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Author Topic: Dense Cloud - Where do the rings come from?  (Read 1380 times)

WSurvey

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Dense Cloud - Where do the rings come from?
« on: July 21, 2018, 01:00:18 PM »
Hello,

Attached is an example of a dense cloud generated using 2 x 20 Mpx photos from a P4 Pro. An accurate camera calibration is in use and they are part of a larger set. Relative position and orientation should not be a problem.

Why do the rings appear? They change in number and amplitude depending on the settings but are always there.

SAV

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Re: Dense Cloud - Where do the rings come from?
« Reply #1 on: July 23, 2018, 07:15:39 AM »
Hi WSurvey,

These 'rings' commonly appear in areas with insufficient overlap or low number of images overlapping (e.g., at the edges of surveys).

Regards,
SAV
« Last Edit: July 24, 2018, 12:03:14 PM by SAV »

WSurvey

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Re: Dense Cloud - Where do the rings come from?
« Reply #2 on: July 23, 2018, 08:56:14 AM »
Hi WSurvey,

These 'rings' commonly appear in areas with insufficient or low image overlap (e.g., at the edges of surveys).

Regards,
SAV

Hello SAV,

Thanks for replying. There is sufficient overlap between this pair of photos, but I think I understand what you meant. They generally appear towards the edges of jobs where only 2 images overlap. If you generate a dense cloud where a particular piece of ground is covered by 3 or more images you don't tend to see them. Or, at least, their presence is clouded (pun intended).
 
I was kind of interested to see if anyone was aware of the mathematics of their appearance; are they an unavoidable part of the depth extraction from 2D images?

To me, it seems that if they can't be avoided then there is an imprecision about the whole depth-from-2D enterprise. Otherwise something is lacking in my equipment /techniques.

stihl

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Re: Dense Cloud - Where do the rings come from?
« Reply #3 on: July 23, 2018, 05:08:24 PM »
Like SAV said it's an overlap issue. So to answer your question whether or not it can be resolved; yes, just fly with a higher overlap.
If you feel that the overlap is insufficient on the borders of your project area (because of these circles) simply increase your project area so that you have an overshoot above the actual project area.

JMR

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Re: Dense Cloud - Where do the rings come from?
« Reply #4 on: July 24, 2018, 11:03:20 AM »
In my opinion this is not a matter of amount of overlap. Actually, if you overlap two photos more than 80% you will likely start to see things worsening fast. So rather than amount of overlap, it is a matter of amount of images overlapping.
The circles are called "moiré" and they are a consequence of interference between the two spatial frequencies of the two overlapping images. This moiré will become less noticeable when other paterns superimpose in the same place as more photos are overlapping.
It is somehow like when a few rain drops start falling on water and wave circles are clearly visible, but once the rain becomes heavier, the circles get less noticeable.

Regards

WSurvey

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Re: Dense Cloud - Where do the rings come from?
« Reply #5 on: July 24, 2018, 01:02:39 PM »

The circles are called "moiré" and they are a consequence of interference between the two spatial frequencies of the two overlapping images. This moiré will become less noticeable when other paterns superimpose in the same place as more photos are overlapping.

Regards

Hello JMR,

Thanks for the explanation.