LogC Gamma and Look-Up Tables

The sensor always outputs a 14-stop, wide-gamut data set, the digital equivalent of a film negative. The camera cannot record this raw data, however (that’s the one big difference to the Alexa) – some form of processing has to take place in camera, and how much is done is up to you, the cinematographer.
First, the data is debayered, transforming the raw data into a LogC image. This image can be recorded and handed over to editorial for further processing. But the LogC image is very malleable and can be interpreted in many different ways. How to communicate your intentions when you created the image?
A Look-Up Table (LUT) is a matrix of transformations that turn a LogC image into the picture you intended by defining the levels of contrast and colour that need to be applied. It can be used in various ways. By applying your LUT of choice to the viewfinder image and/or the monitor output you can accurately judge whether your lighting, exposure and white balance choices result in the desired effect. If you are not achieving the desired results, you can either
1) alter the way you are shooting the scene, or
2) alter the way the camera renders it, or
3) decide to shoot it “as is” and make alterations in colour correction, ie. “fix it in post”.

Method 1 should always be the first approach. Once you have decided on a look or number of looks for the project at hand, it is best to remain consistent in the way you expose the image. Use the tools provided by viewfinder (zebra, false colour) and monitor (waveform, vector scope) to accurately set the exposure, and the camera controls (iris, ND-filters, ISO level, shutter speed, white balance) as well as lighting, blocking, framing and filters to create the desired image. The less manipulation an image requires to achieve the desired effect, the better.

Method 2 should be used to define a project’s look or looks, but not as a tool for scene-by-scene correction. Looks can be used to define how the camera renders different situations such as day and night, or to denote different story-telling elements such as flashbacks or dream sequences. Following tests and approval of the looks by the director and producer, LUTs can be generated in camera or by using Arris AmiraColorTool software and quickly applied when shooting.

Method 3 can be extremely useful, as colour correction allows the tweaking of decisions made on set in a controlled environment and also offers tools such as power windows or in-scene transitions which are not available in-camera. But as it can never be guaranteed that the cinematographer is present during colour correction, this tool should only be employed when it is not possible or practicable to achieve the same result in-camera.

When recording a LogC image, the LUT applied for viewing is also embedded in the image as metadata. This ensures that whenever the footage is viewed further down the line, it is seen with the same LUT used for viewing the image on set and in camera and faithfully reproduces the cinematographer’s intentions. Overlaying a LogC image with a LUT is a non-destructive process and offers maximum flexibility not only to correct but also to change the image in post. This is a great advantage, as viewing conditions are not always ideal, or when running & gunning we simply don’t have time to make all the necessary adjustments, but it also offers anyone who has access to the colour correction desk the possibility to change and re-envisage our shots. In an ideal world, we would always be invited to be present during colour correction, or given the opportunity to approve changes made, but in many cases this doesn’t happen.

It is also possible to record the image with LUT applied – in this case, the look is baked in and the options in colour correction are greatly reduced. Details lost in over-exposed highlights, for example, which could be retrieved from a LogC image are irretrievably lost if the LUT is baked in. Theoretically, one could use this method to protect one’s aesthetic intentions, but as by so doing one sacrifices so many possibilities to perfect the image, I would seldom recommend this method.

One objection to using Log gamma I have heard before is that on a documentary the budget is not available for the full-blown colour correction necessary to turn flat, dark, technical images into broadcastable pictures. And that the director cannot edit properly with this type of footage. Neither of these objections hold water any more as long as your post house is using the latest version of their software of choice. Avid 8.1 and FCPX 10.1.3 (I have not tested Premiere yet) automatically apply the LUT selected while shooting and embedded in the metadata to each clip. This process is not like a colour effect that needs to be applied individually and slows performance down, it runs in the background and (by all accounts, I’ve not performed any tests) does not affect how the system runs. The LUT can also be disabled if one needs to evaluate the LogC image.
By using this feature it is possible to go through the entire editing process including playout without once noticing that the footage was not shot with Rec709 Gamma. So the only reason to actually record your footage with a Rec709 LUT burnt in is if your producer insists on using an older Avid version!

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