ARRI Forum

Hi all,

I've have a question related to the logC color managment. I've read the paper "Alexa LogC Curve - Usage in VFX" and I have a question about tone mapping and Rec 709 conversion.
Acording to the paper: after aplying the LogC curve there is a tone-mapping and the final color space conversion (from Alexa wide gamut).
There are two options for output color space in the Alexa web lut_generator:

Color space - none: 1D-LUT
Color space - Rec 709: 3D-LUT

So I have two LUTs generated by the web lut_generator:

AlexaV3_K1S1_LogC2Video_EE_nuke1d.cube
AlexaV3_K1S1_LogC2Video_Rec709_EE_nuke3d.cube

The 3D LUT combines tone-mapping + color space conversion in one 3D-LUT and the 1D-LUT includes only tone-mapping (correct?).
There is a color matrix described in the paper for conversion from tone-mapped ALEXA wide gamut RGB into ITU Rec. 709 RGB (1.485007 ...).

So if I understand the paper correclty, following workflows should generate same results?

A. linear source image -> Alexa logC curve -> 3D-LUT
B. linear source image -> Alexa logC curve -> 1D-LUT -> Rec 709 color matrix conversion (1.485007 ...)

But the results are different (tested in Nuke). It looks like there is a more complicated color conversion from Alexa wide gamut to Rec 709 than the simple color matrix mutliplication provided in the paper...

Thank you for the clarification,
Martin

Hello,

The conversion steps are
Log C -> Tone Map -> Display Linearization -> RGB Matrix conversion -> Display Encoding

You can not apply a color matrix to non-linear display data. The tone map LUT you get from the LUT converter yields an image for display on a standard video monitor. Five years ago there were some systems around that could apply a 1D LUT only. The tone map LUT has a gamma correction for 2.4 built in. You have to undo this gamma correction and apply after the matrix.

Hence, when your display is a standard video monitor (ITU Rec 1886)
Log C -> Tone Map -> x^2.4 -> RGB Matrix conversion -> x^(1/2.4)

This should get you pretty close to the 3DLUT. In the camera (and in the 3DLUT) we do not use a simple power function but something that approximates the power function with a limited slope around 0. It shouldn't matter when you do your processing in 16 bits ore more.

Best

Thank you, it works.

We sometimes need an inverse LUT (texture and matte painting), and for the 3D-LUT it can't be computed.
I've tested the 1D-LUT in Nuke and it is close enough for our purposes...
Thanks again