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Question about LUTs

Lut rec709 grading

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#1 Kendrick Gray

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Posted 05 August 2016 - 04:50 AM

My plan is to shoot in Slog3 for my short film. I'd like to use a creative lut to help me grade it but I've gotten confused. I know about rec709 being the broadcast standard, but what about for the web? Websites like Vimeo and Youtube? What happens when you apply a creative lut and leave it as it. Will what ever you are displaying it on correct it? Maybe I'm going about this the wrong way because I don't understand it at all lol.  I never had to deal with this considering this is my first time using Slog.

 

Apply a rec709 LUT first and grade manually vs adding a creative LUT and correcting it hmmm.


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#2 Phil Rhodes

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Posted 05 August 2016 - 05:38 AM

Theoretically, computer displays (and things like tablets and cellphones) use a separate standard usually referred to as sRGB, as opposed to 709. sRGB actually uses the same red, green and blue colour primaries as 709, so the colour performance is identical in terms of the deepest available shades. The luminance encoding is slightly different, so that if you grade something on a 709 display and then put it out on the web, the darker shadowy parts of the picture will seem a bit over-bright compared to how they looked on TV. The opposite situation quite often comes up when people grade on computer monitors then view it on a 709 display such as a TV, where the blacks suddenly look crushed. It's not a massively severe problem and people fail to notice it all the time, but you can fix it.

 

The problem occurs when people view YouTube material on a TV. Whether the YouTube servers notice what they're displaying on, or whether TV's YouTube app notices corrects the images is hard to determine. This is something that big studios hit all the time when they release trailers on YouTube and I've heard complaints about it.

 

As to LUTs. I presume your workflow will go somewhat like this:

 

- Record SLog3 in camera. Also consider which colour gamut you'll use. Often Sony cameras say something like "Slog3/SGamut3.Cine", and you need to know both.

 

- On set, monitor either using the camera's built in LUT capability, or using a display with LUT capability. You can use either a straight conversion to 709, assuming your display is 709, or something that's creative that you made in Resolve or whatever, but either way it's likely to output an image intended to be displayed on a 709 monitor. You can generally think of a creative LUT as converting whatever you're shooting to 709, via some creative decisions you made.

 

- In post, apply that same LUT in your grading app or in the monitor you're using while grading. If you did a really good job shooting it, the tweaks will be minor. If you had an off day, were rushed, or whatever, you may need to do more work, but the point is that you're actually working on the original untouched SLog3 material, so you have maximum flexibility.

 

There are a lot of side issues and complexities with all of this, so treat this as a starting point for discussion. What gear are you thinking of using, in terms of camera, monitoring and grading?

 

P


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#3 Kendrick Gray

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Posted 05 August 2016 - 06:34 PM

First thanks for replaying because I can never get anybody to explain this correctly to me. I just bought a Sony FS5 and I'll be using the Odyssey 7Q+ to record straight to 4K prores 422 HQ. The thing is this, I keep hearing rec709 this and that. "Make sure your videos are in rec709". But when I ask if I uploaded my video to youtube or vimeo not in rec709 how will it look... they'll say it will look like you edited it...

 

If this is true why do I keep getting told to make it rec709. The short film will only be sent off to film festivals. Then after that Youtube and Vimeo. This whole thing is confusing to me because I'm missing the basics I guess. For example...

 

I'll see a video in Rec709, then I'll see a HDR video saying this is beyond Rec709. So I ask myself, how can I see this HDR video on a REC709 display lol. What am I missing??? 


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#4 Phil Rhodes

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Posted 05 August 2016 - 06:49 PM

how can I see this HDR video on a REC709 display lol. What am I missing???

 

You're not missing anything. You can't see HDR video on a Rec. 709 display.

 

The differences between sRGB and Rec. 709 are... well, I hesitate to say slight, but a lot of people don't notice. The inaccuracies between various monitors are probably larger.

 

You have a lot of capability in the Odyssey. You will presumably be using it to record the material and you can have it output a LUT that you designed in Resolve, while still recording the untouched SLog3. Is that what you had in mind?

 

Clearly you need to be careful it isn't recording the picture with the LUT burned in.

 

P


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#5 Kendrick Gray

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Posted 05 August 2016 - 11:43 PM

Oh yeah of course not. Im just afraid some of the luts I have aren't REC709. I was looking at "James Miller Deluts". Am I looking to deep into this? Is Rec709 simply just darkening the black and brightening the whites lol. If thats the case -_-


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#6 Robin R Probyn

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Posted 06 August 2016 - 04:55 AM

Maybe you are worrying too much .. Rec 709 is the  recommended (the the Rec bit) display levels for TV,s world wide.. its very outdated now and only allows for about 7 stop dynamic range.. camera,s can now capture alot more ..about 14 stops in log mode..

But at the end of the day you will have to get your image back to within those levels.. by grading.. the thing is you can start off with alot more info in your log footage..esp in highlight area,s.. then you just "massage" it all back down to REC 709.. thats it really.. nothing mystical or to be worried about.. no one wants to look at the Slog image,esp in the VF.. as its very washed out so apply just a straight 709 LUT.. or one you have made/bought yourself.. personally I use the built in 709 LUT in the VF and ext monitor for the dir.. easy..  as phil says .. just be very careful you are not recording the LUT onto your slog ! and that you are not recording any VF/monitor info from the camera SDI onto your rushes on the ext recorder.. eg frame marker lines.. turn off all display for what every video out port you are using to go to the ext recorder.. 


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#7 Phil Rhodes

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Posted 06 August 2016 - 05:11 AM

Is Rec709 simply just darkening the black and brightening the whites lol

 

Kinda. It's doing that in a very specific way. The idea is that if you plug your device into a 709 display (which most of them are by default, unless they're computer monitors) it is designed to look reasonable and not screw anything up. 709 in that situation is really just the manufacturer saying "this is what we consider normal to be when you display it on a 709 monitor".

 

You will also find some monitors that support things like DCI P3 (used for digital cinema), Rec. 2020, and others. To get a theoretically correct result you'd then need (for instance) an SLog3/Sgamut3 to Rec. 2020 LUT, which probably exists.

 

There's nothing stopping you coming up with a LUT that makes it look like whatever you happen to like. Try it. The only issue is that if you come up with a LUT that looks like it does what you want, you should shoot a lot of tests with it, including strange situations such as heavy overexposure and underexposure, bright colours, and other stuff. It's easy to come up with a LUT that looks like it does what you want but which does strange things on certain kinds of image.

 

P


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#8 Robin R Probyn

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Posted 06 August 2016 - 05:22 AM

Im not sure if the Fs5 has it.. but Slog3/SGamut3.Cine is probably going to be the easiest to grade than just Slog3 setting, without the .Cine.. (hope that makes sense!)  its not a huge gamut but still bigger than P3.. and the primaries all line up with the  Rec 709.."triangle"


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#9 Kendrick Gray

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Posted 06 August 2016 - 07:47 AM

The FS5 does have SGamut3.cine fortunately :D. It pretty much has everything the fs7 has picture profile wise.  Thanks for the advice guys because I was really beginning to think I wasted my money on Luts. 


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#10 Patrick Messer

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Posted 10 August 2016 - 07:54 AM

An amazing and free LUT-based tool to change between different Log Formats and Rec709 is the LUTcalc by Ben Turley. https://cameramanben...Calc/index.html


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#11 Kendrick Gray

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Posted 12 August 2016 - 11:30 PM

Thanks!


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