E-10 A-61122/A-61124 March 1999
Screening/Dithering
Screening (dithering) is a method used to simulate gray levels.
Screening is expressed in terms of levels, ranging from
2-level to 64-level, with several levels in between.
Screening is a process that groups neighboring pixels together
into a super-pixel. The size of the super pixel is determined by
the screening level chosen (i.e., when a 64-level screen is
chosen, the super-pixel contains 64 pixels; the super-pixel is 8
pixels wide and 8 pixels high). Each pixel within the super-pixel is
assigned a value; either black or white. It is the ratio of black
pixels to white pixels within the super-pixel which simulates
varying gray levels. The smaller the screening level, the smaller
the super-pixel, resulting in higher resolution; similarly, the higher
the screening level, the larger the super-pixel, resulting in lower
resolution.
NOTES:
• Screening is effective for documents containing only
photographic images. Mixed mode/error diffusion is
recommended for documents containing both text and
photographic images. Screening tends to decrease the quality
of scanned text.
• Do not use the Noise filter with screening.
• When screening and compression are used together, negative
compression (less than 1:1) is more likely to occur.
Refer to Chapter 6,
Scanner-unique Commands
for a description
of the LX/Y/Z command.
