TCPRDITHER(5) COMMAND REFERENCE TCPRDITHER(5) NAME tcprdither - Tektronix color printer default dithering matrix file SYNOPSIS /usr/lib/tcpr/tcprdither.default DESCRIPTION The file /usr/lib/tcpr/tcprdither.default is the default dithering file for the tcpr (Tektronix color printer) system. The file contains a set of default ordered matrices used by the processing command (processing(1)) to do dithering on a Generic Rastor Format (GRF) image file. To understand dithering it is necessary to understand how screen colors become ink colors on a Tektronix color printer. The screen of a color terminal has three primary colors: RGB (red, green, blue). Each can be varied separately to produce an almost unlimited variety of colors and intensities. The RGB of the screen is easily converted into the CMY (cyan, magenta, yellow) color units used by Tektronix color printers. The resolution of the color printer exceeds that of the terminal screen. However, the color printer has one limitation the color terminal does not. It can place, or not place, a dot of color for each pixel location on the paper, but can not vary the intensity of a dot. Each color is simply present or not present in a given pixel location. By itself this allows only eight different combinations, two of which are black and white, on the color printer. But the high resolution of the color printer makes it possible to vary the dots in a larger area to create the desired color of the terminal screen. In an area of 8x8 it is possible to have up to 64 different levels of a single color intensity. Since there are three colors that can be independently varied in the same manner, the user can produce 64 to the third power, or more than a quarter million different colors and shades. Black is included since it is represented as a combination of all three ink colors, though Tektronix color copiers use a separate black ink for a truer black. The eye tends to average the whole area and see the desired colors. There are limitations. The larger the area used to create an average color, the lower the effective resolution. Fortunately, even an 8x8 is only about 1/20 of an inch in each direction (or 1/400 sq. in.) on the Tektronix 4692 color printer. The key to greatest effectiveness is to come up with the best pattern for distributing the dots so that they blend together well, especially when the intensity is Printed 4/6/89 1
TCPRDITHER(5) COMMAND REFERENCE TCPRDITHER(5) low and there are very few dots in an average area. In a large area, we also do not want to see lines, larger multi- pixel dots, and other obvious patterns that might detract from the overall effect. In the dithering process, a dither pattern is a square 2n matrix placed over the pixel map in a location-dependent manner and replicated over the entire image. The dither intensity values go from 0 to 255 and are carefully distributed so low, middle, and high values are not all in one area, column or row. In any pixel location, if the pixel intensity (also 0 to 255) for a color exceeds the dither value, the pixel includes that color and a dot is put on the paper. This process is known as ordered dithering. Note: The largest dither value must be no larger that 254 so that 255 (maximum intensity) exceeds it and is always on. It is largely the dither pattern that determines the quality of the printer copy. The dither values must be evenly distributed from 0 to 254 so the number of on pixels in the dithered area will increase exactly and proportionately as the intensity increases. With dither patterns less than 16 x 16, not every number in the range can be represented, thus limiting the intensity variations to discrete jumps of 4 (for 8 x 8), or 16 (for 4 x 4). The tradeoff is more effective colors with a larger dithering matrix or more detail and less obvious dithering patterns with a smaller dithering matrix. To produce the optimum dithering effect, the mathematical magic square is used. A magic square is special in that all its rows, columns, and diagonals add up to the same value. This places the pattern side-by-side over the entire picture area so that any area of the same size, whether or not it coincides with the location of an original square, will still have the same properties. This assures uniform distribution and representation of all values, regardless of how the pattern lines up with the image. The file /usr/lib/tcpr/tcprdither consists of a set of four dithering matrices. One for 16x16, 8x8, 4x4, and 2x2 magic squares that are used by the processing command to do ordered dithering. Some pictures will not look as good as others, and this can be due to a choice of colors, level of detail, or dither size. Try all four sizes of default dithering matrices, with and without rotation. Sometimes a simple rotation of the dither pattern causes the patterns to line up more favorably with the picture. The user has the option of supplying a new set of dithering matrices to the processing command by overriding the default Printed 4/6/89 2
TCPRDITHER(5) COMMAND REFERENCE TCPRDITHER(5) with a command line option (see processing(1)). Use this option if the default dithering patterns do not achieve the desired effect. If the results are still not satisfactory, a user can design their own patterns. Some reliable sources of alternate dithering patterns are other manufacturers of devices that use ordered dithering and mathematical reference books that show other magic squares. The dithering matrix patterns are placed in a file using a set of GRF tag entries. The file must contain at least one dithering matrix. The default file /usr/lib/tcpr/tcpr.default contains a complete set of four. Any new dithering file or the tcprdither default file must contain the following GRF tag entries: ditheringmatrix [d,16,16] Specifies a 16x16 table of dithering values follows this GRF tag. This tag is not required if 16x16 dithering is never done. ditheringmatrix [d,8,8] Specifies a 8x8 table of dithering values follows this GRF tag. This tag is not required if 8x8 dithering is never done. ditheringmatrix [d,4,4] Specifies a 4x4 table of dithering values follows this GRF tag. This tag is not required if 4x4 dithering is never done. ditheringmatrix [d,2,2] Specifies a 2x2 table of dithering values follows this GRF tag. This tag is not required if 2x2 dithering is never done. headerend This GRF tag is required and marks the end of the last dithering matrix. It must be the last GRF tag entry in the file. FILES /usr/lib/tcpr/tcprdither.default Default path to file of four standard dithering matrices (16x16, 8x8, 4x4, 2x2) used by the processing command (processing(1)). SEE ALSO processing(1), screencopy(1), x_grf(1), st_grf(1), Printed 4/6/89 3
TCPRDITHER(5) COMMAND REFERENCE TCPRDITHER(5) tcprconf(5mdqs), tcprgamma(5), grf(5), tcprserver(8mdqs). REFERENCES For more information, consult the Users Manual for your system. Printed 4/6/89 4
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