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Color Management with Cinema

Color management is a process that helps achieve more predictable and consistent color. However, many of the core concepts involve color science and are often unfamiliar to even experienced users. In this article, we'll introduce the key technologies and best practices as they pertain to cinematic and broadcast distribution.

OVERVIEW

The process starts by assessing which devices will typically be used from camera to computer display to television or projector. For example, one could capture with a RED EPIC®, color grade with a reference-quality monitor, then output to a home theater display:

The key is being able to understand the capabilities and limitations of each device along this imaging chain, and then to adjust their output accordingly. This process involves some combination of profiling, calibration and software control. With cinema, the limiting device is almost always the projector or home theater display, so the process is relatively straightforward and universal.

COLOR SPACES

A device's capabilities are quantified and visualized using something called a color space, which is a three dimensional region containing all producible colors. These are typically defined so that horizontal and vertical directions describe saturation and luminance changes, respectively.

To make color spaces easier to visualize, these are typically represented as a two dimensional slice at 50% luminance. The background colors are qualitative and depict a device-independent reference space with the full range of colors in human vision.

Virtually every projector or computer display creates imagery by combining primary colors in various ratios and intensities. The type and response of these primary colors varies, and determines the most extreme colors a given device can produce. Since most displays utilize three primary colors, their color spaces appear with three vertices (as triangles).

COMPARING DEVICES

Color spaces are most powerful when they're used to reveal which colors are reproducible among two or more devices. When going from a larger to a smaller color space, not all colors will be reproducible, and will have to be compressed into the smaller color space. This process is called "gamut mapping," and can influence intermediate colors and make gradations appear to saturate abruptly. Knowing when this is likely to happen requires being familiar with the common color spaces relevant to digital cinema. These include:

  • ITU Rec.709. This is the international standard color response of high-definition television. It has a relatively small gamut and is almost identical to the commonly used sRGB color space with website publishing.

  • DCI-P3. This is a newer standard for color response with digital cinema projection, and was designed to closely match the full gamut of color film. Its gamut is therefore relatively large, and has more pronounced greens and reds compared to Rec. 709.

  • Adobe RGB 1998. This is primarily used in the stills and publishing world, but is relevant to cinema because many of the same high-end computer displays closely approximate this gamut.

  • CIE XYZ. This is a device-independent space that aims to encompass all the colors perceivable by the human eye using studies from 1931. Other color spaces are typically shown as subsets of this space, as depicted by the colored region in the diagram above. It's a standard encoding for DCI-compliant cinema projectors, and is useful for distribution since it encompasses virtually any output device. For these reasons, it's also an important way to encode theatrical archives. Sometimes XYZ is also denoted as X'Y'Z' when referring to the gamma-encoded values.

    • How do digital cameras fit in among the above color spaces? Many are actually capable of producing imagery which fully utilizes wide gamut output spaces. The eventual gamut is typically limited by the working space that's used during color grading, and by how much saturation is adjusted in post not necessarily by the camera sensor itself. Triangular color space diagrams are primarily useful for encoding and display devices; the primaries with digital sensors also aren't simple vertices. With capture devices, color differentiation is often instead the distinguishing characteristic, and RAW files, lower noise and better dynamic range all help.

    CINEMA COLOR PIPELINE

    A standard cinema workflow may incorporate and convert between two or more of the above color spaces. The most influential conversions typically happen in the following stages:

    1. Developing. This stage interprets the image code values as visible colors. With RED¨, the REDCODE RAW file gets developed and shown on-screen using DRAGONcolor/REDcolor or REDlogfilm plus custom LUTs.

    2. Grading. This stage applies creative color grading using a wide gamut, calibrated display. This is typically done within a standard working space that encompasses the expected output devices. With cinematic releases, the working space is typically DCI-P3; when only broadcast distribution is expected, this is typically Rec709. With REDCINE-X PRO¨, the display should be calibrated to match the chosen working space.

    3. Mastering. This stage encodes the final color-graded imagery as a single distribution master using device-independent XYZ color. Although no device can fully reproduce XYZ colors, this ensures future compatibility with wider gamut projection technologies, and avoids unnecessarily limiting the color possibilities in advance.

    4. Projection. This stage converts the colors from the master into the projector's native, device-specific color space. Any mastered colors which are not reproducible are gamut mapped by the projector itself.

    The key with any color management pipeline is to pay close attention to color mismatches. With cinema, this is most likely to occur during color grading if the display has a narrower gamut than DCI-P3, or during projection if the master's colors have a wider gamut than the projector...

    GAMUT MISMATCHES

    Colors can change adversely when the color space of the master is larger than that of the projector. For example, many high-end displays used for color grading and print publishing are capable of more saturated greens, cyans and blues than standard DCI-P3 cinema projectors.

    The projector then has to perform gamut remapping to convert the out of gamut colors into ones that are reproducible by the projector. This generally appears as a selective desaturation of the most extreme colors, but can also have other effects, such as hue shifts, a loss of texture within those colors, and abrupt changes within otherwise smooth color gradients. In the example below, the clipped reds get closer to orange after remapping:

    Out of Gamut Reds
    Gamut Remapped by Projector

    To minimize these artifacts, color grade footage within a color space that is large enough to encompass the capabilities of all anticipated output devices, but no larger. Out of gamut colors are also much less common than one might initially assume, even with otherwise highly colorful scenes.

    Ultimately though, predictable color requires a projector that is well-calibrated to match reference output. Since the response of projectors and home theater displays varies as those products age, these need to be calibrated periodically to maintain consistent color. Older projectors also may not perform gamut remapping internally. In either case, this can cause colors to appear more or less saturated than intended:

    Wider Gamut
    Master
    Gamut Match
    Wider Gamut Projector

    In the example above, the "wider gamut project" scenario is common when sending footage to a DCI-P3 projector that had been prepared for broadcast using Rec. 709. Similarly, the "wider gamut master" scenario could happen when broadcasting footage for Rec. 709 television display that had been prepared for theatrical projection using DCI-P3.

    REDCINE-X PRO

    All colors shown within REDCINE-X PRO use the monitor's native RGB color space. One can therefore essentially see on-screen what they'll be getting on the output device by calibrating their display to match the output color space. Since Rec. 709 is a common output and reproducible by most displays, calibrating to this space is probably the most common approach for broadcast distribution.

    However, with some productions, REDCINE-X PRO is used strictly to read and convert REDCODE® RAW files. In those cases, colors will change substantially in third party software or in conjunction with a post-production house, so the choice of color space within REDCINE-X PRO is less important. To achieve the best starting point for color grading though, select DRAGONcolor in REDCINE-X PRO and specify Rec. 709 as the color space in any subsequent software. Also see the article on REDgamma versus REDlogfilm for more on preparing files for color grading.

    Controlling Color Rendition in REDCINE-X PRO

    The color space setting in REDCINE-X PRO also influences how colors are rendered, but not in the same sense as Rec. 709, Adobe RGB 1998 and other common color spaces. DRAGONcolor (or REDcolor) effectively acts as a color engine that converts between the camera's native gamut and monitor RGB, which is assumed to be close to Rec. 709. As a result, the gamut produced by DRAGONcolor depends on the capabilities of the particular monitor, and cannot necessarily be plotted on a standard color space comparison diagram. Unless one is trying to match older footage, always use the most up-to-date color science, such as REDcolor3 instead of REDcolor2.

    PROFILING & CALIBRATION

    Regardless of software choice, having a well-calibrated display is essential for color grading. Calibration is typically performed using an external measurement device that attaches to the front of the display, and reads a sequence of colors generated by the calibration software. Some high-end displays come with these devices or perform these calibrations automatically. Otherwise, commonly used third-party devices include the Datacolor Spyder and X-Rite i1Display. Optimal results with neutral grays throughout the tonal range typically requires a display with a high bit depth internal LUT.

    If no calibration device is available, a quicker but less accurate option would be to use one of the color space presets, if available. Many broadcast-oriented displays come with a Rec. 709 preset option, for example. The key is to ensure that any displays used for editing are capable of reproducing the full gamut of the intended output device.

    However, even a perfectly calibrated display won't necessarily match a calibration target. Several off-screen factors can also influence appearance, including ambient light level and color temperature, coloration of objects behind the display and even the clothing one is wearing.

    DISCUSSION

    Before putting these concepts into practice, it's important to set expectations. Even with the best color managed workflow, it is impossible to fully reproduce colors among all devices regardless of the specific camera, display, projector or film stock. The best one can achieve is to make the whole process more controllable and transparent. In the end, that's the goal of color management.

    Color management also isn't new to digital capture; scanned film required careful color control, but with digital, the whole process is more controllable from start to finish. The process is also simpler because color temperature is not baked into the image like it is with film. One is therefore primarily concerned with differences in contrast ratio and color saturation. Color management also isn't something that necessarily needs to be a constant concern. Once all of the best practices are in place, it is often a ñset it and forget itî process when color grading for cinematic output.

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