This section describes the mechanics of transferring an image or map composition from a data file to a hard-copy map.
Halftoning is the process of converting a continuous tone image into a pattern of dots. A newspaper photograph is a common example of halftoning.
To make a color illustration, halftones in the primary colors (cyan, magenta, and yellow), plus black, are overlaid. The halftone dots of different colors, in close proximity, create the effect of blended colors in much the same way that phosphorescent dots on a color computer monitor combine red, green, and blue to create other colors. By using different patterns of dots, colors can have different intensities. The dots for halftoning are a fixed density—either a dot is there or it is not there.
For scaled maps, each output pixel may contain one or more dot patterns. If a very large image file is being printed onto a small piece of paper, data file pixels are skipped to accommodate the reduction.
The following hard-copy devices use halftoning to output an image or map composition:
- Tektronix Inkjet Printer
- Tektronix Phaser Printer
See the hard-copy device manual for halftone printing information.
Continuous Tone Printing
Continuous tone printing prints color imagery using the four process colors (cyan, magenta, yellow, and black). By using varying percentages of these colors, it is possible to create a wide range of colors. The printer converts digital data from the host computer into a continuous tone image. The quality of the output picture is similar to a photograph. The output is smoother than halftoning because the dots for continuous tone printing can vary in density.
There are different processes by which continuous tone printers generate a map. One example is a process called thermal dye transfer. The entire image or map composition is loaded into the printer’s memory. While the paper moves through the printer, heat is used to transfer the dye from a ribbon, which has the dyes for all of the four process colors, to the paper. The density of the dot depends on the amount of heat applied by the printer to transfer the dye. The amount of heat applied is determined by the brightness values of the input image. This allows the printer to control the amount of dye that is transferred to the paper to create a continuous tone image.
The following hard-copy device uses continuous toning to output an image or map composition:
- Tektronix Phaser II SD
The printers listed here do not necessarily use the thermal dye transfer process to generate a map.
See the hard-copy device manual for continuous tone printing.
Contrast and Color Tables
ERDAS IMAGINE contrast and color tables are used for some printing processes, just as they are used in displaying an image. For continuous raster layers, they are loaded from the ERDAS IMAGINE contrast table. For thematic layers, they are loaded from the color table. The translation of data file values to brightness values is performed entirely by the software program.
RGB to CMY Conversion
Since a printer uses ink instead of light to create a visual image, the primary colors of pigment (cyan, magenta, and yellow) are used in printing, instead of the primary colors of light (red, green, and blue). Cyan, magenta, and yellow can be combined to make black through a subtractive process, whereas the primary colors of light are additive—red, green, and blue combine to make white (Gonzalez and Wintz, 1977).
The data file values that are sent to the printer and the contrast and color tables that accompany the data file are all in the RGB color scheme. The RGB brightness values in the contrast and color tables must be converted to cyan, magenta, and yellow (CMY) values.
The RGB primary colors are the opposites of the CMY colors—meaning, for example, that the presence of cyan in a color means an equal lack of red. To convert the values, each RGB brightness value is subtracted from the maximum brightness value to produce the brightness value for the opposite color. The following equation shows this relationship:
C = MAX - R
M = MAX - G
Y = MAX - B
MAX= maximum brightness value
R = red value from lookup table
G = green value from lookup table
B = blue value from lookup table
C = calculated cyan value
M = calculated magenta value
Y = calculated yellow value
Although, theoretically, cyan, magenta, and yellow combine to create black ink, the color that results is often a dark, muddy brown. Many printers also use black ink for a truer black.
Black ink may not be available on all printers. See the manual for your printer.
Images often appear darker when printed than they do when displayed on the display device. Therefore, it may be beneficial to improve the contrast and brightness of an image before it is printed.
Use the programs discussed in Enhancement to brighten or enhance an image before it is printed.