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Tuesday
Jun142011

What the LUT?

Since we're all now working in a linear workflow with RAW data, (right?) you might start hearing more about something called a LUT.  

A LUT is a color profile that can be applied to footage to see it as it should appear in your final result.  Think of a LUT as a "lens" which you look at your work through.  LUT stands for Look Up Table. There are 2 types of LUTS.  1D LUTs and 3D LUTs.  Since we don't want this color space baked into the file, we use a LUT to see the image as we will deliver it.  Therefore, It's important for 3d rendering and compositing to see the final look of the footage as it is intended to be seen, with the LUT on top. 

A 1D LUT can be thought of like the "curves" tool in photoshop.  It can affect the brightness and contrast, but doesn't really shift colors around. It bends the luminance information of an image.  sRGB or Gamma 2.2 is a 1D LUT.   rec709 is another 1D LUT and is very similar to sRGB but the blacks are lifted slightly. Cineon files also use 1D LUT to get the logorithmic data to something viewable. 

Now for the 3D lut.  Imagine you have an image, and you plot the colors of that image in 3D space using Red for Z, Green for Y and Blue for Z. Now, imagine that you apply corrections to shift those 3d points in space, affecting red green and blue differently.  Now, if you took the original colors, and compared it to the shifted colors, and got the difference, that would be a 3D LUT. Now, of course it doesn't capture every color since that would be impossible.  instead, it captures a "look up table" of those values so it can be applied to any other image and give similar results. The data in between is interpolated.

 

3d max can load a 1D lut into the viewer so that the renderer and material editor can be viewed properly.  This file format is a .lut file and is an Autodesk file format that comes from their hi end finishing systems like Smoke and Flame.  Vray can load a 3D lut into it's VFB (Virtual frame buffer) and it supports .cube files. Bring up the color correction controls by pressing the left most button in the Vray VFB.  From there you can load a .cube file.  Make sure to turn off the sRGB button and turn on the LUT button to see the correct results.

 LUT Examples

Here are a few examples of LUTs and what each does to the image.  The left is from Nuke and the right is from Vray- just to show how the results are the same.

RAW R3D

First lets start with a RAW linear image to remind everyone what it looks like before any adjustment. This image is from the RED camera in it's original R3D format. 

 

 

 

sRGB

This image has the typical sRGB 2.2 gamma applied.

 

 

 

 

AlexaV3_EI0800_LogC2Video_Rec709_LL

This LUT can be downloaded from the Arri Digital web site and is designed for use with the Arri Alexa camera.

 http://www.arridigital.com/technical/luts

 

 

HD Vid to Print

This LUT i downloaded from the Light Illusion web site.

 http://www.lightillusion.com/freeluts.htm

 

 

 

Log to Print

Another LUT I grabbed from the Light Illusion web site.

 http://www.lightillusion.com/freeluts.htm

 

 

 

 The hard thing about some of these is knowing what the input color space was when it was made.  The last one from Light Illusions might not be using the proper input color space since the RED camera  doesn't give you a cineon source.  I changed the input source in Nuke until I got something I liked.

When I write a blog, I usually know a lot about the topic.  This topic is different.  Since I started reasearching LUTs I've gained a lot of knowledge, but I still have more to learn.  First of all, finding LUTs is pretty hard.  There is also a lot of LUT formats out there.  Many of them don't work in 3dsmax or Nuke even though the file extension is supported.  I feel like Im writing this pre-maturely, but I've learned enough to pass on a working knowledge of LUTs and that what this blog is here for.

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Reader Comments (1)

Hi, thanx for the article, I am having troubles with saving my renders with LUT corrections, max saves image without LUT correction applied, as a result i get raw with all the other corrections V ray frame buffer does but not the LUT... Wats wrong??? Really need advise)))

April 21, 2014 | Unregistered CommenterNariman

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