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Silicon Imaging SI-2K
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Old December 4th, 2010, 09:42 AM   #1
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Si2k and greenscreen

Alex, Rohan, any other SI2K operators, any clues for greenscreening with the SI2K would be appreciated rather urgently. We have a shoot on tomorrow, only managed to get the screen rigged and de-wrinkled and lit today.

We have a screen adequately and evenly lit, a Canon 7D captures a briliant green, the SI2K's green is quite dull, almost undersaturated. We can get a "fair" key but hair separation eludes us.

For a dumb idiot like me, where on the false-colour exposure display would you set the green? We have it in the gray presently. Should we bring it up into the yellow?

Our first tests with daylight corrected lighting we so-so, not as clean a separation and a bit of noise in the background plate which when it is taken out then appears in the main image.

We put green gel on the lights and that improved things but there is still something not quite right. The green is still quite undersaturated.

Any assistance greatly appreciated.
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Old December 4th, 2010, 02:08 PM   #2
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Hi Bob,

Have done a bit of greenscreening without issue (except the first one which was shot the day after I first received the Mini). In fact one of the production houses said it was the easiest keys they ever pulled.
What program are checking it on?

It sounds like you are doing the right things with an evenly lit screen - I would bump the exposure up, it does sound like you are a bit under if you are getting noise.
Keep doing those black balances.

Make sure your subject is separated from the background so there is no spill, especially if you have green gels on the lights. Of no help to you right now but I have been lighting the background with KinoFlo green tubes - they are brilliant.

What Look are you using? I've done most of mine with the default - yes the colours don't 'pop' like the over saturated DSLR but it's clean.
The in camera keyer is great for checking composition - it's not a 'perfect' keyer but it gives you a really good indication of whether your foreground is separated from your background or not.

If your background & subject are well lit there should be no problem pulling a great key.
Give me a call if you need to (although without seeing the set up it's a bit hard to offer specific advice)
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Old December 4th, 2010, 04:22 PM   #3
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Rohan.


Thanks for your prompt reply.


Yes. I am using the default look and black balance is something I find I should do often. We did do the tests with a camera black-balanced at startup but I did observe in the keyiing a little of the horizontal banding in the noise, so we had probaly gone about 20 minutes into the test without re-doing it.
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Old December 4th, 2010, 06:50 PM   #4
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Black Balance

It is imprtant to not do a black balance immediately on camera head power up. The camera should be powered on for a few minutes to allow it to reach a steady state temperature. If the temp is not varying, such as indoor conditions, there shold be no need to rebalance again.
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Old December 4th, 2010, 11:46 PM   #5
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Bob, sorry - didn't see your message in real time.

SI2K produces one of the cleanest images among the video cams, so you really should not be getting any noise issues.

What you see is probably more like blotches of the matte from the uneven lighting after keying.

I personally never light with green gels/tubes because the green spill is hard to fight the way it is.

Instead, I use fluos in softboxes or bounced off umbrellas. They usually create a very soft lighting that is ideal for greenscreen.

For the greenscreen itself, I do not like using fabric - it is always too wrinkly. Rather, I'd recommend using paper roll greenscreen - gives me great results.

Check B&H for Savage brand. Since you are down under, you probably have your local supplier that sells an equivalent product.

Try not to put your subject any closer than 4ft to the greenscreen, or spill will haunt you in post.

Green screen should be lit 1-2 stops darker than the foreground action/objects.

As for pulling the key. Use After Effects with Keylight and Key Correct.

Do NOT expect to pull clean matte by just using one key on everything - this is usually impossible. Instead, you'll have to treat different areas of the image with different key settings.

Hair is of course hard to process. Anything fuzzy produces a conundrum for the keyer.

However it can be done - look at my site for the Topher's music video Hero. The Japanese girl's long hair is flying in the wind with the office window panel in the background. That was pulled from a greenscreen matte - there was no office, just a greenscreen for everyone - and Topher and the girl were never filmed together - although you can see them composed in the same frame in some shots.

Good luck.

Last edited by Alex Raskin; December 5th, 2010 at 11:10 AM.
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Old December 5th, 2010, 03:33 AM   #6
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Thanks Alex and Rohan for your advice.


We went ahead and shot anyway and brightened up the greenscreen a bit. Here's what we now have. We also used a wonderful Canon HDTV lens which gave us just a bit more than the Angenieux T2.2. Gavan shot with it outside of the range it vignettes.
Attached Thumbnails
Si2k and greenscreen-bail-me-car-scene.jpg   Si2k and greenscreen-bail-me-car-scene2.jpg  


Last edited by Bob Hart; December 5th, 2010 at 03:35 AM. Reason: added images
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Old December 5th, 2010, 11:08 AM   #7
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Photos show that greenscreen, unfortunately, was not lit evenly as it should. The 2nd pic's back window will be rather difficult to pull matte on.

I do not see hair as problem in your example, unless it was flailing in the wind. Yes, keying will probably kill a couple of strands but you can make it look natural if it is uniform throughout the shot (not like hair is appearing/disappearing.)

Also. Seems like consensus is that the greenscreen should be lit about 1-2 stops darker than the foreground. I'm not sure that was the case on the set in question.

Also. What's with the very obvious banding on green, and with the severe aliasing around the kid driver's nose in 1st shot etc. ? This doesn't look like SI2K... Did he use Cineform RAW on FS1 or FS2, or on Adaptive setting??
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Old December 5th, 2010, 11:43 PM   #8
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Hello again Alex.


Australian rules - kid is passenger, right-hand drives over here.

I got caught out badly with the rear side windows on the Toyota Echo. They have about a half-stop neutral density tint in the glass and I did not see it. This was one problem area in an earlier test.

The banding ? I did not see any on Premiere but it is likely there to be seen on a larger display. The time between set blacks drew out a bit and the cam temp went up to 46 in the hot shed we shot in. This camera ( number 178 ) seems to be a bit more prone to this than the other newer camera over here.

The aliasing. That ambushed me a bit. I think it might be related to using the Canon HDTV B4-mount lens, which fits up to the camera via a special IMS mount. The mount contains a big piece of optical glass which mimics the effect of a prism block. There might be CA there.

The lens also seemed to be soft when opened up, zoomed in to about 132mm and looking through two 4x4 filters, Tiffen ND9 and Schneider Tru-cut.

Camera settings were 2048 x 1152 @ 25P, daylight preset, adaptive setting.

This is my first adventure into chroma-keying.

Last edited by Bob Hart; December 5th, 2010 at 11:45 PM. Reason: error
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Old December 6th, 2010, 12:20 AM   #9
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What was your gain setting? I always shoot at -3dB gain to minimize noise.

CFHD RAW compression setting: Adaptive - why? This can give you inferior results. Why not force it to Quality3, which equals to Cineform Filmscan1. If your system cannot handle it, then it is not suited for production. Adaptive is just not good enough for guaranteed quality greenscreen work...

Driver: put him back in the left-hand seat where he should be. This critically affects the matte quality....OK, messing with you here :)

But hey, just relax and use what you got, you'll be fine with multiple keys and garbage mattes.
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Old December 6th, 2010, 08:15 AM   #10
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Alex.


Normally I shoot at -3db gain. On this one because of our limited power budget for lighting, the camera was on 0db gain.

Adaptive? I admit I have not tried the other settings in a good while. The cameras have been used a few times for long takes like a music event. I should test again as I now have a SSD in a dataport.

Driver in the left seat? Fix that soon enough by flipping the image. We really enjoy the benefits of tech these days.

We have the traditional takes, in this case because of a third person in the car, a medium wide, crowded two-shots, close-ups and reverses. The shots with rear side window won't be dominent.

The recorder in use is the P+S camera body.


Ari.

Thanks for your input. Yes we let the camera warm up for about 15 minutes whilst we set lights. In hot conditions we shoot in here, there can be a variation in the camera temp if the takes are a bit long. To be on the safe side, I black balance if the temp changes up and down more than another ten degrees after warm-up.

The DOP, whose history is in film, distinctly likes the images from the SI2K for their dynamic range.

Last edited by Bob Hart; December 6th, 2010 at 08:15 AM. Reason: error
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Old December 7th, 2010, 10:26 AM   #11
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Alex, Bob

Interesting that you both shoot at -3Db when you can, does this significantly reduce noise or is it to reduce depth of field? I've never really tested it as I was under the impression that it increased the noise slightly ( I am not sure where I heard that...) as it was a movement away from the camera's native ISO of 160. Am I missing something here?
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Old December 7th, 2010, 08:12 PM   #12
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Bob,

What's the time of day and setting for the shot?
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Old December 7th, 2010, 08:39 PM   #13
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Quote:
Originally Posted by Mark Deeble View Post
Alex, Bob

Interesting that you both shoot at -3Db when you can, does this significantly reduce noise or is it to reduce depth of field? I've never really tested it as I was under the impression that it increased the noise slightly
If one can win "slight" noise reduction by lowering gain, one should do it :)

DOF cannot be affected by that - electronic gain in signal processing chain is not the same as lens aperture.

...or did you mean that we are forcing the lens aperture opening this way? Nah, this is just to get a cleaner (less noise) image.
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Old December 8th, 2010, 12:39 AM   #14
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What I meant is that employing -3DB, effectively reduces the sensitivity of the camera, which allows you to reduce the t-stop, so the aperture is wider open, hence less depth of focus (?)
If you gain ( maybe that should be 'achieve') less noise at -3DB is there a payoff anywhere else in the image? - sorry if this is elementary stuff, but my background is in film.
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Old December 8th, 2010, 07:18 AM   #15
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Mark.



I would like to claim to be a wise old man but truthfully, with -3db, I am slavishly following common doctrine relating to more conventional video cameras with a view to minimising any sensor noise.

I have been accustomed to slower stocks or tungsten balanced 320ASA with dalyight filter, so I guess it is me trying to remain close to what I know. I have also been playing with 35mm groundglass adaptors on video, which also perform closer to slower stocks light-wise.

The SI2K does not have any inbuilt ND filters. There are circumstances where one wants to be lighter weight and the mattebox and filter set stays off. In that event -3db is desirable to be as little into the diffraction zone of the lens as possible when having to close the aperture.

This is speculative on my part but -3db may also provide a hedge against sensor noise if one forgets to black balance after a significant camera temp rise.



Peter.


The setting is morning at about 0830 Perth summer time. The juvenile offenders are headed for their hearings in Children's Court after having been on supervised bail.

To be consistent with the lighting, it is intended to shoot the background plates from a vehicle travelling on northbound streets and freeways in the early morning.

There is also a bit of a stylised mood thing going on with the lighting and colour grade when it happens.

I cannot claim any credit for the lighting scheme. I know only just enought to get myself in strife so have had serious help on this one.


Key was 1.2K HMI, gelled half CTO I think.
Rim lights were 650watt tungsten fresnels.
Greenscreen were 3 x redheads with green gels.

Available power was a bit limited - 240v single phase supply to house only.



Alex.


Regards the wrinkles in the 20' x 10' greenscreen, DOP Gavan was onto a good trick. He has one of those Danos Direct "wait there's more" little steam cleaner gadgets with its own small vacuum cleaner head on a hose. I had my doubts but they were unfounded. After the fabric screen was hung and pulled to mild tension, we ran the stream cleaner over it and the wrinkles straightened out. I thought the creases would remain however they pretty much came out as well after the fabric dried off.

Last edited by Bob Hart; December 8th, 2010 at 07:34 AM. Reason: error
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