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TOPIC: Am I losing any color information?

Am I losing any color information? 3 months, 1 week ago #214865

Hi.

Let's say I have some footage shot with Slog3. But the exposure was done badly. In a waveform monitor the blacks should be quite a bit above 0, but instead it is very close to, but not below 0.

Now, if I apply a Slog3 to Rec709 LUT to correct the exposure and color, the blacks will go way below 0 in the waveform monitor. Does that mean that I have lost that information or can I after applying the LUT lift the blacks back to normal and it will be fine?

C.E
HP Workstation Z620, LW 14.5 64-bit, Win 10, CPU: 2 x Xeon (R) E5-2660. 8 Core/HT 2,20 GHz, RAM: 32 GB, Graphic: NVIDIA Quadro K2200 4 GB of GPU memory, Decklink mini monitor 4k, SSD: Samsung 850 EVO 256 GB.

Re: Am I losing any color information? 3 months, 1 week ago #214867

  • hugly
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Hello,

When applying the LUT prior to Colour Correction, clipping vanishes existing values. You can avoid this by using CC prior to the LUT in the effects chain (and adjusting the CC accordingly).
It's better to travel well than to arrive...
Last Edit: 3 months, 1 week ago by hugly.

Re: Am I losing any color information? 3 months, 1 week ago #214965

I did some testing yesterday and I think I need to rephrase my question. I assumed that when applying the LUT (in my example), the blacks would be crushed and end up below 0.

But in my test I took the Slog3 to Rec709 LUT and applied it on a Rec709 image. In the image there was a person with a dark, bluish suit with dark, hardly visible stripes on it. After applying the LUT the suit became all black as expected, but I was able to lift back the black level and retrieve the dark stripes of the suite. It had slightly different color (expected too), but it seems that the information at least is not completely vanished or lost.

So that must mean that you can use a Slog3 to Rec709 LUT to a Slog3 image even if it is underexposed. At night it must be hard to overexpose a few steps I guess?

C.E
HP Workstation Z620, LW 14.5 64-bit, Win 10, CPU: 2 x Xeon (R) E5-2660. 8 Core/HT 2,20 GHz, RAM: 32 GB, Graphic: NVIDIA Quadro K2200 4 GB of GPU memory, Decklink mini monitor 4k, SSD: Samsung 850 EVO 256 GB.

Re: Am I losing any color information? 3 months, 1 week ago #214976

  • hugly
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cadenza.editore wrote:
So that must mean that you can use a Slog3 to Rec709 LUT to a Slog3 image even if it is underexposed.

No doubt, but still I'd suggest using the CC to correct for underxposure prior to the LUT in the chain. Make some tests with both setups and you'll see the difference.
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Re: Am I losing any color information? 3 months, 1 week ago #214991

  • arniepix
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If by "losing information" you mean are you crushing all of the detail out the blacks, the easy way to check is to try lifting the blacks back to zero a after you've applied the LUT. What do you see?

You don't necessarily need to use the standard LUT, either. You can simply use the curves or lift/gamma/gain controls to stretch the black and white points to where you want them. That's often the prefered process when you've got material that's been either improperly or "artfully" exposed.

Think of the LUT as merely a starting point, but it's a generic one for what someone decided was "normal" looking footage.
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Re: Am I losing any color information? 3 months, 1 week ago #215000

  • jwrl
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Follow on from arniepix's comment about LUTs: the person who created that LUT hasn't seen your media. If your footage is exposed as expected for that camera, you have no colour balance issues and you are using a LUT designed for that camera then you will get pretty close to what you want. If not, you could actually make things harder for yourself.

Learning to use the colourgrading tools in Lightworks will give you more control over what you're doing and will allow you to make more confident and informed decisions.
Last Edit: 3 months, 1 week ago by jwrl.

Re: Am I losing any color information? 3 months, 1 week ago #215048

It was quite a while since I encountered this in practice. I got some footage for a job, but didn't know what camera had been used. The information I got was "a Sony". So I understood that it had to be a slog 2 or 3. At that time I didn't have any LUT so I made all the corrections manually. When I later got the full information about the format and found the right LUT, I compared the results and found that the colors wasn't exactly the same between my work and the LUT. I worked with curves. When you do that you can choose to do it with Y plus RGB or Y without RGB. I don't remember which method I used and if that was the cause in the difference I saw. Maybe the LUT contains some extra color correction in the mapping?

Anyway, thanks for your answers. After my tests I understand more. It is difficult to know if the dark, not so good picture, is because of high ISO grain, bad white balance, bad compression or the effect of a LUT.
But now that I have the LUT the best thing is to just experiment.

C.E
HP Workstation Z620, LW 14.5 64-bit, Win 10, CPU: 2 x Xeon (R) E5-2660. 8 Core/HT 2,20 GHz, RAM: 32 GB, Graphic: NVIDIA Quadro K2200 4 GB of GPU memory, Decklink mini monitor 4k, SSD: Samsung 850 EVO 256 GB.

Re: Am I losing any color information? 3 months, 1 week ago #215053

  • hugly
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A picture is worth a thousand words.

I've recently made a demo with BMPCC6K demo footage while testing some LUTs shipped with Resolve Studio. It shows a simple way how to roughly match shots for exposure using CC with a technical 3D LUT and adjusting only the RGB curve on CC.


Please note: As you can see, with CC prior to the LUT you can bring back highlights apparently clipped/crushed by the LUT. That wouldn't be the case with reversed effects order, crushed highlights would stay crushed. The same applies of course also to dark areas.
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Last Edit: 3 months, 1 week ago by hugly.
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