An Introduction to LUTs

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In today’s digital cinema workflow, you’d be hard-pressed to avoid exposure to Look Up Tables. Whether you’re a director, DP, editor or colorist, Look Up Tables—or LUTs—are something you need to be aware of, because they play a prominent role in many on-set, editorial, and finishing workflows. LUTs have many uses, from calibrating monitors or giving your footage the look of a particular film stock, but can also be a source of confusion for people. What exactly is a LUT? How does it work? Why should I care? If you’ve ever asked yourself any of these questions than I urge you to read on, as I attempt to clear up some of the confusion.

What is a LUT?

It has become increasingly more commonplace, even at the prosumer level, for cameras to offer log gamma curves and flat picture profiles to preserve the maximum amount of dynamic range (please see our Understanding Log-Format Recording article for an in-depth look at log formats). Flat images, however, aren’t all that fun to work with on set, especially for directors, DPs, and producers who want to have a better sense of what the final product is going to look like. This is where Look Up Tables come into play.

A Look Up Table (LUT) is essentially nothing more than a set of mathematical instructions used to transform one set of RGB pixel data into another. These sets of instructions can be used in several ways, including mapping a source signal to a different color space, emulating a particular film stock, or adding a creative look to your footage, and does so in a way that is precise and repeatable. It is, after all, only numbers—give the LUT the same set of input data and the output will, in theory, always be the same. Could you make these same adjustments manually in your color-grading software? Absolutely, but the process won’t be nearly as quick, easy, or offer such precise, repeatable accuracy.

1D versus 3D LUT

When working with LUTs, you’ll quickly notice that there are 1D LUTs and 3D LUTs, with the key difference being the complexity of the adjustments you’re able to make. Let me try to explain without getting too technical (for your sake and mine). A 1D LUT handles each color (red, blue, and green) separately, with matrices mapping every possible output value for a given input value. For making basic color balance and contrast adjustments, this is usually sufficient. However, by treating each color separately, a 1D LUT lacks the complexity needed for serious grading work. For this, we must turn to 3D LUTs.

A 3D LUT is much more complex than a 1D LUT, mapping all color values together in a three-dimensional cube rather than using matrix combinations. Since having a specific output value for every input value would make it much too large to be useful, a 3D LUT instead uses input and output points for each color axis, with colors between these points interpolated by the system; the greater the number of points, the greater the accuracy. The most common number of points for each color axis is 17, producing what is called a 17 x 17 x 17 (17^3) 3D LUT cube. It is important to note that because there is interpolation involved, the same 3D LUT file may give you slightly different results between different software, but this isn’t something you should be overly concerned about.

LUT Types and Uses

Now that we have a general understanding of what a LUT is, let’s take a look at the different types of LUTs—or rather, the different reasons we create and use them. I tend to divide LUTs into three general categories: Calibration LUTs, Display LUTs, and Creative LUTs.

Calibration LUT

A Calibration LUT is used to correct inaccuracies in displays and ensure that a reference monitor or projector adheres to a standard color space, such as Rec. 709. By creating an appropriate Calibration LUT for each monitor, you ensure that they are displaying your images in a consistent and accurate way, so everyone on set or in the grading suite is seeing the same image.

Display LUT

While a Calibration LUT is focused on calibrating the display of the monitor, a Display LUT is used to convert from one color space to another, and is essential for on-set monitoring of log footage. Rather than viewing flat, desaturated log images on set, the Display LUT maps the log footage to the color space of the monitor, letting you view images with contrast and saturation resembling that of the final product. Some monitors and cameras even have built-in Display LUTs, with others offering the ability to load your own 3D LUTs. For monitors that don’t support LUTs, you can add a LUT in an incoming source signal using an external device like AJA’s LUT-box.

An example Display LUT would be a Sony S-Log2 to Rec. 709 LUT. This LUT could be general in nature—that is, being used with any camera that supports S-Log2 gamma—or specific to a certain camera, such as the Sony FS7. When possible, it’s best to use a camera-specific LUT, as each camera has its own nuances in color, contrast, and dynamic range. Applying an S-Log2 LUT designed for the FS7 to footage shot on an a7S II, for example, might yield some undesirable results.

Creative LUT

A Creative LUT goes beyond normalizing the footage to a specific color space by also making creative adjustments. These LUTs are great for mimicking the look of a particular film stock, popular effect, or other stylized look. It’s not uncommon for a director, DP, and colorist to work together to create custom Creative LUTs before production begins that can be used on set to monitor what the final images will look like more accurately. This LUT can also be applied to dailies and then brought back into the mix during post production to serve as a starting point for the final grade. Perhaps even more so than Display LUTs, creative LUTs should be camera-specific.

Emulation LUT

Okay, I know I said I divide LUTs into three main categories, but I need to give special mention to Emulation LUTs, which are used when you’re going to be printing to film for distribution. By apply the LUT in combination with a Display LUT, it will allow you to see how the film will look once printed to the film stock you have. During the color-grading process, you remove the Emulation LUT prior to exporting and printing.

LUT Creation and Acquisition

Display and Creative LUTs can be created using supporting editing and color-grading software or from a dedicated third-party software and plug-ins. DaVinci Resolve, for instance, lets you save your color adjustments as a 1D or 3D LUT file. If you’re not a seasoned colorist and creating a custom LUT just isn’t your thing, you can often find downloadable Display LUTs on camera manufacturers’ websites, or purchase a pre-built package of camera-specific Creative LUTs from third-party companies. I have personal experience with VisionColor’s ImpulZ LUTs, which offer a wide assortment of 35mm film emulations that give my S-Log2 footage an instant filmic look.

If you’re fortunate enough to have a DIT on set running real-time LUT creation software, you can even create LUTs live, on set. This lets directors and DPs explore grading ideas live, on set, and export them for use in dailies creation and final grading.

LUT Limitations and Final Thoughts

A LUT is not intended to be a one-step color grading and monitoring solution and should not be used as one. It can, however, be a good place to start—especially if it’s adding a look that was pre-determined before production began and was used on set. It would be great if it were as easy as throwing a LUT onto your footage and calling it a night but, unfortunately, it isn’t that simple. Remember, LUTs are just applying blanket mathematics and are unaware of the intricacies of each specific shot. A LUT that processes the desired results for a bright outdoor scene might give terrible results for a dimly lit interior. Likewise, if you’ve intentionally overexposed a shot by two stops to get cleaner shadows, then you’ll probably want a Display LUT on your monitor that brings the exposure back down by two stops. For these reasons and more, it’s not unusual to use multiple LUTs for different shooting conditions—both on set and during grading.

LUTs can be powerful tools when used appropriately, and are virtually essential for workflows involving log footage. My advice for you is to use them to your advantage and never let them begin to be a hindrance. If you’re applying LUTs during color grading, always consider them a starting point and never an end point.

While this has been a very general introduction to LUTs, I hope it has shed some light on what they are and the different ways they can be used. Everyone’s needs are different, so I urge you to create and experiment with different LUTs to find how they best fit into your workflow.

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