on Set LUT's/Looks On Camera

[saavedrarolan]

Hi I'm shooting a feature where we are using an Alexa Mini as the main and an AlexaXT as the secondary cameras.

At the beginning of the project the DP sent me a reference LUT he used in his last film made by a post house, and asked me if we could use them in camera. We are working with a live grade system, but since we just have one LUT box, he wanted to get the second camera signal directly with his LUT applied on camera through the SDI output.

When he sent me the LUT, he did it in three formats: .xml, .dat and .3dl. I loaded the .xml on the AlexaXT and transform the .3dl into an .adl at Pomfort's Live Grade so I could load it into the Mini. Here appeared the first issue; they didn't look similar at all. The most surprising thing was that the Alexa XT's output looked like if there were two LUT's burned in at the same time: also by the way you had to set up the menus, it gave me the impression that the look you could apply worked on top of the Rec 709; this have any sense?

So, the rest of the questions would be, how could I translate a LUT so I could use it on camera (which software)? It's a common practice to load LUT's on camera when a LUT box it's our of the budget? How could I, for example, get a primary color grade exported on .xml so it could be compatible with the camera?

[Oliver Temmler]

Hi there,

the ALEXA XT (or Classic) and ALEXA Mini color processing are not the same. The Mini can use a 3D LUT, whereas the XT supports only a 1D Tonemapping LUT + Matrix. You can get a close match if you limit the 3D LUT to only apply an Offset on the Log C and a 1D/Master Tonemap curve.

If your XT output looks like a look applied on top of the Log C 2 Rec 709 LUT, then you probably exported it that way. The camera can only apply one look file.

Anyway, I suggest that you change the setup:
Load the 3D LUT into the ALEXA Mini and run a Log C outpu from the ALEXA XT through your LUT box, controlled by LiveGrade.
That should give you consistent results as both LiveGrade/LUT Box and Mini can use the same 3D LUT.

[saavedrarolan]

Thanks a lot for the info Oliver.

In the case of the first escenario; making a limited 3D LUT that could match more or less the Alexa XT 1D LUT, so both cameras could be monitored with an already modified output, how and where I could make my own costumed LUT. Right now, the only way I´m aware to make .xml for Alexa are the ARRI LUT GENERATOR but it does´t give much options... There´s any software that can import a third party made LUT (for example from davinci or scratch) and export or convert it to an .xml compatible with Alexa?

Let´s put another example so I can explain myself: we will be shooting a new film with an Alexa XT and an Alexa MINI. Due to location conditions we can´t afford a live grade system, so the DP wants to prepare some LUTs that work on both cameras the same way. He and the colorist made this LUT´s at a color grading suite, for example scratch. So the question would be, which steps or software I would need to be able to load this grades as LUT´s on camera?

[Jan Heugel]

Hi there,

I'll answer the second part and leave the other for Oliver.

But it's almost the same what Olli said...
In that "prepared LUT case" I'd use a LUT Box on the ALEXA XT and for ALEXA Mini integrate the LUT with the ARRI Color Tool to a *.aml. Changing LUTs in the LUT Box would need a Computer then, but you can use the 3D LUTs straight out of your grading tool.

Best,
Jan

[Oliver Temmler]

Right now, the only way I´m aware to make .xml for Alexa are the ARRI LUT GENERATOR but it does´t give much options... There´s any software that can import a third party made LUT (for example from davinci or scratch) and export or convert it to an .xml compatible with Alexa?

The LUT Generator allows you to produce 1D or 3D LUTs to go from Log C to e.g. Video and optionally include an ALEXA xlm Look file.
The ARRI Look Creator https://www.arri.com/camera/alexa/tools ... k_creator/ is used to generate the ALEXA Classic/XT look files. But I think you were referring to that.

ALEXA Classic and XT use a combination of
- Printer Light color correction, applied to the Log C image,
- Saturation adjustment, applied during the color space (matrix) conversion
- and Lift/Gamma/Gain, applied to the Video image (after the Log C to Rec 709 Video conversion).
I think the only tools that can create Looks for the ALEXA color processing are ColorFront Express Dailies (and OnSet) and Pomfort LiveGrade.

You can try to approximate a LUT using other tools, but it’s rather difficult.

[saavedrarolan]

Thanks a lot Oliver! That piece of info was very useful.