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Chroma Keying on a DSLR?

Chroma key greenscreen DSLR

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#1 Richard Plucker

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Posted 03 March 2014 - 02:46 PM

Hello all,

I know it's very difficult to get a good key with footage coming from a DSLR due to the 4:2:0 compression, soft edges, etc. I'm currently shooting church announcements videos on a black background, but I'd like to start using a green screen if possible. Right now, I'm using a 60D with a Canon EF-S 18-135mm f/3.5-5.6 IS STM lens. I will be using Premiere Pro CC and AE CC in post. Is there any way to accomplish this using my current setup?
Thanks in advance. 


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#2 Richard Plucker

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Posted 03 March 2014 - 02:54 PM

Update: I also access to a Panasonic P2 if that would help. 


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#3 Phil Rhodes

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Posted 03 March 2014 - 02:55 PM

You might want to look into the transcoding tool 5DtoRGB, which will do a better job turning the h.264 out of the camera into ProRes with interpolated colour information. It can't put back precision that isn't there, of course, but it can make things look better within the limits of the information you have.


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#4 Richard Plucker

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Posted 03 March 2014 - 03:17 PM

Capture.PNG You might want to look into the transcoding tool 5DtoRGB, which will do a better job turning the h.264 out of the camera into ProRes with interpolated colour information. It can't put back precision that isn't there, of course, but it can make things look better within the limits of the information you have.

Thanks for the reply, Phil.
I tried using 5DtoRGB but the converted files have a lot more contrast, and I'm losing details in the shadows. I tried converting to ProRes and DNxHD.


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#5 Phil Rhodes

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Posted 03 March 2014 - 03:25 PM

Hm, that really shouldn't be the case - let me make a couple of calls...

 

P


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#6 Sean Cunningham

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Posted 03 March 2014 - 05:50 PM

It's likely the "full range" setting.  I forget now if Canon shoot full-range or broadcast range but I'd set it to broadcast and see if that does a better job.  You also need to be mindful of your color management within something like After Effects because the video is natively at a different gamma than your computer's display and in a different color space (rec709 versus sRGB).  You can ask AE to "compensate" or treat the values as-is with respect to your working color space and then you manually adjust the footage to look appropriate.


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#7 John Miguel King

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Posted 04 March 2014 - 09:04 AM

Throw some minus green in the kicker light. Maybe a quarter, not much. Even go as far as having two kickers, one on each side of the subject. It'll make the key a little bit cleaner. Plus also aim to expose the green at a similar stop to the skin tones, as that's where the codec retains more info. Definitely test until you find the right recipe.


Edited by John Miguel King, 04 March 2014 - 09:06 AM.

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#8 John Miguel King

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Posted 04 March 2014 - 09:13 AM

Forum won't let me edit, here goes my full reply:

Throw some minus green in the kicker light. Maybe a quarter, not much. Even go as far as having two kickers, one on each side of the subject. It'll make the key a little bit cleaner. Plus also aim to expose the green at a similar stop to the skin tones, as that's where the codec retains more info. Definitely test until you find the right recipe. You want to expose the green as little as possible, so the spill is reduced. But as we're talking of a codec which does not retain density equally at all luminance levels, you'll be forced to ramp up this exposure until density is just right. It won't be too far from your skintones.

Oh, and get the subject as far from the green as possible! The closer it is the more green polution you'll get. Which in this case might be quite a lot due to the need to overlight the green.
 


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#9 John Miguel King

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Posted 04 March 2014 - 09:35 AM

And one last advice: have the entire subject in focus. Get your DOF calculator out and close the aperture until you're 100% certain the entire subject is sharp.

These are all, or should be, basic principles of greenscreen work. But with the added complexity of a bad codec you gotta be twice as careful.


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#10 Adrian Sierkowski

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Posted 04 March 2014 - 11:03 AM

I also prefer to kep my green screen 1/2 to 1 stop under to help control spill.


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#11 John Miguel King

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Posted 04 March 2014 - 11:56 AM

I also prefer to kep my green screen 1/2 to 1 stop under to help control spill.

I'd say you can go even lower if you're raw/log and get away with it. I've seen it done with Arriraw at 3 - 4 stops under and the key pulled alright. I wasn't around for post so can't say fo sho.


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#12 Adrian Sierkowski

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Posted 04 March 2014 - 11:59 AM

Of course; but how often are you shooting Raw on a DSLR? The biggest issue I find with under-ing the chroma too much is the green goes from 1990s clothing and chroma key color, to oh, I might exist roughly as something else on set, and you get into doing garbage mattes. Though I constantly surprised by how easy it is to pull a key in something like After Effects. I remember when it was a big[ger] deal to do with specialized very expensive software/hardware.


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#13 John Miguel King

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Posted 04 March 2014 - 12:06 PM

Of course; but how often are you shooting Raw on a DSLR? The biggest issue I find with under-ing the chroma too much is the green goes from 1990s clothing and chroma key color, to oh, I might exist roughly as something else on set, and you get into doing garbage mattes. Though I constantly surprised by how easy it is to pull a key in something like After Effects. I remember when it was a big[ger] deal to do with specialized very expensive software/hardware.

My fault, I was theorising from the point of view of a perfect capture medium, not a DSLR. When I've done it myself (on Epic most of the times) I've played it safe and done just as you suggest, 1 stop under. But I do wonder if this is at all necessary with a true raw medium?. It'd be a matter of increasing gain and saturation in the green channel when pulling the key, which would then be solely limited by the noise being created and how this affects the key.

On the good side, finding the sweet spot on this after some testing, could result in very little spill. :D


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#14 Adrian Sierkowski

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Posted 04 March 2014 - 12:09 PM

I don't think that'd work since green woudl be mixed into many things as a component color, so you'd start effecting other parts of the image. Who knows though-- i often look at Resolve's tracking, and other motion tracking software and thing in time we'll just be able to "tell" it to remove this or that from the frame and it'll autotrack it around-- no green required.


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#15 John Miguel King

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Posted 04 March 2014 - 12:24 PM

I don't think that'd work since green woudl be mixed into many things as a component color, so you'd start effecting other parts of the image. Who knows though-- i often look at Resolve's tracking, and other motion tracking software and thing in time we'll just be able to "tell" it to remove this or that from the frame and it'll autotrack it around-- no green required.

It'd be to create the alpha layer, so in theory whatever happens to other elements in the frame is not important, as they'd remain unnafected in their layer. Now, if you are suggesting that it'd make they key harder because the green would not be clean, then it'd be a definite no go, i think.


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#16 Thomas Worth

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Posted 04 March 2014 - 06:55 PM

Thanks for the reply, Phil.
I tried using 5DtoRGB but the converted files have a lot more contrast, and I'm losing details in the shadows. I tried converting to ProRes and DNxHD.

 

5DtoRGB doesn't clip shadows/highlights if you choose Full Range. It will clip if you choose Broadcast Range with full range material (like from a Canon DSLR). That's because it assumes the maximum range is within 64-940 (16-235 in 8 bit) and scales accordingly.


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