitude you've chosen to swim at, to give you the quality you would like.
So if you're scanning at 1m altitude, then the pitch between each camera might 0.5m... whatever works out...
So with the limitations of the camera, the lighting, the vis, etc. etc. etc. having 2 or more cameras on a pole allows you to get 2 or more perfectly parallel paths, which means the quality of the scan between them will be as good as it can be, give these limitations, JUST because its perfectly parallel, and at the perfect pitch between camera paths. It just as if you're a perfect swimmer, swimming perfect straight paths, perfectly parallel...
IF you require better quality scans, you can change the altitude, the camera/path pitch, the lighting, the cameras, etc. etc.
But the fact remains, that if you happened to have 2x, 3x, 4x, of your cameras available, perhaps with their own strobes, IF you put them on a pole, that satisfied the pitch you required for your side-to-side overlap.
So 2x of your Nikon rigs, with their strobes, would be faster at scanning than using just 1... And you'll get the solid data down the middle of the swath...
And if you could sync them, then you get all the benefits you described earlier, as a bonus.
We're limited on time underwater, so using 2 gopros, which is not impossible to have available, allows us to make better use of that dive.
Again, i'd absolutely LOVE to get some sync'd 45mp cameras on a carbon fibre pole, with a lighting array!
I can supply my bank details on request
Topic: Underwater Photogrammetry with GoPro Hero 12 (Read 30032 times)
