Print Modes

Snap to Primaries

While error diffusion is useful for solid images, it is not adequate for line drawings since lines appear intermittent due to "holes" in the dither pattern. The primary mode supports direct generation of lines using the primary colors (red, green, blue, cyan, yellow, magenta, black, and white).

Snap to Black and White

The monochrome mode maps each non-zero pixel to black. This mode works well for line drawings where a constant (black) intensity is desired for each line. This mode does not work well for solids modeling or filled polygons as every non-white pixel maps to black.

Color Ordered Dither or Monochrome Ordered Dither

In order dither, the intensity of each point (x,y) in a pixel matrix depends on the desired intensity at that point I(x,y) and an 8×8 dither matrix. The value of each cell (i,j) in the dither matrix is computed by:

i = x modulo 8
j = y modulo 8

If I(x,y) > D(i,j), the point corresponding to the (x,y) is intensified; otherwise, it is not.

Color Error Diffusion or Monochrome Error Diffusion

The actual intensity of each dot in the output print is determined in a complex manner. A color map index value is obtained for the source pixel currently being processed. Residual errors which have accumulated from previously processed output dots are added to the color map index value to obtain a desired color map index value. The desired color map index value is then tested against a value equivalent to half bright. If the desired value is greater than half bright, this output dot will be turned on; otherwise it will be turned off. If this output dot is turned on, a new error value equal to the desired color map index (minus full bright) is accumulated in adjacent output dots. If the output dot is not turned on, only the desired value is accumulated in adjacent dots. The result of this process is that errors in dot intensity are diffused (or accumulated) over adjacent output dots. This process is repeated for each dot being expanded from the source pixel. When the source pixel expansion is complete, a new color map index value is obtained for the next source pixel, and the process is repeated.

The error diffusion method works well for most color intensities. Certain color intensities result in generation of unwanted patterns. This is most noticeable with gray (r=g=b) in the range of 0.3 to 0.7. Note that this unwanted pattern problem is discussed in ACM Transaction on Graphics, vol. 6, no. 4, October 1987.

Disappearing Lines in Monochromatic Ordered Dither

One result of the dithering method used is that single pixel width lines can disappear. When the pixel is copied from the ordered dither pattern (as discussed above) portions of the source pattern are empty (white). With certain conditions the slope of a single pixel line can be such that it intercepts all black or all white pixels in the dither cell locations being copied. This results in a disappearing line. A similar problem results in a line appearing as random size strings of dots.

This mode was designed to be used with solids and polygons rather than with lines. If the bitmap you desire to print consists of lines you should use monochrome mode, possibly with no background.

Differences When Printing Single Planes

The two modes, snap to black and white and monochrome ordered dither, have quite different effects when printing from a bitmap which consists of monochromatic foreground and background. Essentially monochromatic ordered dither mode tries to approximate the actual display as closely as possible in shades of gray. As a result, a display that consists of white text on a black background will be printed faithfully. That is, the black background will be printed full black, while the white letters will not be printed (white being the absence of subtractive color). Conversely, snap to black and white mode will print the foreground (that is, the letters) in black and not print the background. The resulting prints will (correctly) appear to be reversed images of each other.

Controlling Print Orientation

The default print orientation is left to right across the length of the paper (equivalent to the LaserJet landscape mode). You can cause the HP-UX commands pcltrans and screenpr to orient the print across the width of the paper by using the -R option.

Print Size and Clipping

By default, the printer will scale the image to the size of the paper used. However, the user can select the destination size of the image by using the -x, -y, -d, and -h options of the pcltrans and screenpr HP-UX commands.

Paper size is automatically sensed by some devices, such as the PaintJet XL printer. However, the PaintJet XL300 requires the -m option to select a page size because the device does not automatically sense what size media is installed.

Refer to the Starbase Reference manual for detailed option information on the pcltrans and screenpr HP-UX commands.