Tonal Range Primer

2013/10/21 14:06:19Click: 853

By Steve Hullfish, The Art and Technique of Digital Color Correction


1. Vector: A position or coordinate in space or a direction between two coordinates. On a vectorscope, the vector is the specifi c position of a color in the twodimensional circle defi ned by the vectorscope. The “targeted” vectors on the vectorscope are the three primary colors — red, green, and blue — and the secondary colors between them — magenta, cyan, and yellow.


2. Legal: For video-based images, legal means that the brightness and color saturation of an image does not exceed minimum or maximum levels that have been determined for a specific delivery channel for a video. This usually implies broadcast, but can also pertain to duplication. Each duplicator or broadcaster sets his or her own specifi c requirements for video levels, but in general, these levels adhere to national and international standards, which state that the darkest portions of the luminance of a picture cannot fall below 0 IRE for NTSC digital video (and most other international video of any type) or 7.5 IRE for composite NTSC in the United States. The brightest pixels are not to pass 100IRE when monitoring luminance only,or, when combined with chroma, cannot pass 110 IRE. (There are other ways to measure the signal other than IRE, such as in millivolts.) Also, as our delivery systems become more and more digitally based, gamut is also included in legal levels. Not all waveform monitors or vectorscopes can monitor gamut levels. These gamut levels are the legal amounts, or values, of certain colors. It is possible for the luminance of an image to be well under legal levels, but, because of a combination of saturation and luminance, the legal gamut levels can be exceeded.In addition to legal levels,there is a second, similar term called valid levels.


3. IRE: One of the units of measurement that can describe a composite analog video signal’s amplitude (brightness) where 0 IRE generally represents black and white extends to 100 IRE. 1 IRE is equal to 1/140 of a volt or 7.14 millivolts in NTSC,though in all other systems, it corresponds to 7 millivolts. IRE stands for the Institute of Radio Engineers which defined the unit.


4. Gamut: The complete range of colors that can be captured, displayed, or broadcast by a device or a system of devices. Most cameras or color-correction devices have a much wider gamut (range of colors) than those that can be used further on in the production stream. For example, the gamut of colors later in the production stream that could require a limited gamut can be those recorded to tape, burned to a DVD, encoded for the Web, broadcast from a TV transmitter, or viewed on a TV set. So there are multiple gamuts that have to be considered.


5. Valid Levels: Levels that remain legal when transferred, translated,or transcoded  between formats.


6. Tonal Range(singular): This is sometimes also called the dynamic range, luminance range, or contrast range, though these terms can have slightly different technical definitions. The tonal range is the difference between the brightest and darkest areas of an image. The tonal range of an image, and how those tones are spread throughout the tonal range, defines its contrast. For some applications of this phrase, tonal range indicates the actual number of levels of tones that a recording medium can record (256 per channel in the case of RGB 8 bit, or 1,025 per channel in the case of RGB 10 bit).


   Tonal Ranges(plural): The three commonly used tonal ranges that are used to breakdown the description (and control) of an image are shadows, midtones, and highlights. Sometimes shadows are referred to as blacks, pedestal, setup,lift, or even lowlights. Midtones are often referred to as gamma, but also as grays or mids. Highlights are sometimes also referred to as whites,gain, luma, or even video.


7. Waveform Monitor: A waveform monitor displays the amplitude level — brightness and darkness — along the vertical axis with the dark parts of an image near the bottom and the brighter parts of the signal near the top. Technically, the horizontal axis of the waveform displays “time,” but practically speaking, the horizontal axis of the waveform corresponds to the horizontal placement of picture elements across the image with no regard to the vertical placement of elements in the image.


8. RGB Parade Mode (Waveform): The RGB parade is simply a display option of a standard waveform monitor. Colorists rely heavily on the RGB parade viewing option on a waveform because it displays the individual levels of the red,green, and blue channels of an image. Each of these channels is displayed in its own cell horizontally with red, green, and blue in a “parade” from left to right across the screen. Each of these cells is essentially identical to the regular display of information on a waveform monitor, except that the values only pertain to the amount of that one color in the image. A variation on this display is the YRGB parade display that you will see throughout this book, which has four cells instead of three: the first being luminance (Y), followed by red, green, and blue。


9. Vectorscope: The vectorscope displays chrominance and hue. The saturation, or gain, of the chroma, or color, is measured by how far it extends from the center of the scope. Neutral images (black, white, and all levels of gray) register as a dot in the middle of the vectorscope. Hue is indicated by the position of the trace around the perimeter of the circle. Vectorscopes have graticules that show eachof six different colors (red, green,blue, magenta, cyan, and yellow) in a different, fi xed vector (position) around the vectorscope.


10. Graticule: Graticule is the overlay on the scope that indicates levels and positioning information. The graticule does not change unless the user changes it. The graticule is usually customizable to display various scales and to provide information on how the trace signal is being displayed. It is analogous to the legends and lines on a chart or graph.


11. Trace: The trace is the part of the waveform or vectorscope display that reacts to the incoming video information. The trace is a representation of your video image on the waveform or vectorscope.


12. Cell: This term is used to describe one of the three (or four) separate images on the RGB parade  waveform corresponding to the individual red, green, blue, and, in the case of a YRGB display, luminance signals.