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Help setting a multicam 3D scanner

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Re: Help setting a multicam 3D scanner
« Reply #10 on: 28 / November / 2018, 12:39:28 »
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Wow! From my understanding that would be a pretty neat way to sync CHDK and digiCamControl! I would suggest it in their forum, but I don't even know how to word it...  CHDK integration through PTP??  :blink:

Little update: Did a quick test shooting a several second exposures in complete darkness and triggering a flash manually. Quality is better than I expected, there's some noise in the DNG even at base ISO 80, but I think it's the inherent noise of such a small sensor. At least noise reduction didn't kill most of the detail, so it's reasonably sharp and detailed. Didn't see much change in noise patterns from 16s up to 0,3s.

In any case, manually syncing the flash with exposures shorter than 1 sec proved to be quite tricky. The flash option is still alive, but I guess I'll have to look for a better sync method.

I'll keep you posted on new developments. Thx!

Re: Help setting a multicam 3D scanner
« Reply #11 on: 28 / November / 2018, 14:33:21 »
The chdkptp ptp sync method is very clever but also, fundamentally, straightforward.

In a much simplified nutshell, at a (different for each camera) point in time both the PC operating system and camera operating system times are measured (in ms) and recorded in PC memory. For the camera it is the count of ms that have passed since it was turned on & for PC it is the count of ms from a particular date in history that I don't remember. 

At the present time a new PC system ms time count is measured & recorded (on the PC). In order to make a calculation (on the PC) of what the unique present time camera system count (since turn on) should be (at this present time) we take the difference of both PC system time counts (present less historic) and add that to the original camera count.

It takes a finite time to send this calculated unique count from the PC to each camera (one after another) so an unmodified calculated count sent to any camera would already be less than that of the camera operating system count at the time of receipt. The unique PC calculated count for each camera is therefore increased by a number of ms that is equal to (or greater) than the maximum period required to send the unique count to all cameras.

On receipt (at the camera) of this unique PC calculated count, the camera waits until its operating system ms count is equal to the one sent to it by the PC before opening the shutter. 

There's a bit more to it than that but, even if an equivalent were possible in digicamcontrol, it'd probably take the author quite a while understanding the detail & coding it.


The time to automatically move the mouse and click a button is probably not insignificant.

You might need to be careful wiring those remote shutter cables together http://www.agisoft.com/forum/index.php?topic=2081.msg11290#msg11290


Edit: there is a link to a breezesys article in that Agisoft thread that includes a diagram with the addition of a diode (for each camera) when wiring the focus/shutter release cables together which (presumably) limits the potential for current flow (or voltage application) between cameras that could, potentially, cause problems/damage.
« Last Edit: 29 / November / 2018, 05:29:38 by andrew.stephens.754365 »

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Re: Help setting a multicam 3D scanner
« Reply #12 on: 28 / November / 2018, 16:01:28 »
In any case, manually syncing the flash with exposures shorter than 1 sec proved to be quite tricky.
"A few seconds" was just my WAG, so if you get acceptable quality with longer exposures, I don't see any reason not to do that.

The main issues you see with long exposures are
1) Amp glow: Generally shows up as a purple glow, more pronounced at one edge or corner. Roughly between models released in 2009 and 2012, Canon significantly reduced the impact of this. Cameras after the change have minimal amp glow even in 1 minute exposures.
2) Dark current (https://www.photometrics.com/resources/learningzone/darkcurrent.php)
3) Hot pixels (also described in the above link)

These can be significantly mitigated by dark frame subtraction (https://en.wikipedia.org/wiki/Dark-frame_subtraction). On most powershots, the Canon firmware automatically performs dark frame subtraction based on exposure length and possibly other factors like temperature and ISO. When this happens, you will see "busy" on the screen for the same amount of time as the exposure took. CHDK can force this on or off for most cameras. You can also perform DFS by creating dark frames and subtracting them later in your workflow.
Don't forget what the H stands for.

Re: Help setting a multicam 3D scanner
« Reply #13 on: 29 / November / 2018, 07:16:47 »

The chdkptp ptp sync method is very clever but also, fundamentally, straightforward.
I wouldn't call it straightforward myself.... But I got the gist of it ;)
Quote

You might need to be careful wiring those remote shutter cables together http://www.agisoft.com/forum/index.php?topic=2081.msg11290#msg11290
Thanks for pointing that out, I'm in the process of designing the rig armature, so will keep that in mind.

"A few seconds" was just my WAG, so if you get acceptable quality with longer exposures, I don't see any reason not to do that.

The main issues you see with long exposures are...

Luckily (for now) didn't see much of those. There is some noise, but pics came out decently clean. And RAW quality is in a whole different level against the contrasty, badly sharpended and heavily denoised Canon post-processing. Thank god (you guys?) we have that! In any case, I won't be able to judge if the quality is good enough until I process a scan in Photoscan, but it looks way better, and the flash triggering option starts to take the lead (for simplicity and compatibility with the Nikon system).

I'll keep digging into this, thanks again!


 

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