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Log grading in Premiere, avoiding intermediate clipping without messing up the colour science


UncleBobsPhotography
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Sorry if this is a basic question, but I'm fairly new to log grading and couldn't seem to find a good answer.

I'm colourgrading some clog3 by using the bt709 CanonLog3 to bt709 Wide DR 33 LUT provided by Canon.

To keep the colour science correct, the LUT should be applied before any expose adjustments/contrast adjustments since these adjustments are made for adjusting linear values and not logarithmic values. However, applying the LUT clips my highlights, and I am not able to recover them afterwards. The problem disappear if I do the expose compensation first, but even though the results looks fine in this instance, this is not the technically correct way of doing it and might give funky results in different situations. 

What is the best way to solve this? Is there some setting to ask Premiere to use a wider value range during calculations? Or is the solution as usual to switch to Davinci Resolve?

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I've attached an example. The video on the left is technically wrong, but has a decent highlight roll-off around her shoulders, the one on the right applies the LUT first, but ends up with blown highlights on her back. The clipping is obvious in the right histogram even though it's far from maxing out.

example.JPG

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Don't worry too much. It's fine to make preLUT grading in creative colour correction. Especially if we're talking about basic gamma curve adjustments - just check they don't rotate color primaries on vectroscope.

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On 8/6/2021 at 3:30 AM, UncleBobsPhotography said:

Sorry if this is a basic question, but I'm fairly new to log grading and couldn't seem to find a good answer.

I'm colourgrading some clog3 by using the bt709 CanonLog3 to bt709 Wide DR 33 LUT provided by Canon.

To keep the colour science correct, the LUT should be applied before any expose adjustments/contrast adjustments since these adjustments are made for adjusting linear values and not logarithmic values. However, applying the LUT clips my highlights, and I am not able to recover them afterwards. The problem disappear if I do the expose compensation first, but even though the results looks fine in this instance, this is not the technically correct way of doing it and might give funky results in different situations. 

What is the best way to solve this? Is there some setting to ask Premiere to use a wider value range during calculations? Or is the solution as usual to switch to Davinci Resolve?

You can put a “fast color correct” before the layer with LUT and drop the input highlight or shadows. This effectively does what you’re describing.

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8 hours ago, Mark Romero 2 said:

Does PP not have any sort of LOG exposure wheels?

Or does it have anything similar to the Color Space Transform in Resolve?

Because in Resolve, one method was to use the CST to convert to linear, use the (linear) lift gamma gain tools to adjust luminosity, convert from linear BACK TO LOG, then apply the LUT.

Those are some good questions, unfortunately my Premiere skills are not good enough to answer them. I am not aware of LOG exposure wheels in Premiere or how CST in Resolve works, but that doesn't mean Premiere hasn't got it. I am sure Premiere has some way to deal with it correctly.

Using a different colour space while editing and then converting to bt709 (if that is what I need) in the end might help. I recorded these clips in bt709 clog3 which only comes with bt709 conversion LUTs from Canon, but if I record in CinemaGamut clog3 I'll have many more options.

 

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I'm not an expert in this, but one thing i have been gleaning from pro Canon users is that the official LUTs are not very good for some reason, and that highlight rendition is a particular area of concern.  Many people are using third party LUTs for input.

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