TCPRGAMMA(5) COMMAND REFERENCE TCPRGAMMA(5) NAME tcprgamma - Tektronix color printer default gamma correction intensity values SYNOPSIS /usr/lib/tcpr/tcprgamma.default DESCRIPTION tcprgamma.default is the default gamma correction file for the tcpr (Tektronix color printer) system. A terminal screen uses an intensity range of zero to 255 for each color. The in-between intensity values are matched to the color display hardware so there is a continuous transition from dark to full brightness. This transition curve does not vary greatly among displays of similar brightness, even those from different manufacturers. Half intensity (128) internally gives a brightness on the screen that is considerably lower than full intensity. It appears to be only about one-fourth as bright. However, a Tektronix color printer is an on-off color device. Its levels of intensity are varied by using ordered dithering over square areas of many pixels. The intensity of the color printer copy decreases more evenly, but more slowly than on a terminal display. For example, half intensity on paper is represented by writing half the pixels in a dithered area. This is much more than "half bright" on a display. Only about one-fourth of the pixels in an area need to be on to achieve the visual affect of a half-bright display. Gamma correction alters the brightness curve by declining rapidly at the top and more slowly at the bottom. This is done by a scaling function, implemented in the processing software as a look-up table. The tcprgamma file contains three tables of 256 values each. These are the default values of the processing command (processing(1)) if no user-supplied file is given. The additive colors RGB (red, green, blue) of the display and the subtractive colors CMY (cyan, magenta, yellow) of the printer inks are quite pure, so the three tables are usually the same. Gamma correction scales down the intensity values before dithering. This turns on less pixels in an area, bringing the actual intensity down to where the apparent brightness is correct. The default values supplied in the tcprgamma file are designed to match a high quality video display to a Tektronix color printer. The color printer is a neutral intensity device, so the gamma corrections are made for the video display. Printed 5/12/88 1
TCPRGAMMA(5) COMMAND REFERENCE TCPRGAMMA(5) The user has the option of suppling their own table of gamma correction values in a file for use by the processing command (processing(1)). Use this option when the default table does not produce optimum results due to differences in colors, dithering matrices, or because a more or less intense copy is desired. The most scientific approach to creating your own gamma function is to get the gamma function of your display from the manufacturer or measure it with a spectral photometer. You need enough discrete values to approximate discrete integer values from 0 to 255 and also to scale the output to cover the range of 0 to 255. All independent (original) and dependent (gamma-corrected) values must be rounded to integers. There must be exactly one entry in the table for each original intensity value over the range of 0 to 255. A less scientific but much simpler approach is to draw a graph of the existing corrective curve. To bring up the brightness in the center, draw another curve that is still smooth, but higher in the center, then derive the new values from the graph. It is important that 0 and 255 always map into themselves. Otherwise, there are extremes of brightness that are not representable. Varying the curves for the three colors separately requires more understanding of the subtractive CMY process. To increase the additive RGB red intensity, for instance, the CMY magenta and yellow values should both be increased. It is recommended that a reliable source of information on the subject be consulted before making changes to gamma tables, especially if the colors are to be adjusted separately. The gamma correction values are placed in the file using a set of Generic Rastor Format (GRF) tag entries. Any new gamma correction file or the tcprgamma default file must contain the following GRF tag entries: colorunits {cmy|rgb} This tag is required. Specifies the color coordinate system used to encode the gamma intensity values in the file. The gamma intensity values can be either in the CMY or RGB color coordinate system. gammacorrection {[d,256]|[d,3,256]} This tag is required. Precedes the table of gamma correction intensity values. A table of 256 gamma intensity values is included by use of a GRF tag of the form gammacorrection [d,256] Printed 5/12/88 2
TCPRGAMMA(5) COMMAND REFERENCE TCPRGAMMA(5) followed by a 256 entry table of gamma intensity values. This results in the table being duplicated for all RGB (red, green, blue) or CYM (cyan, magenta, yellow) entries depending on the color coordinate system used. A GRF tag of the form gammacorrection [d,3,256] specifies a separate table for each coordinate of the color system used. If the RGB coordinate system is used, a table of 256 red values followed by a table of 256 green values and a table of 256 blue values follows the GRF tag entry. headerend This tag is required. Marks the end of all GRF header information. It must be the last GRF tag entry in the file of gamma correction values. FILES /usr/lib/tcpr/tcprgamma.default Default path to table of gamma correction intensity values. SEE ALSO processing(1), screencopy(1), x_grf(1), st_grf(1), tcprconf(5mdqs), tcprdither(5), grf(5), tcprserver(8mdqs). REFERENCES For more information, consult the Users Manual for your system. Printed 5/12/88 3
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