HP DeskWriter C printer driver development - development of Macintosh driver for HP color ink-jet printer - Technical

Hewlett-Packard Journal, August, 1992 by William J. Allen, Toni D. Courville, Steven O. Miller

Grey Balancing

In theory, an equal mixture of cyan, magenta, and yellow inks would produce a neutral (white, grey, or black) color. In practice, this does not occur. The particular inks used by the HP DeskWriter C and DeskJet 500C printers, when mixed in equal proportions, typically produce a color with a slight greenish cast. To compensate for this effect, grey balancing is performed before halftoning.

Grey balancing reduces the amount of cyan used for neutral and near-neutral colors. Decreasing cyan increases the relative amounts of magenta and yellow. Magenta and yellow inks together make red, which is the opponent color of green. One can think of the cyan reduction as an increase in red, which compensates for the greenish cast. Unfortunately, reducing the amount of ink on the page makes the color lighter, so the cyan reduction must be balanced against the loss in darkness. Experimentation showed that a cyan: magenta:yellow ratio of 2:3:3 produces good neutral and black colors over a wide range of papers.

The adjustment is made by first computing the saturation of the color.2 The following equation yields a value for S between 0 and 1:

S = (max(c, m, y) - min(c, m, y)) / max(c, m, y).

In this equation, c, m, and y represent the amounts of cyan, magenta, and yellow inks, respectively. Larger values of S indicate more saturated colors, while 0 indicates a neutral color.

The amount of cyan ink is adjusted based on S. If S is 0, cyan ink is reduced to two thirds of its original value. If S is 1, cyan is unaffected. Cyan ink is adjusted linearly between 100% and 67% for intermediate values of S.

Intensity Slider

Possibly the greatest challenge in creating a plain paper color printer is delivering high-quality output over a range of environmental conditions (temperature and humidity) and on a range of media, from copier paper to high-quality cotton bond. Two problems occur when fully saturated colors are printed on some types of paper, or at high humidities. Color bleed occurs when adjacent colors run into each other and mix. Ink can even bleed through the paper and appear on the back side. This can occur even if shingling is used. Bronzing occurs when too much ink is laid down on the paper and the dye sits on top of the paper fibers rather than soaking in. This overabundance of ink crystalizes on the surface and the crystals reflect light. This causes a shiny reflective surface that actually gets lighter as more ink is laid down.

As it turns out, when these problems arise, reducing the amount of ink used in printing the colors usually results in higher-quality output. Use too much ink and you get the problems described above; use too little, and the output looks dull and washed out.

Other factors affect the user's perception of quality and color accuracy, such as room lighting, computer display variations, personal taste, and the type of data being printed such as line art versus an image. Rather than attempt to characterize all of these factors, we supply a control that is analogous to the brightness control on a television set. This is called the intensity slider.