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Author Topic: No points found for photo with detail  (Read 4133 times)

cbnewham

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No points found for photo with detail
« on: December 26, 2019, 10:38:53 PM »
My projects are usually 400 to 1200 images in size. I have noticed for a few of them that no points are found for a few photos where there is plenty of detail. I've tried masking any background or blurry foreground out and I've also tried varying the total number of key points at the start but it makes no difference - the same photos always come back with no points - that is, zero points, not just points that weren't matched - during the initial processing phase and hence can't be matched.

I attach an example (the attachment is 25% of the original photo size). Other similar photos from the same area of this sculpture with similar photographic viewpoint produce hundreds of points.

Why would this be?


Alexey Pasumansky

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Re: No points found for photo with detail
« Reply #1 on: December 27, 2019, 01:52:30 PM »
Hello cbnewham,

How different are the viewpoints for the overlapping images for the same area? Have you tried to perform the matching of only a few images for the same area (including the problematic one) - do you observe the same behavior?
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Alexey Pasumansky,
Agisoft LLC

cbnewham

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Re: No points found for photo with detail
« Reply #2 on: December 28, 2019, 03:50:45 PM »
I currently haven't got an example to hand with similar views, but here is one with the same area of the subject. One photo with no points and the other with many. There's definitely something odd going on with how Metashape is detecting points. I also don't understand why Metashape seems to like clustering points around one area of a photo rather than distributing them more evenly if the photo has detail features across the entire photo - I've seen that happen more than a few times.



Mak11

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Re: No points found for photo with detail
« Reply #3 on: December 28, 2019, 03:59:56 PM »
Metashape isn't "clustering" points. The points you see in one image are exactly matched in another frame (or more). If a particular image (or area in an image) doesn't have any points it means that the program could't find another image containing the same features. Which leads to two possible causes:

- Not enough overlap in your photos
- inconsistent lighting (too much specular reflection which seems to be the case here)

Mak


cbnewham

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Re: No points found for photo with detail
« Reply #4 on: December 28, 2019, 09:40:02 PM »
My understanding is that the blue points are the matched ones, the grey ones are unmatched.

The photo with no points has no specular reflection.

Mak11

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Re: No points found for photo with detail
« Reply #5 on: December 28, 2019, 10:27:08 PM »
Does your project have other images which are close to the one with no matching points? How was the shooting patern?
Lack of overlap in addition to poor lighting conditions are the principal causes of unsuccessful alignment.

Mak

cbnewham

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Re: No points found for photo with detail
« Reply #6 on: December 29, 2019, 12:13:43 AM »
My projects have more overlap than is good for them (because they are sculptures with heavy undercutting) and the lighting is always good. Anyway - if no features are detected (the photo I provided) then no matches will be found, of course.

As I said before, I think there is a problem with Metashape - unless feature detection is different to how I imagine it works.



chrisd

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Re: No points found for photo with detail
« Reply #7 on: December 30, 2019, 05:42:36 AM »
... lighting is always good.

The lighting appears fairly inconsistent from the images you have provided. You may wish to consider using supplemental lighting to improve results.

cbnewham

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Re: No points found for photo with detail
« Reply #8 on: December 30, 2019, 12:48:45 PM »
I don't know what you mean by inconsistent. The images are naturally lit and of surfaces which are not smooth and have detail. Feature detection is based on details in the images and the fact that there are no points being detected suggests something strange is going on.

chrisd

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Re: No points found for photo with detail
« Reply #9 on: December 30, 2019, 04:09:39 PM »
There are dark or low light areas in many of the images circled below although I don't think this is entirely the problem as you show you are getting some points
The stone appears to be somewhat translucent ... I think I see subsurface scattering in at least one image?

Do you use RAW images ... the extra few bits of image data can help.


More than likely there a few factors contributing to the problem.


Alexey Pasumansky

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Re: No points found for photo with detail
« Reply #10 on: December 30, 2019, 04:58:00 PM »
Gray points mean that the tie point has been detected, but was not used for alignment (marked as invalid). So they are not original key points.

Key points are not displayed in the photo view mode.


You may observe similar behavior, if you add completely different random image to the overlapping image set and try to run Align Photos operation - no tie points will be detected on that additional image.
According to the screenshots, the point of view is quite different, are there any other images similar to the one where no tie points are detected?
Best regards,
Alexey Pasumansky,
Agisoft LLC

cbnewham

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Re: No points found for photo with detail
« Reply #11 on: January 03, 2020, 11:46:49 AM »
Thanks Alexey.  I need to find a better example. Will post again when I find one.