ProRes 444 XC Log to Lin

[Nicolas Hanson]

I get ProRes 444 XC log material all the time and I use to balance it out and do the log to lin transform manually. Any LUT's that will set this material back to the linear as shot condition?

[Thomas Singh]

A gift from me to you  

AlexaV3_K1S1_LogC2Video_Rec709_EE_davinci3d.cube

[Marc Wielage]

Log as shot on what camera? Log is different on just about every camera, because each manufacturer kind of has their own idea of what that is. I find it useful to try out the built-in LUTs within programs like Baselight and Resolve, then find a way to reverse-engineer what they do only using curves and primaries. That way, I find you can have more control than just with a LUT. 

The stock Alexa LogC -> Rec709 can be pretty harsh, but Arri has a free service on their website so that if you wanted to, you could make custom adjustments to the toe or shoulder to get different results. Another thing that's useful is to have the DP shoot standardized charts (like the DSC Chroma DuMonde), and see how those react with your own software. Once you can get a predictable result from a chart, the rest is just customizing the look for a DP's particular technique and style. A LUT per se isn't necessary, but I have no problem using a technical LUT for something like dailies, where quick turnaround is necessary. Custom LUTs are also useful in some situations, like turning over a look to the VFX team. 

[Nicolas Hanson]

The ProRes 444 XC log material is shot on Alexa (Mini + full body) I've used the LUT creator on Alexa's site and end up with almost the same results as the built in software LUT. I just want to know if applying this LUT will put the footage back to the linear condition that was defined by the DOP on set. Put another way, is the software built in LUT applying the same transformation as the camera is doing on set?

Often, the director is asking me to set the image back to the original state (what he saw in the monitor on set ' ....yes, I know...), so will applying this LUT be the right way to approach this request?

Edited March 31, 2017 by Nicolas Hanson

[Orash Rahnema]

13 hours ago, Nicolas Hanson said:

I just want to know if applying this LUT will put the footage back to the linear condition that was defined by the DOP on set.

Sorry to jump in but there's something not clear to me on this.

You want to "normalize" your log footage, not transforming to linear.

By  normalize i mean transform from log gamma image to a display gamma image (eg. rec709)

Transform it into a linear state is something different and Davinci does it automatically under the hood so the math works or you do iso you can render it out linear (without the working gamma applied) so if someone needs it to do comp or mixing it with some other linear material it won't have trouble with it.

 

13 hours ago, Nicolas Hanson said:

Put another way, is the software built in LUT applying the same transformation as the camera is doing on set?

Davinci LogC to Rec709 isn't exactelly the same as what you get as a rec709 signal out of an Alexa mini or Amira, there are slightly difference but to me are not noticeable if not looked side by side.

So if the dop wants to see the rec709 as on set, you can safely use the davinci transform.

There is a bigger difference with the Alexa plus/XT (not sure SXT) as they use a 1D lut and not 3D.

 

[Nicolas Hanson]

8 hours ago, Orash Rahnema said:

Transform it into a linear state is something different and Davinci does it automatically under the hood so the math works or you do iso you can render it out linear

Thanks for your explanation. So by normalization you mean correcting the clips (e.g with a log-lin LUT) to set it back to the initial state. How do you transform the logarithmic clip to linear in DaVinci and what is the pro/cons compared to normalization?

[Orash Rahnema]

This are 3 logc images, the flat one is the log as you know, the "normal" one is transform with a logCtoRec709 lut and the third one, the darkest, is the linear image.

Basically the light hit the sensor and the camera see a linear image, where the light with 0 energy return 0 as a value in the sensor, light value of 0.1 return 0.1, 0.5 is 0.5, and so on until 1.

Mathematically this thing is perfect as the math it's easy, BUT, the law of light physics says that to gain a stop of light you have to double the power, so if we keep using the straight linear output from camera we end up with an image with most of the energy packed and wasted in the low range, where the noise live in the sensor.

To avoid this energy waste the camera manufacturer transform the linear values that the sensor see with a logarithmic function so the can distribute in a more efficient way the energy collected by the sensor, creating images that are flat and well distributed in exposure.

In this way we have images that holds detail and information across the whole range, but it's flat and ugly.

Rec709 (and other like p3 or rec2020 and so on) is a gamma function that tells how the display bend the signal and make it pleasing, plus remain within broadcast standards.

It's only a display gamma tho, not a camera gamma.

Rec. stands for recommendation, in fact it's not a low, the only law it used to be with the color gammut, but the actual gamma curve was decided by the association of broadcast around the world to make something that was working for everybody once we started to get worldwide broadcast signal and to get all the display similar.

So, the tv manufacturer took this recommendation and built their display using this gamma function (or close to it)

The broadcast cameras used to record using this recommendation as well simply because they were going live straight to broadcast.

 

So getting to today, we have cameras that record log to give us the best possible dynamic range and broadcast that display a "standard" gamma function.

With a lut we can make the log image look like it has been shot with a rec709 camera.

But no one is stopping us to send the log image on tv.

It really all comes down to, what i see on a rec709 monitor is what i will see on tv, so it's up to the colorist to make the image that he wants.

 

Ok, I'm done and I'm not even sure that is clear, writing about this kind of stuff not in my native language is not easy at all, so I'm sorry if you can't understand, if you need I can try to make it clearer

Orash

 

[Nicolas Hanson]

Orash, great explanation but I'm fully aware of this. What I'm curious about is how you define the difference between normalization and transforming the image from any log favour. 

[Orash Rahnema]

Sorry!

As i said previously english isn't my first language. I complittely misunderstood your previous question.

Well, hopefully will be usefull to someone else!

To me normalization really is whatever takes a log footage into a pleasing "normal" contrast. no difference if is a rec709 lut tranform or a simple contrast expansion.

[Nicolas Hanson]

4 hours ago, Nicolas Hanson said:

Transform it into a linear state is something different and Davinci does it automatically under the hood

I'm sure it is! :-) What I don't understand is that your previously wrote that transform is something different than normalization so can you please explain what a transform is if it's not "taking log footage into a pleasing "normal" contrast" one or the other way?

[Orash Rahnema]

Oh... Next time i will get someone ti read my posts before, i think we would all save a lot of trouble!

There is no difference, its a Word that i got used to use at work and It stuck with me in those explanation, i believe. 

Both are some kind of transfomation/translation or whatever you want to call It. 

When a lut is used, so where math is involved, i am used to use the Word "transform", so logtolinear or logtorec709 is the same, anotherone could be going from a 2.4 gamma to 2.2

But, normalizing an Image It doesn't have to bè precise if there is no need to. 

Its just expanding contrast and make the Image normal, but maybe without following any rules. 

At l'east these are my thoughts about it. 

What i wanted to say in the very First post is that there is a massive difference is saying make an Image linear and rec709.

Edited April 2, 2017 by Orash Rahnema