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Microsoft Livecam Studio flickers

The place for questions, problems, comments and tips regarding the camera calibration.

Microsoft Livecam Studio flickers

Postby david138 » Mon Apr 07, 2014 6:58 pm

I got a Microsoft Livecam Studio today.
When setting it up in David 3.10 the livepicture shows horizontal black and white "moving strips".
I tried different frequencies (60, 75 Hz) of the projector (Acer K11 connected via HDMI) without any positive results.
The Exposuretime of the camera can only be set to 1/64, 1/128, 1/256.. an so on.

I think these different frequencies cause the problem.

Any idea to solve this?
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Re: Microsoft Livecam Studio flickers

Postby MagWeb » Wed Apr 09, 2014 11:39 pm

You're right, this is most likely a problem of interference of framerate and refresh rate.

Don't own the K11 - maybe it allows a different refresh rates if you set the second monitor to a lower resolution than the native projector resolution? Same for the cam: Maybe it allows different exposure times at lower resolutions. In theory both components should run on a speed where (projector's frequency) * (exposure time) = some integer value {e.g. 60 * 1/30 = 2 }

If that's not possible:
Stripes should not be a major problem if they do not move. Well, you mentioned some flickering so I assume they do. But DAVID grabs the patterns in a not sorted way . The black and white patterns aren't that important. The fringe patterns (sine-wave patterns) instead do the either job, AFAIK . So the simple :? thing you've to do is to find a setting where the stripe is at the very same position at every used cam's frame. There are advanced parameters which have some influence on the point when DAVID grabs an image (e.g. PatternHoldingTime). Means: If your stripe needs, let's say, 2 sec to be at the the same position again, you should force DAVID to wait 2 seconds to grab the next fringe image... easily said but hard to do - needs some tinkering.

@ Sven: Assuming a simple white projection and doing a comparison of some grabbed frames of that scene, it should be possible to find a "pattern" of frequency-mismatches (change of all-over brightness and/or moving fall-offs) in a "pre-calibration" step. This possible changes should be based on the interference and therefore it should be a constant thing which could be used to correct incomming images further on. Could be usefull...
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Re: Microsoft Livecam Studio flickers

Postby Sven » Tue Apr 15, 2014 6:46 pm

Hi Gunter,
Good idea. However I think it's not worth the effort... nearly all cameras can be set to the correct exposure times.

@David: If you would not like to purchase an industrial camera, I would recommend the webcams from Logitech. First choice: C615.

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Re: Microsoft Livecam Studio flickers

Postby MagWeb » Tue Apr 15, 2014 10:19 pm

Hi Sven,

here are some cams (also the good old first DAVID cam) which still show some brightness changing even at a theoretical "correct" exposure time.
Industrial as consumer cams are not built to grab a pulsing projection but mainly for stable light conditions. If there is some (faster or quicker) change of the image's brightness there will be waves in resulting scans.

There are multiple things (hard- firm- and software) involved to build a single frame which is the input for DAVID and each component might cause an error. I've to state: There ARE errors in the finally grabbed frame after a pattern command passed the whole pipeline. DAVID doesn't has any access to any of component which created the final frame.

Thanks to Godehardt I've a uEye cam at hand. This cam provides more than the normal cam parameters. I can set it to provide stable images of a projection or make it flickering like hell on the same exposure time by changing its pixel clock rate. Drivers for simpler cams do not give any access to this or similar parameters. Its not a question of exposure but of firmware parameter as the hard coded clock (or similar things).

The single input frame is the one and only thing DAVID relies on. But has DAVID to trust that input? You did great efforts to get rid of lense distortions. You did great things to get rid of color balance errors. I don't understand why you trust a assumed "stable" image (= comparable set of images). IMO No - DAVID should compare a set of images too. IMO this should be the next step to a calibrate and to get rid of "waves", "ripples" or whatever you may call it.

Think it would be worth ( against a alternative option where all hard- firm- and software parameters are known and where would be no need to do some calibration - but that's not DAVID)

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Re: Microsoft Livecam Studio flickers

Postby Sven » Wed Apr 16, 2014 9:58 am

Hi Gunter,

You are right, the same idea can be used to optimize setups where there are minor interferences, causing waves.
With rolling shutter cameras, this is more complicated than global shutters.
First step might be to try to compensate for overall image brightness variations (global shutter).
Alternative: Use a camera like your uEye (with allows very fine adjustments of exposure time or clock rate), and try to find and set the ideal camera timing automatically.
We'll see...

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