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Author Topic: underwater photos don't align  (Read 4069 times)

teds

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underwater photos don't align
« on: February 29, 2016, 04:57:19 AM »
I'm trying to align photos taken underwater. PhotoScan is able to able to correctly find points and match them, but it always marks them as "invalid". For example, in the attached image, you can see that there are many common points in the two images that are correctly paired, but no "valid" pairs.

Does anyone have any ideas what I may be doing wrong?

My one guess, is that the camera has not been calibrated underwater, and the reported data in the EXIF is wrong, as I assume those values are reported for above water. I have entered approximate focal length values for underwater, but that did not help. 

I have a few hundred photos in this set, so I don't believe lack of information is the problem.

thank you for any tips or advice...
--ted

SimonBrown

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Re: underwater photos don't align
« Reply #1 on: February 29, 2016, 02:35:06 PM »
The camera and lens combination I use underwater isn't calibrated either, but the resulting images do align.

So I do not think the issue is with calibration.

Are you applying any post-shoot manipulation to the images?

What ISO are you working at when shooting the images?

Are the images in sharp focus?
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teds

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Re: underwater photos don't align
« Reply #2 on: March 02, 2016, 01:08:26 AM »
Simon,
Thanks for your reply.

When I first tried this process, my camera was (incorrectly) adding GPS information to the EXIF file. This was incorrect information (as my camera cannot get a GPS signal under water) and was confusing PhotoScan. So I have removed all the GPS sections of the EXIF data.  Other than that, I am not doing any manipulations to the photos.

The photos are reasonably sharp and in focus (as far as underwater photos go…). I am using a point in shoot camera with an automatic ISO setting (it varies I believe based on the lighting conditions).

When I manually inspect the photo matches, they look good. Many of the photos have hundreds if not thousands of points correctly matches between photos, but they all are recorded as having “0” valid matches! The photographed areas are captured from multiple angles, and all have good matches found.

It seems that whatever PhotoScan is using to determine if a match is valid or not is too strict for my implementation?  Does anyone know how this works? Is this based on there being a consistent transformation between two photos, or does it use an overall model and look for consistency? Any tips on things to try would be greatly appreciated.

Thank you,
--ted

eddiethemartian

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Re: underwater photos don't align
« Reply #3 on: March 03, 2016, 04:56:36 PM »
From your screenshot it looks like you're not getting many matches for most of the images.  The pair of photos we can see have a number of matches but not much overlap, and only two images have more than a hundred or so points in common with the selected one.  Perhaps you just haven't overlapped your images enough?

Image quality tends to have less of an effect on alignment than coverage does.  Post up a few photos that show your coverage of a particular area, maybe that would give us something to go on.

Eddie
« Last Edit: March 03, 2016, 04:59:38 PM by eddiethemartian »

Mohammed

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Re: underwater photos don't align
« Reply #4 on: March 03, 2016, 11:20:52 PM »
As i see in your screenshot there's not enough overlap images, also the below image was too much oblique than the other image that make very hard for the sw to detect the match points correctly.

SimonBrown

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Re: underwater photos don't align
« Reply #5 on: March 06, 2016, 12:07:33 PM »
The photos are reasonably sharp and in focus (as far as underwater photos go…). I am using a point in shoot camera with an automatic ISO setting (it varies I believe based on the lighting conditions).

As others have said, overlap is critical. And a well exposed & lit image will be as sharp as anything taken on the land...

I have just run some material off of a Gopro video through Photoscan. The biggest issues was a lack of sharpness (low light/slow shutter/fast panning) in some frames resulting in a lack of images that overlapped and were technically acceptable.

Do you have a subset of the images that do align? If yes, then perhaps have a look at those and compare them to what does not?

Without overlap there is nothing to be recovered, so another dive is probably in order.
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