4-bit Printout

Abstract

Printer size variables are the same for 4-bit as for 1-bit images. Ideally, a printer should convert the three additive colours (Blue, Green, Red) to the complementary subtractive inks (Yellow, Magenta, Cyan). But deficiencies in the available inks means that Black ink has to be used also, and the Black component removed from the other three inks. A calculation for this conversion is defined. Printing then requires separation of the four ink components as 8-bit grey-scale sub-images, and then half-toning each to 1-bit sub-images. The 4-bit intensity variables are defined, with examples of colours, separation, and half-toning. The spreading of ink when a half-tone is printed causes dot gain, which acts in a similar manner to display gamma. The flowchart (Fig. 35.1) shows a 4-bit image file sent to a printer by an image editor.