Y′CBCR Color Model

In the RGB color model, all three channels contribute to the perception of brightness. In the early 1950s, this was a problem when developing a three-channel color television system that would be compatible with existing black-and-white televisions. The solution was to encode a single channel that represented luminance—light intensity as perceived by humans—which existing black-and-white televisions could decode. Color televisions would receive the same luminance channel and two additional color channels that could be decoded back into RGB color for display.

Luminance

Luminance is a measure of physical light intensity modified by the spectrum sensitivity measured in human vision. In other words, red, green, and blue contribute to the perception of brightness, but not equally. Since two of the cone cell types in the eye favor light in the green portion of the spectrum, the luminance channel is weighted to mostly represent the green color channel. This makes the luminance channel appear to have the equivalent brightness that we see in a color image.

The Y′CBCR color model derives luminance from the Commission Internationale de l’Eclairage (CIE) XYZ color system, in which the Y component represents luminance (X and Z represent color components).

In video systems, a gamma adjustment is applied to the RGB color channels to make efficient use of the bandwidth available for carrying and recording signals. The gamma-adjusted channels are called R′G′B′, and the derived black-and-white channel is called Y′, or luma. For more information about gamma, see About Gamma.

Color Difference Channels

The color channels, CB and CR, are derived by subtracting Y from the R and B signals and are sometimes referred to generally as B-Y and R-Y, or color difference channels. Analog video systems such as Betacam SP use PB and PR channels, and digital video systems use CB and CR. Each color channel format performs the same function, but the underlying specifications are slightly different.

In digital video, the color channels are typically sampled less often than the luma channel, reducing the video data without noticeable loss of image quality. The ratio of sampling between the Y′CBCR channels is often written as 4:2:2, 4:1:1, and so on, depending on the sample rates used. For more information about how color is represented in various video formats, see An Overview of Video Format Characteristics.

Luma

Luma describes the brightness of video, from absolute black, through the distribution of gray tones, all the way up to the brightest white. Luma is completely separate from the color of your video. In fact, if you viewed the luma of a video clip by itself, you would see a grayscale image completely devoid of color.

Luma is measured by Final Cut Pro as a digital percentage from 0 to 100, where 0 represents absolute black and 100 represents absolute white. Final Cut Pro also allows you to see super-white levels (levels from 101 to 109 percent) if they exist in your clip. Although super-white video levels are not considered to be broadcast-safe, many consumer camcorders record video at these levels anyway.

Figure. Diagram showng a gradient extending from black to white to super-white.

Note: In analog video, luma is measured in IRE units. These IRE measurements are irrelevant in Final Cut Pro because it deals only with the digital signal that exists in your computer as a straight percentage from 0 to 100 for NTSC and PAL video. For more information, see How Analog Video Signals Are Measured.

Blacks, Midtones, and Whites

In the Final Cut Pro color correction filters, most of the controls that you use to correct your clips are divided into controls over blacks, midtones, and whites. These represent different overlapping ranges of luma values in your image.

Figure. Diagram showing a gradient of black going to white.

Blacks make up the minimum range of luma in your clip. If you looked on a smooth gradient from black to white, controls that affect the blacks will affect your picture in the leftmost three-fourths of the gradient, from black to gray. The effect that controls have over the blacks of an image starts to diminish at approximately 75 percent luma, shown above. This excludes the brightest parts of your image.

Figure. Diagram of a gradient showing midtones.

Midtones make up most of the gray tones of an image. On the same gradient, controls that affect the midtones will affect the middle half of the gradient, excluding the deeply white and black parts. The effect that controls have over the midtones of an image starts to diminish at 25 and 75 percent luma, shown above. This excludes both the brightest and darkest parts of your image.

Figure. Diagram of a gradient showing whites.

Whites make up the maximum range of luma in your clip. On this gradient, controls that affect the whites affect the rightmost half of the gradient, from gray to white. The effect that controls have over the whites of an image starts to diminish at approximately 25 percent luma, shown above. This excludes the darkest parts of your image.

When you use controls that affect only one of these ranges, all changes made to the hue, saturation, and luma levels of your picture happen exclusively in the area that falls within that particular range of luma. This allows you to perform very targeted color correction only where it’s needed, such as subtly manipulating the hue of the highlights without touching the shadows, or vice versa.

Chroma

Chroma describes the color values in your clips, ranging from the absence of color to the maximum levels of color that can be represented. Chroma has two properties, hue and saturation.

Hue

Hue describes the actual color itself, whether it’s red or green or yellow. Hue is measured as an angle on a color wheel.

Figure. Diagram showing a color wheel and hues measured as angles.

Saturation

Saturation describes the intensity of a color, whether it’s a bright red or a pale red. An image that is completely desaturated has no color at all and is a grayscale image. Saturation is also measured on a color wheel, but as the distance from the center of the wheel to the edge.

Figure. Diagram of a color wheel showing saturation.

As you look at the color wheel, notice that it is a mix of the red, green, and blue primary colors that make up video. In between these are the yellow, cyan, and magenta secondary colors, which are equal mixes of the primary colors. These colors are most intense at the outer rim of the wheel and gradually desaturate to pure white at the center, indicating the absence of color.