Fundamentals of embedded video, part 2

[Part 1 explains the basic components of video signals. Part 3 looks at video from a system level, discussing video sources and displays. It will be published Monday, October 8.]

Color Spaces

There are many different ways of representing color, and each color system is suited for different purposes. The most fundamental representation is RGB color space.

RGB stands for "Red-Green-Blue," and it is a color system commonly employed in camera sensors and computer graphics displays. As the three primary colors that sum to form white light, they can combine in proportion to create most any color in the visible spectrum. RGB is the basis for all other color spaces, and it is the overwhelming choice of color space for computer graphics.

Gamma Correction

"Gamma" is a crucial phenomenon to understand when dealing with color spaces. This term describes the nonlinear nature of luminance perception and display. Note that this is a twofold manifestation: the human eye perceives brightness in a nonlinear manner, and physical output devices (such as CRTs and LCDs) display brightness nonlinearly. It turns out, by way of coincidence, that human perception of luminance sensitivity is almost exactly the inverse of a CRT's output characteristics.

Stated another way, luminance on a display is roughly proportional to the input analog signal voltage raised to the power of gamma. On a CRT or LCD display, this value is ordinarily between 2.2 and 2.5. A camera's precompensation, then, scales the RGB values to the power of (1/gamma).

The upshot of this effect is that video cameras and computer graphics routines, through a process called "gamma correction," prewarp their RGB output stream both to compensate for the target display's nonlinearity and to create a realistic model of how the eye actually views the scene. Figure 1 illustrates this process.

Gamma-corrected RGB coordinates are referred to as R'G'B' space, and the luma value Y' is derived from these coordinates. Strictly speaking, the term "luma" should only refer to this gamma-corrected luminance value, whereas the true "luminance" Y is a color science term formed from a weighted sum of R, G, and B (with no gamma correction applied).

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