LUT for linear exr to rec709

[Matt Keith]

Hello all.

We are trying to adopt a more complete linear work flow in the 3D department of the vfx company that I work for.

The problem that we currently have is that 3D software ( in our case Maya rendering with Vray) renders out linear exr files. In order to use any of the LUT that are generated by the Arri LUT generator, (https://www.arri.com/camera/alexa/tools/lut_generator/) I need to apply a "pre" LUT to convert my linear renders into logc images that the arri LUT is expecting. Does anyone know how I can go about doing that?

Any advice greatfully recieved.

Kind regards,
Matt

[Andrew Britton]

You should be able to create a LUT from that generator that performs both operations. Have you tried these settings?

Load preset - Advanced

"Camera Data"

Gamma - Normalized sensor linear

Range - Extended

"Conversion Parameters"

Scaling - Photometric

"Image Destination"

Gamma - video

Range - legal

"LUT Parameters"

3D - Forced

Bits - 16

File Format - Nuke (makes a .cube file if I remember right which maya/VRay can use)

Sent from my iPhone

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[Matt Keith]

Thanks for the promt reply Andrew!

I'm out the office at the moment but I'll give this a go as soon as I'm back at my desk.

Matt.

[Andrew Britton]

If it doesn't work, email me and I might be able help out. 

Sent from my iPhone

[mei...@gmail.com]

Dear Andrew,

I have tried the lut generator, from normalized sensor linear to video, but it fails to get a correct outcome.

The output lut generate an over-expose image.

Is that because there is difference between "scene linear" and "normalized sensor linear"?

I have the situation here same with Matt, hope find out solution asap.

Thanks,

Meiwa

[Sean Cooper]

Hi Meiwa,

It's a bit hard to remotely debug from the info you've provided, but the general answer is that you'd want to bring your Linear source (presumably in Arri Wide Gamut primaries) into a V3LogC space. Depending on the tools at your disposal you should be able to do this in most grading applications and in Nuke's native "colorspace" node, or uses OCIO's ACES 1.0.1 config. Then use the ARRI Lut Generator, but coming from LogC source instead of Sensor Linear.

If you give a little more info I could try to shape my answer a little better.

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