Author Topic: Workflow for Multi-mission, overlapping, UW scans.  (Read 1821 times)


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Workflow for Multi-mission, overlapping, UW scans.
« on: September 08, 2021, 11:13:19 AM »
Here's my workflow I'm trialling for dealing with multi-mission scans that also overlap a lot (too much)...
This has previously caused HUGE align times (3d), especially in the 'Selecting pairs' stage, most likely due to the excessing overlap.  Aligning every photo (19,000 so far) from scratch just isn't practical any more, so I'm reducing overlap as i go, and adding the next scan's photos to the growing alignment of the previous scan's photos...

The project:
I've been scanning an underwater dive site using 2x 12MP GoPros on a 3m pole, 'flying' 3-6m above, cameras normal to the bottom/slope/rocks... The 2x GoPros just make it 2x quicker to scan... It then produces a scan with two parallel, but not synchronised, time-lapse photo camera tracks, that have good to too much overlap side-to-side, depending on flying height, and good to too much overlap going forward, depending on how fast I'm swimming, drifting and the flying height again...  Sadly you can only set the GoPro's time-lapse photo interval to 0.5, 1, 2, 5, 10, etc seconds...  I'm using 2, though 3 would be better i think!
Due to the nature of diving and the dive site, each trip there and scan means i land up scanning over parts previously scanned, and it's amazingly easy to miss patches, not know where you are, and not swim in straight lines.  Some of the big features, house sized boulders need a spiral scan, the rest mostly a 'mowing the lawn' u-pattern.
Conditions are usually sub-optimal... greenish water sometimes, not that great vis (10-15m), and sun shimmering on the shallower rocks.  So each scan can look quite different... a 'different' colour. doh..
A 'good' scan dive gives me 1,500 photos from each GoPro... So 3,000 per scan.  The reduce overlap to 8 reduces this down by about a third to 2,000...
Due to these conditions, and the fact that I'm simply not looking for mm accuracy, only a 3D model of the rock formations, sand patches, corals, etc. I'm not pushing Metashape Standard with high quality settings, which has sometimes actually given better alignments for my crappy images, and which obviously saves time too.

Enable Keep Key Points in Preferences, to allow adding photos to a previous alignment.

For each scan pair (from both GoPros combined):
Add Photos and create Folder with scan name.
Align Photos:  Low - GenPre - RefPre-Est - 20,000 - 2,000 - ExclStatTiePts - AdptCamModFilt
Save as 'Aligned' (just in case)
Select any Unaligned - Reset Alignment - Align Selected
Gradual Selection: - ReconUncert = 15 - ProjAccuracy = 15  Then Optimise Cameras
Check Region, Resize.
Build Mesh from Sparse.
Reduce Overlap: Surface Coverage = 8
Select disabled Cameras and Remove.
Optimise Cameras
Generate Mesh from Depth Maps to
Save as "Scan 'n' reduced"
...Repeat for all other previous or new scans...
This reduces a 3,000 photo scan with too much overlap to 2,000.

Combining 'reduced' scans:
Load "scan 1 reduced"
Append "scan 2 reduced"
Remove all of scan 2's parts leaving its folder of now unaligned cameras only.
Move that camera folder from chunk 2 to chunk 1, next to 1's camera folder (already aligned).
Repeat the above steps from 'Align Photos', Ensuring Reset Alignment is NOT set.
Saving after alight as "1+2 aligned"
Optimising and reducing as above
Saving after optional meshing as "scans 1+2 reduced"
...Next, repeat, appending "Scan 3 Reduced" etc... Until you have "Scans 1+2+3+4+5+6+7+8+...."

This has reduced the total number of photos needed for the area covered by about a half i think.

This then give me the mesh/model of the summation of all those overlapping scans, without the 3 days of aligning all the photos in one go.

Please feel free to rip into this workflow with any suggestions...  ;D ;)
Yes, I've only gone for low, 20k, 2k... only pushed gradual selection to 15... chosen 8 as my overlap...  Should i change any of these for this type of job???

See my Sketchfab page in my Signature below for previous versions of this model of Manta Point I'm creating...
I'll post some photos later...  Waiting for the alignment after adding scan 7 of 8...