Using Display LUTs

Color supports the use of 3D look up tables (LUTs) for calibrating your display to match an appropriate broadcast standard or to simulate the characteristics of a target output device (for example, how the image you’re correcting will look when printed to film).

Color is represented on CRTs, LCD flat panels, video projectors, and film projectors using very different technologies. If you show an identical test image on two different types of displays—for example, a broadcast display and a video projector—you can guarantee there will be a variation in color between the two. This variation may not be noticeable to the average viewer, but as a colorist, you need a predictable viewing environment that adheres to the standards required for your format, and to make sure that you aren’t driven crazy by changes being requested as a result of someone’s viewing the program on a display showing incorrect color.

There is also variation within a single category of device:

All these variables inevitably result in significant color variation for any image going from one viewing environment to another. One solution to this is calibration using LUTs.

What Is a LUT?

Simply put, look up tables (LUTs) are precalculated sets of data that are used to adjust the color of an image being displayed with the gamut and chromaticity of device A to match how that image would look using the gamut and chromaticity of device B.

The gamut of a particular device represents the total range of colors that can be displayed on that device. Some types of displays are capable of displaying a greater range of colors than others. Furthermore, different video and film standards specify different gamuts of color, such that colors that are easily represented by one imaging medium are out of bounds for another. For example, film is capable of representing far more color values than the broadcast video standard.

Chromaticity refers to the exact values a display uses to represent each of the three primary colors. Different displays use different primary values; this can be seen on a chromaticity diagram that plots the three primaries as points against a two-dimensional graph representing hue and saturation within the visible spectrum. Since all colors represented by a particular display are a mix of the three primaries, if the three primary points vary from display to display, the entire gamut of color will shift.

Figure. Illustration of a chromaticity diagram.

While the chromaticity diagram shown above is useful for comparing displays on paper, to truly represent the hue (color), saturation (intensity of color), and lightness (luminance from black to white) that defines a complete gamut, you need to use a 3D color space.

When extruded into 3D space, the gamut and chromaticity of different devices create different shapes. For example, the standard RGB color space can be represented with a simple cube (as seen in the ColorSync Utility application):

Figure. Illustration of an extruded RGB cube.

Each corner of the cube represents a different mix of the R,G,B tristimulus values that represent each color. The black corner is (0,0,0), the opposing white corner is (1,1,1), the blue corner is (0,0,1), the red corner is (1,0,0), and so forth. The RGB color cube is an idealized abstraction, however. Actual display devices appear with much different shapes, defined by their individual gamut and chromaticity.

Figure. Illustration of a transformed RGB cube.

To accurately transform one device’s gamut to match that of another involves literally projecting its gamut into a 3D representation and then mathematically changing its shape to match that of the other device or standard. This process is referred to as characterizing a device and is the standard method used by the color management industry. Once calculated, the method of transformation is stored as a 3D LUT file.

Once a device has been characterized and the necessary LUT has been calculated, the hard computational work is done, and the LUT can be used within Color to modify the output image without any significant impact on real-time performance.

When Do You Need a LUT?

The following examples illustrate situations in which you should consider using LUTs:

  • If you’re matching multiple displays in a facility: LUTs can be useful for calibrating multiple displays to match a common visual standard, ensuring that a program doesn’t look different when you move it to another room.
  • If you’re displaying SD or HD video on a nonbroadcast monitor: You can use a LUT to emulate the Rec. 601 (SD) or 709 (HD) color space and gamma setting that’s appropriate to the standard of video you’re viewing.
  • If you’re displaying video or film images using a video projector: You can use a LUT to calibrate your device to match, as closely as possible, the gamut of the broadcast or film standard you’re working to.
  • If you’re grading images destined to be printed to film: You can use a LUT to profile the characteristics of the film printing device and film stock with which you’ll be outputting the final prints, in order to approximate the look of the final projected image while you work.

Important: LUTs are no substitute for a high-quality display. In particular, they’ll do nothing to improve muddy blacks, an inherently low contrast range, or a too-narrow gamut.

When Don’t You Need a LUT?

If you’re color correcting video and monitoring using a properly calibrated broadcast display that’s compatible with the standard of video that you’re displaying, it’s not generally necessary to use a LUT.

Generating LUTs

There are several ways you can generate a LUT.

Create One Yourself Using Third-Party Software

There are third-party applications that work in conjunction with hardware monitor probes to analyze the characteristics of individual displays and then generate a LUT in order to provide the most accurate color fidelity possible. Because monitor settings and characteristics drift over time, it’s standard practice to periodically recalibrate displays every one to two weeks.

If you’re creating a LUT to bring another type of display into line with broadcast standards (such as a digital projector), you’ll then use additional software to modify the calibration LUT to match the target display characteristics you require.

Have One Created for You

At the high end of digital intermediate for film workflows, you can work with the lab that will be doing the film print and the company that makes your monitor calibration software to create custom LUTs based on profiles of the specific film recorders and film stocks that you’re using for your project.

This process typically involves printing a test image to film at the lab and then analyzing the resulting image to generate a target LUT that, together with your display’s calibration LUT (derived using a monitor probe and software on your system), is used to generate a third LUT, which is the one that’s used by Color for monitoring your program as you work.

Creating LUTs in Color

In a pinch, you can match two monitors by eye using the controls of the Primary In room and generating a LUT to emulate your match directly out of Color.

You can also export a grade as a “look” LUT to see how a particular correction will affect a digitally recorded image while it’s being shot. To do this, the crew must be using a field monitor capable of loading LUTs in the .mga format.

To create your own LUT
  1. Arrange your Color preview display and the target monitor so that both can be seen at the same time.

  2. Load a good evaluation image (such as a Macbeth chart) into the Timeline.

  3. Display the same image on the target display using a second reliable video source.

  4. Open the Primary In room and adjust the controls appropriate to make the two images match.

  5. Choose File > Export > Display LUT.

  6. When the Save LUT As dialog appears, enter a name for that LUT into the File field, choose a location to save the file, and click Save.

    By default, LUTs are saved to the /Users/username/Library/Application Support/Color/LUTs directory.

Important: If your project is already using a LUT when you export a new one, the currently loaded LUT is concatenated with your adjustments, and the combination is exported as the new LUT.

Using LUTs

All LUTs used and generated by Color are 3D LUTs. Color uses the .mga LUT format (originally developed by Pandora), which is compatible with software by Rising Sun Research, Kodak, and others. If necessary, there are also applications available to convert LUTs from one format into another.

LUTs don’t impact processing performance at all.

To use a LUT
  1. Choose File > Import > Display LUT.

  2. Select a LUT file using the Load LUT dialog, then click Load.

    Note: By default, LUTs are saved to the /Users/username/Library/Application Support/Color/LUTs directory.

    The LUT immediately takes effect, modifying the image as it appears on the preview and broadcast displays. LUTs that you load are saved in a project’s settings until you specifically clear the LUT from that project.

To stop using a LUT
  • Choose File > Clear Display LUT.

To share a LUT with other Color users, you must provide them with a copy of the LUT file. For ease of use, it’s best to place all LUT files into the /Users/username/Library/Application Support/Color/LUTs directory.