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Primary Color Adjustments for Video and Cinema

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In this chapter Alexis Van Hurkman examines the common methods you can use to make primary color adjustments to affect an overall image.
This chapter is from the book

As discussed at the beginning of Chapter 3, the human visual system processes color signals separately from luminance, and as a result, color conveys a completely different set of information. Color is used by what Margaret Livingstone refers to as the “what” system of the brain to identify objects and faces. Other studies support the idea that color plays an important part in speeding object identification and in enhancing memory recall.

For example, in their article “Revisiting Snodgrass and Vanderwart’s Object Databank: Color and Texture Improve Object Recognition” (Perception Volume 33, 2004), Bruno Rossion and Gilles Pourtois used a set of standardized images first assembled by J.G. Snodgrass and M. Vanderwart to determine whether the presence of color sped subjects’ reaction times for object identification. The study sorted 240 students into separate groups and asked them to identify one of three sets of test images: black and white, grayscale, and color (such as those images shown in Figure 4.1). The resulting data showed a clear increase in the speed of object recognition by nearly 100 milliseconds with the addition of color.

Figure 4.1

Figure 4.1 One of a set of 260 line drawings used to test the differences in object identification speed for black and white, grayscale, and color images.

Similarly, in “The Contributions of Color to Recognition Memory for Natural Scenes” (Wichmann, Sharpe, and Gegenfurtner, Journal of Experimental Psychology Learning Memory and Cognition, 2002), subjects were reported to have performed 5−10 percent better at memory retention tests that used colored images than they did with grayscale images.

Beyond these purely functional benefits to color, artists, critics, and researchers over the centuries have called attention to the emotional signifiers of various colors and the importance that color exerts on our creative interpretation of visual scenes.

For example, not many people would dispute that orange/red tones are high-energy colors and that an abundance of warmth in the art direction of a scene will lend a certain intensity to what’s happening, as shown in Figure 4.2.

Figure 4.2

Figure 4.2 An actress in an environment with predominantly warm tones and warm, golden lighting.

Similarly, blue has an innate coolness, and bluish lighting will give an entirely different impression to an audience (Figure 4.3).

Figure 4.3

Figure 4.3 The same actress as in Figure 4.2, performing the same activity, but now the environment and lighting are both deliberately cool and blue, setting a different mood.

In her book, If It’s Purple, Someone’s Gonna Die (Elsevier, 2005), designer, author, and professor Patti Bellantoni cites numerous color experiments with her art students, whom she separated into groups, asking each to create an environment based on a specific color. The resulting color-dominated rooms not only drew a clear emotional response from the students, but over a number of years, successive classes of students exhibited strikingly similar interpretations for identical colors.

In the “backstory” chapter of her book, Bellantoni says, “[M]y research suggests it is not we who decide what color can be. After two decades of investigation into how color affects behavior, I am convinced, whether we want it to or not, that it is color that can determine how we think and what we feel.”

Simple primary corrections won’t unrecognizably alter the art direction and costumes within a scene. However, by correcting, shifting, and deliberately controlling the overall color tone of the lighting, you can create distinct audience impressions about the emotional atmosphere of a scene, the health and attractiveness of your characters, the tastiness of food, the time of day, and the kind of weather, no matter what the lighting of the shot originally was. Figure 4.4 shows two contrasting versions of the same scene.

Figure 4.4

Figure 4.4 Which room would you rather wake up in?

To master these kinds of adjustments, we’ll examine the role that color temperature, manipulation of the chroma component, additive color math, and an understanding of color contrast all play in the use of the color balance and RGB Curve controls present in nearly every professional color correction application.

Color Temperature

All color in a scene interacts with the dominant light source, or illuminant, of that location. Each type of illuminant, whether it’s the sun, practical tungsten or halogen light fixtures, or stage and cinema lighting instruments, has a particular color temperature that dictates the color quality of the light and how it interacts with subjects in a scene.

Nearly every lighting effect dealt with in this book is a result of differing color temperature, or color of light, in various circumstances. Every time you correct or introduce a color cast in an image, you’re effectively manipulating the color temperature of the light source.

Color temperature is one of the most important concepts for a colorist to understand because the color temperature of the lighting in any scene changes the viewer’s perception of the colors and highlights found within. Despite the human eye’s adaptive nature, when the color temperature of the dominant lighting is not taken into account through the use of film stocks, filtration, or white balance, a color cast will be recorded. Sometimes a color cast is desirable, as in the case of “magic hour” lighting or sunset photography. Sometimes it’s not desirable, such as when you’re shooting interior scenes with incorrectly balanced or spectrally varied light sources.

Each type of light source used to illuminate subjects recorded by film or digitally has its own particular color temperature, which in many cases corresponds to how hot that light source must be to emit light. Light emitters can be modeled in physics as black-body radiators, which are idealized light sources that output pure color corresponding to their temperature. For example, the heating elements in some toaster ovens are approximate black-body radiators. The hotter they get, the brighter they glow: first dark orange and then progressively lighter. The carbon rods used for arc welding are so hot that they glow a bright blue-white.

Candles, light bulbs, and sunlight operate at very different temperatures, and as a result, they emit more or less radiation at different wavelengths of the visible spectrum. Thus, comparing two different light sources (such as a household lamp next to a window on a clear morning) reveals differently colored light. Consider Figure 4.5, color-balanced for tungsten, which accounts for the white quality of the interior lighting. This reveals how cool the sunlight coming in through the window is, which by comparison is a vivid blue.

Figure 4.5

Figure 4.5 Mixed lighting reveals strikingly different color temperatures.

The color temperature of a light source is measured in Kelvin (Figure 4.6), named after William Thompson (aka Lord Kelvin), a Scottish physicist who first proposed a scale for absolute temperature measurement. While named for Kelvin, Max Planck was the physicist who developed the principle (called Planck’s law) that, as Wikipedia explains, “describes the spectral radiance of electromagnetic radiation at all wavelengths emitted in the normal direction from a black body in a cavity in thermodynamic equilibrium.”

Figure 4.6

Figure 4.6 Approximate colors corresponding to popular known color temperatures.

The math is complex, but for our purposes the general idea is that the hotter an emission source, the “bluer” the light. The cooler the emission source, the “redder” the light. Consider how the scale in Figure 4.6 matches to light sources and other illuminant standards.

It’s not a coincidence that the color gradient from 1600K to 10000K matches the progression in the quality of sunlight from sunrise to bright, noon sunlight.

“D” Illuminants and D65

A second color temperature standard you may hear mentioned describes the so-called “D” illuminants (also listed in Figure 4.6), which are defined by the Commission Internationale de l’Eclairage (CIE). The CIE defined standard illuminant graphs to describe the spectral distribution of different types of lighting. The “D” illuminants are all intended to describe daylight color temperatures so that manufacturers of lighting fixtures can standardize their products.

Each of the CIE illuminants was developed for a specific purpose. Some illuminants are intended for use as lighting for critical color evaluation; others are meant for use in commercial lighting fixtures.

One illuminant you should memorize is D65 (corresponding to 6500K), which is the North American and European standard for noon daylight. This is also the standard setting for white that broadcast video monitors use in the United States and in Europe, and it is the type of ambient lighting you should employ in your color correction suite. Inconsistent lighting in your environment will cause your eyes to adapt incorrectly to the colors on your monitor, resulting in bad color decisions.

Broadcast monitors in China, Japan, and Korea are balanced to D93, or 9300K, which is a significantly bluer white. This should ideally be paired with matching D93 ambient lighting.

Spectrally Varied Light Sources

The simple color temperature measurements shown in Figure 4.6 are good for describing light quality in general terms, as well as for standardizing film stocks, optical filters, and HDSLR, camcorder, and digital cinema camera white balance controls. However, the spectral distribution of real-world light sources isn’t always so perfect. Different light sources have unique spectral distributions that may include numerous spikes and dips at specific wavelengths of light.

A good example of a spectrally varied light source is fluorescent lighting, which has spikes in its spectral distribution that can illuminate other colors differently than you might expect. An average office fluorescent tube has small but significant spikes in the green and indigo-blue portions of the spectrum that, while appearing perfectly white to the human eye, may lend a greenish/blue cast to unfiltered film and improperly white-balanced video. For example, the image on the left in Figure 4.7 is incorrectly balanced for tungsten, and the fluorescent lighting lends a greenish cast to the image (especially visible in the gray doors). The image on the right is properly white balanced.

Figure 4.7

Figure 4.7 The image to the left exhibits the greenish tint of fluorescent lighting shot with an incorrect white balance. The image to the right is shot using the correct white balance.

Generalizing about the light given off by fluorescent tubes is difficult because there are many different designs, all of which have been formulated to give off different qualities of light. Some fluorescent tubes have been specially designed to eliminate these spectral inconsistencies and produce light with nearly equal amounts of radiation at all frequencies of the visible spectrum.

Other spectrally varied light sources are the sodium vapor lamps used in municipal street lights, which give a severe yellow/orange cast to an image, as shown in Figure 4.8.

Figure 4.8

Figure 4.8 The spectrally monochromatic light put out by sodium vapor lamps produces a harsh, orange light that’s difficult to compensate for.

Other spectrally varied light sources include mercury vapor lamps, which lend an intense off-red tint to shots, and metal halide lamps, which can give off either magenta or blue/green casts.

With a shot that has one of these intensely red/orange light sources as the primary source of illumination, you’ll be surprised at how much of a correction you can make, assuming that the main subjects of the shot are people. Because these light sources have a strong red component, you can generally bring back relatively normal-looking skin tones. Unfortunately, other colors won’t fare as well, so cars, buildings, and other colorful exterior objects may prove troublesome.

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