LUTs, Looks And Scene Files

At a recent AbelCine seminar introducing the new ALEXA Studio camera, I gave a presentation on ARRI’s Look Files and their online LUT generator. Afterwards, several people asked me to describe the difference between a Look File and a LUT. I explained that ARRI’s Look Files were more like Scene Files. It seemed like an easy answer, but it just opened the door to more questions. With more and more cameras shooting in flat-looking Log modes or RAW, the topic of LUTs on set and their implications in post have become big points of discussion. At the same time, there are many cameras that utilize the more traditional Scene Files for control of color and contrast. Furthermore, there are cameras like the ARRI ALEXA and RED EPIC that offer their own unique types of image control. The confusion is understandable. Let me start by explaining some of the terms.


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SCENE FILES, PICTURE PROFILES AND CUSTOM LOOKS

Most traditional HD cameras shoot standard HD (Rec. 709) video, which produces video that could air on television without any postproduction color grading. We often call these types of cameras “broadcast cameras” for this reason. Professional HD cameras allow you to push beyond the limits of standard HD to get more dynamic range, but they still generally conform to the HD standards. Most of these cameras allow you some amount of adjustability over the image, including color, contrast and detail adjustment. We generally call these settings Scene Files, which is what Panasonic calls them, but they’re also called Picture Profiles (Sony EX cameras) and Custom Profiles (Canon XF cameras). These settings can offer a wide variety of adjustments to the camera, including gamma adjustments, color saturation, color matrix adjustments and detail enhancement settings. Settings are created in the camera, and generally they only work with a specific piece of hardware—a particular camera model. We call them files, and not just settings, because you’re often able to save the settings onto a memory card to share between cameras of the same model. Because these cameras all shoot traditional video, any adjustments made in-camera are “baked” into the recorded image. So what you see in-camera is what you get in post. These same adjustments can be achieved with external remote-control units, also called “paint boxes.” Because these files are camera-specific and bake their look into the recorded video, a Scene File is distinctively different from the concept of a LUT.

WHAT’S A LUT?

LUT is an acronym for lookup table, which in computer science is a data structure used to replace a computation. In other words, a lookup table is a defined set of data that’s used instead of actually doing much processing. In the video world, a lookup table does much the same thing; it’s a set of data offsets that, when applied to an image, will alter it in some way. If I have a flat Log C image and I want to get something like a standard HD Rec. 709 image, I can apply this offset, which will do that adjustment for me. I could do this by eye every time, but with a LUT it can be done quickly and accurately. LUTs come in various forms and with various intentions, which only adds to the confusion. First, let’s talk about the forms of LUTs that are out there today.

1D LUTs do the work of modifying an image, but with fairly limited adjustment possibilities. A 1D LUT is composed of three different data mappings: one for red, one for blue and one for green. Each color is treated separately, so a change in red wouldn’t affect a blue value, for example. This limits complex color correction overall, but a 1D LUT can be very useful in many situations.

3D LUTs are more complex and are based on a three-dimensional cube. This cube maps all colors together and gives much greater adjustment potential. However, where a 1D LUT usually maps an output value for every input value, a 3D LUT with the same mapping would be too complex. So a 3D LUT is created with a number of input and output points, usually 17, and any value outside of those points is interpolated.

Besides these two varieties, LUTs can be created for different purposes.

A Calibration LUT, sometimes called a Display or Monitor LUT, isn’t designed to create a look, but instead to make an image “correct.” Let’s say you’re on set and have three different monitors, and they all look a little bit different. We could create a LUT that makes each of these displays adhere to the standard HD Rec. 709 spec, or at least to match each other. This could also be done by adjusting each monitor with its own controls, but given that not all monitors are equally adjustable, using a LUT may be preferred. For digital cinema projection, a Calibration LUT is used to make a digital projection have the same look as a film projection. Software and probes are used to create these types of LUTs. So in simple terms, these Calibration LUTs are designed to make different displays match well.

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