There are two things that are at the core of doing good color grading for video:
Ensuring that the image on the screen looks great and is graded to best tell the story; and
Making sure that the image can be properly reproduced and delivered to a variety of media and screens.
This article will focus on the latter. Understanding the importance of legal and valid gamut and determining the color balance are critical to maintaining proper color reproduction across a variety of media and broadcast methods. Before we examine the concepts of color balance, let’s quickly review the concepts of color space.
The HSL color space
Video is comprised of three color components: red, green and blue (RGB). Various combinations of these colors make up the colors we see. One way to understand the Hue Saturation and Luma (HSL), or RGB color space, is to imagine it as two cones joined at their widest point, as shown in Figure 1).
The waveform monitor
The waveform monitor or rasterizer (scope) is key to providing a legal output of your creative product. Being a brilliant editor and colorist doesn’t mean much if no one will air your product. Even if your product isn’t being broadcast, legal levels affect the proper duplication of your project and the way it will look on a regular TV monitor.
One of the most basic uses of the waveform monitor is determining whether your luma and setup levels are legal. This means that the brightest part of the luma signal does not extend beyond 100 percent and that the darkest part of the picture does not drop below 0 percent.
Determining color balance
Color balance is indicated by the relative strength of each color channel. With neutral (pure black, white and gray) colors, the strength of each color channel should, technically, be equal. The usual goal of color balancing is to achieve an image where neutral colors are represented with all channels being equal. The most common reason for unbalanced colors is how the camera is white balanced on location. For example, if a camera is set up to record in tungsten light, when it is actually capturing a scene lit with daylight, the blue channel will be stronger than the red and green channels. Some camera sensors have a natural tendency to be more sensitive to certain colors in certain tonal ranges. These errors, in sensitivity or white balance, can be corrected by monitoring the image with a waveform monitor and making adjustments to the signal until the signal strength of all three channels is equal when displaying a neutral color.
There are two types of waveform displays colorists consult that are defined as “parade” displays because they show channels of information in a “parade,” from left to right.
The most common of these is the RGB Parade shown in Figure 2, which shows the red, green and blue channels of color information horizontally across the display.
The reference marks are referred to as the graticule. On a waveform monitor, these are the horizontal lines describing the millivolts, IRE or percentages from black to full power (white).
Component video levels are represented in terms of millivolts, with black being set at 0mV and white at 700mvV. This range of video levels is also represented in terms of a percentage scale with 0 percent equal to 0mV, and 100 percent equal to 700mV.
Whereas a waveform monitor normally displays a plot of signal vs. time, a vectorscope, shown in Figure 3, is an XY plot of color (hue) as an angular component of a polar display, with the signal amplitude represented by the distance from the center (black). On a vectorscope graticule, there are color targets and other markings that provide a reference as to which vector, or position, a specific color is in.
In color grading applications, the vectorscope helps analyze hue and chroma levels, keeping colors legal and helping to eliminate unwanted color casts. With the gain, setup and gamma corrections done while monitoring primarily the waveform monitor, the colorist’s attention focuses more on the vectorscope for the hue and chroma work.
The chroma strength of the signal is indicated by its distance from the center of the vectorscope. The closer the trace is to the outer edge of the vectorscope, the greater the chrominance, or the more vivid the color. The hue of the image is indicated by its rotational position around the circle. An important relationship to understand is the position of the various colors around the periphery of the vectorscope. The targets for red, blue and green form a triangle. In between each of these primary colors are the colors formed by mixing those primaries.
The chroma information presented on the vectorscope is instrumental in trying to eliminate color casts in images. As stated earlier, the chroma strength of a signal is represented by its distance from the center of the vectorscope. Because white, black and pure grays are devoid of chroma information, they all should sit neatly in the center of the vectorscope. Although most video images will have a range of colors, they also usually have some amount of whites, blacks and neutral grays. The key is to be able to see where these parts of the picture sit on the vectorscope and then use the color correction tools at your disposal to move them toward the center of the vectorscope.
For nearly all professional colorists, the various waveform displays — Flat, Low Pass, Luma only, RGB Parade and YCbCr Parade — plus the vectorscope are the main methods for analyzing their image. Although experienced colorists often rely on their eyes, they use these scopes to provide an unchanging reference to guide them as they spend hours color correcting. Without them, their eyes and grades would eventually drift off course. Spend time becoming comfortable with these scopes, and what part of the video image corresponds to the images on the scopes.
—Richard Duvall is video marketing manager of Tektronix.