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Author Topic: Rolling shutter option detail  (Read 20915 times)

SAV

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Re: Rolling shutter option detail
« Reply #15 on: September 26, 2017, 05:57:40 AM »
Too bad  ;)

All the best for your future endeavors.



Thank you !

No sorry it is confidential for the next 2 years ! and note that everything is in French...

Regards

BobvdMeij

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Re: Rolling shutter option detail
« Reply #16 on: September 26, 2017, 09:17:35 AM »
Hi Yoann,

There are basically 3 types of shutters used in cameras:
 
1. Mechanical shutter
2. Electronic shutter
3. Hybrid shutter (mixture of 1 and 2)

When it comes to electronic shutters, you can either have a rolling electronic shutter or a global electronic shutter (= total electronic shutter). Most digital cameras use a rolling electronic shutter, which leads to the rolling shutter issue we try to avoid for photogrammetric processing.

Note that some cameras use an electronic shutter for live view (& video) and a mechanical shutter for still photographs (e.g., the camera used on the DJI Phantom 4 Pro).

I am not aware of a 'rolling mechanical shutter' issue because the information from the sensor is not read line by line but in one go, hence no rolling shutter issues.

If your camera is moving during photo acquisition (e.g., camera is mounted on drone/UAV), you have basically four options to mitigate the rolling shutter problem:
1. Use a camera with a mechanical shutter
2. Use a camera with a (hard) global shutter
3. Compensate for the rolling shutter effect in post processing (as it can be done in Pix4D and Agisoft PhotoScan)
4. Change your 'survey design' (i.e., stop for each image) and adjust the camera settings (i.e., higher shutter speed)

Regards,
SAV

Thanks for this detailed answer !

Since, I've red somewhere that mechanical/electronic shutter doesn't mean global/rolling shutter.
So we could have a rolling mechanical shutter (?) or a global electronic shutter (which is actually exist)

May you confirm that point ?

Regards

SAV,

You seem to explain the diffent coming abouts of the Electronic Shutter systems well, but I seem to miss how this is the way for Mechanical Shutter systems? Or are you suggesting that Mechanical Shutters are ALWAYS Global shutters by default?

Yoann Courtois

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Re: Rolling shutter option detail
« Reply #17 on: September 26, 2017, 10:54:00 AM »
Hi Bob and SAV !

The lack of understanding about Mechanical shutter is actually coming from some information I found about the existence of Mechanical Rolling shutter, but without any reliability.

So I spent a bit more time on that point and found this webpage which will, I guess, make you more confident about that topic:
https://www.juzaphoto.com/article.php?l=en&t=mechanical_and_electronic_shutter

In a word:
- Mechanical shutter are basically global, but can be consider as rolling for shutter speed shorter than 1/250. BUT, the rolling effect is so small (1/1000 or less) we cannot suffer from it.
- Electronic shutter are basically rolling, and suffer from a huge rolling effect from 1/10 (basic cameras) to 1/200 (expensive ones). It exists electronic global shutter but those cameras are very expensive and the system limits the quantity of light that can be capture and so increase the noise.

Hope it helps !
Regards
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Yoann COURTOIS
R&D Engineer in photogrammetric process and mobile application
Lyon, FRANCE
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stihl

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Re: Rolling shutter option detail
« Reply #18 on: September 26, 2017, 11:08:36 AM »

Mechanical shutters are always global shutters. The mechanical shutter is found in DSLR cameras and as far as I know refers to the glass mirror inside the camera that flips up or down to let light fall onto the sensor all at once, completely eliminating the rolling shutter distortion because the sensor is read all at once.


In general you'll either have a camera with a CCD or a CMOS sensor. CCD cameras are nearly always equiped with a global shutter mechanism but since CMOS sensors have a couple of advantages over CCD, manufacturers have started building CMOS sensors with a global shutter. Those prices have dropped down a lot and now you can find CMOS global shutter mechanisms in cameras like the sony alpha 7r.

The DJI Zenmuse X4S camera uses a small mechanical leaf shutter which, although the camera is very compact and uses a CMOS sensor, eliminates the rolling shutter distortion.
« Last Edit: September 26, 2017, 11:26:46 PM by stihl »

Yoann Courtois

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Re: Rolling shutter option detail
« Reply #19 on: September 26, 2017, 11:23:56 AM »
Mechanical shutters are always global shutters. The mechanical shutter is found in DSLR cameras and as far as I know refers to the glass mirror inside the camera that flips up or down to let light fall onto the sensor all at once, completely eliminating the rolling shutter distortion because the sensor gets lit up all at once.

That's true for DSLR cameras. But with micro-sensors such as Phantom 4 Pro or Zenmuse cameras, the mechanical system is more looking like what you can see in the link I just shared before (or like leaf shutter). Here is typically what we call rolling shutter, but the rolling effect is negligible because the readout speed if most of the time faster than the shutter speed.

Regards
« Last Edit: September 26, 2017, 11:27:29 AM by Yoann Courtois »
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Yoann COURTOIS
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Lyon, FRANCE
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Dave Martin

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Re: Rolling shutter option detail
« Reply #20 on: September 26, 2017, 03:05:57 PM »
This is straying off-topic, but ....

...Mechanical shutters are always global shutters. The mechanical shutter is found in DSLR cameras and as far as I know refers to the glass mirror inside the camera that flips up or down to let light fall onto the sensor all at once, completely eliminating the rolling shutter distortion because the sensor is read all at once.
....

Sorry but would beg to differ.

The mirror, if fitted, moves out of the way before the shutter is triggered.

If the shutter was effectively taking away and replacing a cover, then the first part uncovered might receive more illumination than the last piece uncovered. Also it is harder to get a mechanism to accurately reverse at high shutter speeds.

What is often done is to use a pair of shutter blinds (see image below). The first blind (the green one) 'drops' and uncovers the film or sensor; it is closely followed by a second blind (the blue one) which re-obscures the sensor. As the first blind goes 'down' it is uniformly followed by the second one so as the 'gap' travels down the sensor, all parts receive an equal duration of illumination. The gap between the trailing edge of the first shutter and the leading edge of the second one determines the exposure time.

If your exposure is, say 1/500 second, it does not mean the whole sensor is exposed in 1/500 second, it means that the second shutter is following 1/500 second behind the first - it may take well more than 1/500 second for the 'gap' to travel from top to bottom of the sensor.  This means that there is a rolling exposure (as Yoann alluded to) - albeit far lower magnitude than some electronic-only shutters with rolling readout over many ms.

Dave


https://i2.wp.com/digital-photography-school.com/wp-content/uploads/2011/07/Curtain-Action.jpg
« Last Edit: September 26, 2017, 03:53:28 PM by Dave Martin »

brianpgreen

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Re: Rolling shutter option detail
« Reply #21 on: September 26, 2017, 10:56:10 PM »
This is clear as mud to me at this point.  I've been following this because I'd like to buy an M210 to use for mapping.  I'm not sure which camera to get.  From what I understand so far the 4s has a CMOS sensor with a mechanical shutter which is triggered for exposures slower than 1/2000 but it will still capture an image line by line like it had an electronic shutter.  If this is the case I'd be inclined to buy the X5s with the larger sensor and interchangeable lenses and deal with the rolling shutter through software compensation.

Am I on the right track?

stihl

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Re: Rolling shutter option detail
« Reply #22 on: September 26, 2017, 11:21:42 PM »
Thanks Dave for clearing that up about the mirror. I was looking at super slow motion videos of the shutter blinds of DSLRs mirror cameras and cameras like the a7r which are mirrorless and together with your post it cleared up a lot.

If there is a small and neglectable rolling shutter distortion with cameras that use a shutter blind, can these be considered global shutter cameras?
« Last Edit: September 26, 2017, 11:39:05 PM by stihl »

SAV

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Re: Rolling shutter option detail
« Reply #23 on: September 27, 2017, 08:56:49 AM »
Hi brianpgreen,

Clear as mud ... great  ;D

This conversation/thread went into quite some detail which might have made it the water muddier than it should  ;)

The X4S camera is basically the same camera as used on the DJI Phantom 4 Pro and Phantom 4 Advanced. No rolling shutter issues when taking still photographs, because the mechanical shutter is used then (needs to be enabled in the DJI GO 4 app though). For video, only the electronic shutter is used and the rolling shutter issue persists.

The X5S camera has only an electronic shutter, hence you will have to use software based rolling shutter compensation to get rid of it.

I personally would go for the X4S. Maybe even think about getting a Phantom 4 Pro instead of the M210. The P4P has the same camera but is much cheaper, easier to carry around and has a longer flight time per battery. IMO, much better value for money. Unless you want to fly 2 cameras at the same time or also have the option to add a thermal camera (Zenmuse XT), then you will be probably better off using a Matrice 210.

All the best.

Regards,
SAV

This is clear as mud to me at this point.  I've been following this because I'd like to buy an M210 to use for mapping.  I'm not sure which camera to get.  From what I understand so far the 4s has a CMOS sensor with a mechanical shutter which is triggered for exposures slower than 1/2000 but it will still capture an image line by line like it had an electronic shutter.  If this is the case I'd be inclined to buy the X5s with the larger sensor and interchangeable lenses and deal with the rolling shutter through software compensation.

Am I on the right track?

cadm8

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Re: Rolling shutter option detail
« Reply #24 on: September 27, 2017, 12:14:29 PM »
I personally would go for the X4S. Maybe even think about getting a Phantom 4 Pro instead of the M210. The P4P has the same camera but is much cheaper, easier to carry around and has a longer flight time per battery. IMO, much better value for money.

Sorry for spamming the thread but, what do you think about the X4S camera upscaling images ie not a really 20MP camera and how could that affect photogrammetry projects?

Regarding P4P camera ( https://phantompilots.com/threads/p4p-image-not-true-20mp.111480 )

Jeremiah_ROWE

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Re: Rolling shutter option detail
« Reply #25 on: September 27, 2017, 05:17:04 PM »
Congrats!

Hi SAV !

Thanks for the support ! I'm now graduated  :D

SAV

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Re: Rolling shutter option detail
« Reply #26 on: September 27, 2017, 05:51:06 PM »
Hi cadm8,

Interesting. I wasn't aware about that, thanks for sharing.
I knew that DJI is using a 1 inch, 20 Megapixel CMOS sensor from Sony, but the 'internal processing/upscaling/downscaling' is new to me. Not sure how big the effect on the resulting photogrammetric model would be though.

But from my own experience I can report that even using JPGs (not DNGs) captured by a P4P you can achieve great photogrammetric modelling results. The P4P is awesome value for money (as of Sept 2017), IMO.

Regards,
SAV


I personally would go for the X4S. Maybe even think about getting a Phantom 4 Pro instead of the M210. The P4P has the same camera but is much cheaper, easier to carry around and has a longer flight time per battery. IMO, much better value for money.

Sorry for spamming the thread but, what do you think about the X4S camera upscaling images ie not a really 20MP camera and how could that affect photogrammetry projects?

Regarding P4P camera ( https://phantompilots.com/threads/p4p-image-not-true-20mp.111480 )

cadm8

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Re: Rolling shutter option detail
« Reply #27 on: September 27, 2017, 05:59:37 PM »
But from my own experience I can report that even using JPGs (not DNGs) captured by a P4P you can achieve great photogrammetric modelling results. The P4P is awesome value for money (as of Sept 2017), IMO.

Totally agree with that. I was greatly dissapointed when having found out about the upscaling, since it seems that the true image is about 15MP. So, if you're reporting that you haven't noticed any blurring or mysterious lower overlaps, I guess it's OK

Jeremiah_ROWE

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Re: Rolling shutter option detail
« Reply #28 on: September 27, 2017, 10:29:03 PM »
This is due to internal distortion corrections. If you compare the RAW file to a JPEG for many many cameras you will find the same thing. It isn't a new issue, or even a problem, just something people worry about when they learn more about digital photography.

Agisoft handles P4 Adv/Pro (and X4s) data very well. When optimizing data using this sensor keep the "Fit rolling shutter, Fit k4, Fit p3, and Fit p4" options unchecked.



But from my own experience I can report that even using JPGs (not DNGs) captured by a P4P you can achieve great photogrammetric modelling results. The P4P is awesome value for money (as of Sept 2017), IMO.

Totally agree with that. I was greatly dissapointed when having found out about the upscaling, since it seems that the true image is about 15MP. So, if you're reporting that you haven't noticed any blurring or mysterious lower overlaps, I guess it's OK

SAV

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Re: Rolling shutter option detail
« Reply #29 on: September 28, 2017, 07:45:15 AM »
Hi All,

Some more info regarding the DJI X4S camera and the camera that is used in the P4 Adv/Pro.

DJI's X4S camera uses a 1-inch Exmor R CMOS from Sony, with 20MP EFFECTIVE PIXELS. So what does effective pixels mean?

I found this online:

Quote
Effective pixels are the pixels that are actually capturing the image data.
A conventional sensor in, for example, a 12MP camera has an almost equal number of effective pixels (11.9MP). The actual, or total, pixel count of a camera sensor includes 0.1 MP left over after counting the effective pixels. These 'leftover pixels' line the edge of an image sensor and are shielded from receiving light but are still used as a reference point which can help in reducing noise.

This would mean that DJI's X4S camera sensor has actually MORE than 20MP, because in the specifications the amount of effective pixels is 20MP.

There is also something called INTERPOLATED PIXELS. Found this info online:

Quote
Some cameras can interpolate the number of sensor pixels. For instance, a 6MP camera may be able to produce 12MP images. In this case, the camera adds new pixels next to the 6 megapixels it captured to create 12 megapixels of information. The file size is increased and this actually results in a better image than if you were to interpolate in an image editing software because the interpolation is done before JPG compression.

Honestly, it does not really help to clear up the 'muddy waters', but it might be helpful to know all these details (for some at least).

As mentioned by Jeremiah_ROWE, P4 Adv/Pro and X4S imagery works well for for photogrammetric modelling in Agisoft PhotoScan. That's what most people need to know ;-)

Regards,
SAV



« Last Edit: September 28, 2017, 07:47:57 AM by SAV »