Projector and Camera Syncing

Discuss about cameras, projectors, calibration panels, turntables etc.
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Micr0
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Projector and Camera Syncing

Post by Micr0 »

Continuing from a discussion started over "there".....

I had though of using the sync signal in the HDMI stream to trigger cameras (ones like the imaging source that have external triggers) and hopefully get a better scan quality. Possibly one that doesn't have the dreaded wavy lines we all know and love. We have found that getting a cameras frame rate as close to your projectors frame rate is the best way to minimize the wavies. my thought was to use a common sync signal to precisely lock the cameras shutter to the projectors display. Here are my thoughts.

1) it should be possible to take the sync signal from the HDMI stream and send it to the trigger input of and IS camera. There are cables made for braking out that signal from pin 2 but I am not sure of the format of that signal. It may be a straight PWM signal at the frame rate output by the video card or it may not. Someday I can try looking at it on a scope and see what it looks like.

2) if the signal on pin 2 can be used directly by the camera it will need to be properly split off from the signal to the projector. You can't just splice into the HDMI cable. Luckily HDMI splitters are cheap and easily available. My question there is: do they maintain the timing between the 2 (or 3) output signals. I tend to think they will but with cheap home hardware you never know.

3) Providing the camera will trigger off of this signal, it is possible that an immediate capture may not be best. It is possible that the projector may need a few ms to fully display the image, or if there is a wheel in the projector it may not in any way be synced to the frame rate. If this is the case a way to add a bit of delay to the cameras triggering would be necessary. the Is cameras may be able to do this a sthey have some good developer tools/software. However is not adding a delay into a sync signal would require processing power and may be able to be done with an arduino. One more rabbit hole I don't really need.

I can get an HDMI splitter on Ebay for about $10 and a breakout cable for another $10. for $20 this may be worth exploring however my instinct is that first, its not this simple, and second that even if it works the returns on the investment of time and $ wouldn't be all that dramatic. I have a very basic understanding of this theory and almost no practical experience, so this is just as likely to result in nothing but I am curious enough to at least give it a try.

Thoughts?
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Micr0
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Re: Projector and Camera Syncing

Post by Micr0 »

It seems I made a mistake. There seems to be no V-sync signal in HDMI. it looks like it's digitally encoded onto the data stream. There are boxes that will break this out but this is where I wound up last time and these boxes aren't cheap.

The thought I'm now exploring is using an HDMI to VGA converter because VGA does output both V-sync and H-sync> the only problem here will be latency. A little would be OK but too much and this experiment falls apart.
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avogra
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Re: Projector and Camera Syncing

Post by avogra »

Uuuh, great idea!
Regarding latency, I remember that if you buy a desktop monitor for gaming, one of the key attributes is, that the monitor adds as little extra latency as possible. So it seems to be the norm that they do add latency :/
For the Imaging Source cameras, you can add a delay between an edge on the sync input and when the actual picture will be taken. That might work to compensate for any latency.

Some time ago, I had the idea to build a fast photodiode-amplifier that you can attach to an oscilloscope. That would be super handy to understand the timing and structure of the projection.
Further, I could imagine, that between the single frames, the projector has kind of a short dark period. One could use a photosensor to trigger the camera on that break optically so the timing of the video source doesn't matter.
Curiousjeff
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Re: Projector and Camera Syncing

Post by Curiousjeff »

I have doubts that this a sync problem.

I made some test today. I will post a little later but to make the story short, I change the frame rate on the pointGrey (global shutter) using the "absolute" feature. This gives you one unit increaments.

Anyway, within the 59-61 fps range, all scans had exactly the same artifact (waves) at the exact same place on the surface (of the calibration board).

If this was a sync problem, I would imagine that the artifact would "move" across the surface from one scan to the other since the lack of sync would make it unpredictable.

Also, strange that the artifacts are more on the panel parallel to the camera. The other panel is nearly perfect.

Jeff
Curiousjeff
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Re: Projector and Camera Syncing

Post by Curiousjeff »

Stop ! I just spent a few hours testing to try to figure out where the waves were coming from.

Now, I went to David 4 and the scan was nearly perfect. No waves at all !

I don't know what is going on, but maybe the new patterns of David 5 + new algorithm is the problem.

Anyway, to make sure of what we are trying to solve, we should post images of The problem.

I must go now, but I will post something more complete soon.

Jeff
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Micr0
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Re: Projector and Camera Syncing

Post by Micr0 »

Curiousjeff wrote:Stop ! I just spent a few hours testing to try to figure out where the waves were coming from.

Now, I went to David 4 and the scan was nearly perfect. No waves at all !

I don't know what is going on, but maybe the new patterns of David 5 + new algorithm is the problem.

Anyway, to make sure of what we are trying to solve, we should post images of The problem.

I must go now, but I will post something more complete soon.

Jeff
When I first got David 5 my scans were much worst than David 4 surface quality wise. It took a while to get it dialed in and I don't remember exactly what I did aside from solving the alignment/distortion problem associated with the optimize dual camera setting. I do agree with you that there may be something to it not being completely a sync issue. I too had noticed that sometimes the waves appear on one panel and not the other. And I likewise noticed that the distortion didn't move within a frame rate range. However, when I was changing the frame rate on my video card experimenting with frame rates. It was surprising how below 63fps I got one result, then at around 64fps not only did the surface quality of the scans get much better. but the rate at which the scan sequence was projected doubled. I haven't had enough coffee to think about this further. Stand by.
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economycar
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Re: Projector and Camera Syncing

Post by economycar »

I was able to split the hdmi signal going to the projector, convert it to vga and get a clean vsync pulse. The pulse was 59.6306hz incidentally (with the nominal projector frequency of 60hz) and the signal had a peak of ~325mV. The imaging source literature says that the pulse needs to be at least 3.3v so I will try a high speed optocoupler to switch the signal. I read that most DLP projectors have at least 30ms of lag on the low end and more realistically 50ms+. I suspect that the lag introduced by the active splitter, d/a conversion , optocoupler, etc should be less than that but I don't know. I have some of the hirose connectors in the mail and I will order some more components and see what comes of it.
Oteck
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Re: Projector and Camera Syncing

Post by Oteck »

I'm getting that too, my older david4 scans were much better than now. I know i'm feeling bitter about this
economycar
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Re: Projector and Camera Syncing

Post by economycar »

Does anyone have a David 4 License that they would be willing to sell/trade? I am going to call HP tomorrow but I am not too optimistic about what will happen. Thanks.
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Micr0
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Re: Projector and Camera Syncing

Post by Micr0 »

economycar wrote:Does anyone have a David 4 License that they would be willing to sell/trade? I am going to call HP tomorrow but I am not too optimistic about what will happen. Thanks.
Unfortunately when you update to David5 all you get is a file that updates your existing USB dongle. So unless someone wants to completely sell their seat of David.......
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