// Written by Nick Shaw - Antler Post

One of the major new features in the latest firmware update for the Odyssey line is the addition of 3D LUT support. This brings to the Odyssey LUT functionality not found in many dedicated LUT boxes. This document gives an overview of 3D LUTs, their purpose,
and their use in the Odyssey.

What is a 3D LUT?

A 3D LUT is a way of transforming an image, where any set of red, green and blue values at the input map to a different set of red, green and blue values at the output. Unlike a 1D LUT, where a given red value at the input will always map to the same red value at the output (like Curves in Photoshop) with a 3D LUT, the output value in each channel is affected by the input values in all three channels.

The LUT is a "black box" insofar as it contains no information about what actually happens to the signal to get from the input to the output, it is just a list of output values for corresponding input combinations.

Because with a 10-bit signal, there are 1024 × 1024 × 1024 possible input combinations, it would not be practical for a 3D LUT to contain a list of the output combinations for the one billion input combinations. Therefore the LUT lists the outputs for a sub-set of the possible inputs, with the outputs for values in between these calculated by interpolation. This is often visualised as a cube, with the x, y and z axes of the cube representing the red, green and blue input values, so each point within the cube represents a particular combination of RGB values at the input.

The Odyssey allows you to apply a LUT to the image on the OLED display, and independently control whether the LUT is applied to each of the SDI outputs, the HDMI output, and the monitoring tools. This is very powerful, as it allows you to, for example, check the original log image from the camera on the waveform (allowing you to check for clipping) while seeing an image with a Rec.709 "video" LUT applied on the Odyssey screen, and simultaneously sending that Rec.709 image to other monitors on set, and the log image on to other devices for further processing.

Currently the LUT is not applied to the recorded image, but this option is planned for the future.

What are 3D LUTs used for?

There are two basic types of 3D LUT. While what they are doing to the image is the same, their purpose is different.

1. Technical LUTs

These are used to transform an image from one format to another. This transform is often from the log format of a Digital Cinema camera, into a "video" format suitable for direct display on a monitor. These LUTs are usually derived from mathematical definitions of the colour space and curve of the particular log format, and often incorporate a "tone mapping curve" or s-curve, which compresses the high dynamic range that a log image can contain, and makes it viewable on a low dynamic range display, while trying to preserve a sense of the dynamic range of the original image.

Technical LUTs may also be used to transform from one log format to a different log format. Again such a LUT would be created mathematically from the definitions of the two formats. This can allow you to apply subsequent processing to an image which was designed for a different format. Such LUTs are used in post pipelines, not for on-set monitoring, so will not be discussed further here.

2. Creative LUTs

These are used to apply a "look" to an image. Where a technical LUT is intended to create a neutral image, a creative LUT builds on this and is used to take a grade created with one image, and allows it to be applied to different images. Often this look is still based on a technical LUT, with grading transforms applied before and after the LUT, and the result combined to produce a new LUT.

It should be borne in mind that the same LUT can give dramatically different results on different images, even if they are from the same camera. Therefore it is generally recommended when creating looks for a production to use broad strokes, creating a small number of looks, perhaps one for day interiors, one for day exteriors, and so forth. Detailed grading should be done on a per shot basis, rather than trying to create a "one size fits all" LUT.

With both technical and creative LUTs, the transform will have been designed to be applied to a particular type of image. So a LUT designed for an ALEXA LogC image, for example, will not have the desired effect when applied to a Sony S-Log image. If you don’t start in the right place, you won’t end up in the right place. All the preset LUTs that are available in the Odyssey are technical LUTs, designed for the output of specific cameras. So it is important to select a suitable LUT for the camera you are using.!Some of the additional LUTs available from the Convergent Design website include exposure compensation. These allow you to open up the iris of the camera by one or two stops, deliberately over-exposing the image, and the LUT compensates for this, pulling the exposure back down to produce a normal looking image. This can be very useful where an image might be noisy in the shadows when "correctly" exposed. The over-exposure records more shadow detail, and the LUT compresses the shadows back down, taking the noise with it. It is important to note that in doing this, parts of the image which would not have been clipped at a "correct" exposure may become clipped due to the over-exposure. What you gain at the bottom end, you lose at the top. But if you are aware of this, very good results can be achieved.

LUTs in post

When a particular LUT has been used in monitoring on a shoot, the normal practice is to apply that LUT to the dailies, so that the image seen in editorial is the same as the one seen on set. The Odyssey records an XML sidecar file with each clip recorded, and the LUT active when a particular clip was recorded is stored in the <view_lut> tag. This can be used to identify what LUT should be applied to what clips.

It is likely that clips will be in groups, all of which use the same LUT, so the LUT can be applied in batches. This can be done using the free DaVinci Resolve Lite, for example. It is not the place of this document to go into detail on the use of Resolve. There are many excellent tutorials available online. This is just an overview of applying LUTs for dailies.

You need to have all the LUTs used on your project available to you in Resolve, so you should add them to Resolve’s LUT folder. You may wish to add all the Odyssey preset LUTs to Resolve, or only the ones required for your project. From the Resolve Project Settings window, opened using the cog icon at the bottom left of the screen, go to the Lookup Tables page, and click the Open LUT Folder button. Copy the .cube files you want into the folder (you may wish to organise them by sub-folder) and then back in Resolve click the Update Lists button and then Apply.

It may be convenient to create a number of timelines in Resolve, each containing a group of clips which require the same LUT. These may be all from the same scene, from the same location, or even from the same shoot day. There is no one correct way to organise your media, but organising it in a way that makes sense to you and to others who will have to work with it later is obviously important. When you have all your media assembled in timelines, you can apply the LUTs, either individually to clips, or to the entire timeline using a timeline node tree. The principle is the same. Each clip already has a single empty node applied to it in the node window at the top right of the screen on the COLOR page. If you click the drop-down at the top right of the node window, and choose Timeline, instead of Clip, you will see an empty node tree, to which you can add a node by choosing Add Serial Node, under the Nodes menu (or pressing ⌥S on a Mac).

If you use a LUT in the Timeline node tree it is applied to every clip in the timeline, downstream of any per-clip grading you do. When you add a 3D LUT to a node, it it applied downstream of other grading in that node. For a quick one-light pass for dailies, applying the appropriate LUT in a timeline node will immediately get most clips to approximately the right place, as that will be what was seen on the Odyssey and other connected monitors when shooting. You can make small per-clip adjustments if needed in the clip nodes. You can then render out dailies for editorial, with the LUT baked it, from Resolve’s DELIVER page.

Grades applied to dailies assembly timelines can be used in Resolve to apply the same grade as a start point for finishing using Resolve’s Color Trace function. A detailed description of the use of that is beyond the scope of this document.

Creating Look LUTs

There are a number of ways to build a creative LUT for use in the Odyssey. Applications such as Pomfort LiveGrade allow you to "paint" a live image from a camera, and then save the resulting grade as a LUT. The latest version of LiveGrade can save LUTs in the native Odyssey format directly, without the use of the CD3dLUTConverter, although you will probably also wish to save the LUT as a .cube file for later use. You can also create a LUT using Resolve, and what follows is a brief overview of how to do this.

It is advisable to start with a representative shot for the scene you will be applying the look to. It needs to be in the same format as the image that the Odyssey will be applying the LUT to, so the easiest thing to do is to record a clip on the Odyssey from the camera you will be using, in the format you will be recording. For example, you might record an S-Log3/S-Gamut3.Cine clip from a Sony camera as ProRes on the Odyssey.

You then bring this clip into Resolve, and perhaps apply one of the Odyssey Preset LUTs suitable for S-Log3/S-Gamut3.Cine material, to give yourself a neutral start point. You then grade your clip to create your look. Do not use any Power Windows, blurs, or other spatial effects, as these cannot be represented by a LUT, and indeed if you try to create a LUT from a grade using these kind of effects, it may well display artefacts.

When you have a look you are happy with, you can save it as a LUT by simply right-clicking (CTRL-Click on a Mac) on the thumbnail of the clip in the timeline of the COLORpage, and choosing Generate 3D LUT (CUBE) from the contextual menu. When you have saved the LUT, you can use the CD3dLUTConverter application to convert the .cube file into a .cdlut file, which can be loaded into the LUT library of an Odyssey 7Q or 7Q+ using an SSD.

Applies To

Odyssey7Q+, Odyssey7Q, Sony FS RAW Record Option, Canon RAW Record Option, ARRIRAW Record Option, POV RAW Record Option

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Created : 2016-07-14 16:37:11, Last Modified : 2016-08-08 13:28:26

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