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Expert
Posts: 180
Registered: ‎07-24-2013
Message 1 of 19 (6,583 Views)
Accepted Solution

SR 4:4:4 with Slog 2 on green screen

[ Edited ]

Any one used this combo on green screen?  Is 4:4:4 nesessary or would XAVC 4:2:2 be OK?  Any wierdness shooting SLOG2 and S GAMUT with green screen?

 

Jack F5 Sydney

Jack
F5
Sydney
digiteyes.com.au
Expert
Posts: 54
Registered: ‎11-19-2012
Message 2 of 19 (6,549 Views)

Re: SR 4:4:4 with Slog 2 on green screen

I've never tried it, but I might need to. Let us know what you decide, why, and how it turns out!

 

Cheers,

 

Brad.

 

Expert
Posts: 2,214
Registered: ‎11-23-2012
Message 3 of 19 (6,495 Views)

Re: SR 4:4:4 with Slog 2 on green screen

[ Edited ]

444 is preferable to 422 for green screen. You will get a more accurate key at 444 and the reduced compression of SStP will mean fewer artefacts to deal with.

Whether that's an issue or not depends on what you need to achieve. I would say that if quality is paramount then use SStP and 444. If dealing with large files and lots of media is a concern then you might want to use XAVC.

If you have an F55 or the R5 then the other factor to consider is whether to shoot at 4K or HD. If you shoot at 4K then you should get a slightly more precise key at 422 4K or raw than at HD 444 due to the increased luma resolution with similar chroma resolution.

 

You shouldn't really need to use log for a green screen shoot as you should have control over your lighting. Using Slog2 will mean your reserving a lot of data for bright highlights and thus reducing the amount of data available for shadows and mid tones. Really it's better not to use SLog2 and use a more conventional gamma where the bulk of the data is used for shadows and mid range and then light accordingly. I'd recommend HG 1 through 4.

If you feel you must use Slog2 then you need to follow the usual exposure guidelines of middle grey at 32% and white at 59%. Don't over expose as your skin tones go flat.

Expert
Posts: 365
Registered: ‎12-03-2012
Message 4 of 19 (6,481 Views)

Re: SR 4:4:4 with Slog 2 on green screen

[ Edited ]

I just recently shot a green screen job with my F5 and used SR 444 w/Slog. I hear what Allister is saying about considering HG 1-4 but our female talent was shirtless(back facing us) and I wanted the very best skin color I could get. Exposed middle gray at 33% as usual and all went well. Handed off the footage at the end of the day but I did a few de-logging tests when I got home using Resolve and the Sony #1 "LC-709" LUT that they released for the F55. After a 5% reduction in Saturation and a slight mid-tone exposure bump the shot looked fantastic. As far as did I "need" to shoot SR 444?, maybe maybe not but thats why I love this camera, it has it and gives me the option without having to rent a additional recorder so might as well use it. This is exactly why I bought the F5, options that will make my clients happy and my life easier.

 

*Update in regards to other green screen thread:

To clarify, I exposed middle grey at 33% for the subject and the green screen was a half stop brighter. Havent heard from the client about any problems.

Expert
Posts: 2,214
Registered: ‎11-23-2012
Message 5 of 19 (6,472 Views)

Re: SR 4:4:4 with Slog 2 on green screen

Well if your using SLog2 exposing middle grey at 32%, white should be 59%. As skin tones are not normally brighter than white then they will fall between 32 and 59%, typically around 45%. Unless you have direct light sources in your shot or other things brighter than white (typically this would be a window, spot lights, clouds or sky, practicals etc) then your wasting 40% of your available data, it's just not being used. Over exposing SLog2 to try to use the range above 59% for skin tones is bad news as your getting into the highly compressed part of the curve and throwing away large amounts of subtle texture data.

Not saying that the SLog won't look good, but it's often not optimum.

Shoot with S-Log2 and your squeezing 14 stops into your finite sized data bucket, Shoot with Rec-709 and no knee  and that same size data bucket is only recording 8 stops, almost double the amount of data per stop. The end result is that skin tones and very subtle textures are recorded more precisely with 709 than Slog2. I recommend Hypergammas 1-4 as this is a half way house where you still have greater mid range precision but still have extra headroom compared to 709. As skin tones and most natural textures fall within 709's color gamut your not normally missing out on much by not using S-gamut.

Expert
Posts: 365
Registered: ‎12-03-2012
Message 6 of 19 (6,457 Views)

Re: SR 4:4:4 with Slog 2 on green screen

Good advice, thanks Alister.

Specialist
Posts: 45
Registered: ‎01-11-2014
Message 7 of 19 (6,105 Views)

Re: SR 4:4:4 with Slog 2 on green screen

Hi folks. Assuming shooting greenscreen in non-log SR444 and one of the HG1-4 HyperGammas, it seems like exposing middle grey at 50ire, skin tones at 60ire, white with detail at 95ire makes sense. Do you guys agree that the greenscreen should read at 40ire or should it read 50ire? I appreciate your thoughts. Thanks.

Highlighted
Expert
Posts: 1,591
Registered: ‎11-19-2012
Message 8 of 19 (6,088 Views)

Re: SR 4:4:4 with Slog 2 on green screen

[ Edited ]

You can shoot green screen in either 422 and 444 but arguabley 444 is "cleaner" for keying but I'd say far more important if you chose to shoot with the slog2 gamma curve is that you rate your camera lower by a full stop so that you are forced to light your scene brighter by one stop. This gives you at least 6db improvement of your signal to noise ratio.

  

By the way this is a perfect and practical case for using the CineEI mode setting it to 640 on the F55, or 1000 on the F5. 

 

Raising your middle grey point by over exposing your green/blue screen scene will eat into your lattitude above middle grey but as mentioned you will lower the noise floor by -6db so have a much cleaner image to work with in post.

 

Regarding using HG's or any other gamma curve, what people need to remember is where that specfic gamma curve mapps middle grey because most people work with the idea of restoring it back to the original value (45-50%) in post. So in the case of green/blue screen work, whatever gamma you decide to use - over expose it by a stop to improve the quality of the image.

Specialist
Posts: 45
Registered: ‎01-11-2014
Message 9 of 19 (6,080 Views)

Re: SR 4:4:4 with Slog 2 on green screen

Thanks Dennis. BTW I do not own an F5, I'm renting one for a shoot this weekend. That's why I really appreciate your collective advice since I have no ability to really dig into using it hand's on, I've been researching all the pros and cons based on you owner/operators which is the best I can do. Anyway...

 

Dennis, I intend to NOT shoot in any slog for the greenscreen shooting. My subject are kids holding white art cards (animations go on cards in post) so my key details are lighting for skin tones and shooting to not blow out white on the cards. So my question is plotting exposures correctly. If middle grey is 50ire, I want skin tone 1 -1.5 stops brighter; my white art cards at 95. So my belief is that the greenscreen should be 1-2 stops LESS than skintone exposure not EQUAL or GREATER. This should key well and not spiil at all in foreground. That's the question I'm concerned with; definetely not over exposing greenscreen, but rather underexposing. Do you agree Dennis? 

Expert
Posts: 1,591
Registered: ‎11-19-2012
Message 10 of 19 (6,053 Views)

Re: SR 4:4:4 with Slog 2 on green screen

[ Edited ]

With reference to my previous post, over-exposing the entire "scene" to improve the noise levels means you establish the ratios you want (which you have done) then either open the iris by a full stop (shooting 1 stop hot), or from the start you light everything 1 stop hot.

 

Without using any in-camera LUTs it can be hard for you or others watching monitors to view "over-exposed" footage (although 1 stop probably won't show too badly) so you can implement the use of the CineEI function which works in conjunction with the in-camera LUT's to compensate what you are previewing. 

 

If you plan to have any live preview of the keying while shooting however you might need to consider LUTing the output of the SDI before it goes for keying. (maybe HDLink Pro box from BMD, etc..)

 

Regarding the contrast ratios between your actual screen and skin/subject I have only done a little green screen work but I can recommend that the contrast ratio between your screen and skin value would largely depend on the spill effect and trying to control it. Generally screens are lit one stop under skin/key values but again there is no hard and fast rule. Test test test - or buy someone with the experience to help you out.

 

Now here is the problem using any HG's or slog gammas without adding an MLUT to preview the output - your waveform monitor or preview monitor will be reflecting the underlaying mapping of the gamma curve you are using, so if you see 50ire on a wavform monitor, actually it is not!

 

If you were to use slog2, seeing 50ire on the WM means your are 18ire above the mapped value for middle grey. With HG7, middle grey is mapped to 40 so again if you saw 50ire on your monitor, that is in fact about a stop over middle grey. The problem is you are recording with a gamma curve and viewing with it. Should you memorize all the mappings or worse go out of your way to plot them before each and every shoot? This is what the MLUT's were designed to take care of. Put everything BACK to regular 0-100IRE scale so you can work with "standard" values when using your waveform monitor and later in post the footage can be graded back to normal levels.

 

This is precisely why I use a light meter for lighting. I don't to give two hoots about the underlying mappings as long as I know my middle grey value, my meter will tell me when I'm 1 or 2, 3, etc.. stops above it and my last check before hitting record is to check where the highlights are sitting on a waveform monitor, just as my safety.

 

 

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