Appeal No. 1998-0671
Application 08/285,328
the image signal is directly thresholded, (no modulator 1), using
varying threshold values formulated from a screen" (numbers in
brackets added) (Br18). The second screening process (referred to as
dithering or halftoning) is discussed in the specification (pp. 2-3)
and in Eschbach '653 (col. 1, lines 44-57) and also satisfies
limitations (a)-(c) of claim 1.
Now we address the question of motivation for modifying the
screening process of the APA (figure 1 and the direct thresholding at
p. 2 of the specification), the Survey article, and the background of
Eschbach '653 to add error diffusion. In the background of the
invention, Eschbach '653 discloses a problem with screening (col. 1,
line 58 to col. 2, line 10):
Dithering presents problems, however, in that the amount of
gray within an original image is not maintained exactly over an
area, because the finite number of elements inside each
halftone cell only allows the reproduction of a finite number
of gray levels. The error arising from the difference between
the threshold value and the actual gray level value at any
particular cell is simply thrown away. This results in loss of
image information. Dithering creates significant image
artifacts because it ignores this error completely. A well
known example is the banding or false contour artifact that can
be seen in smooth image areas. Here, the image input gray
level varies smoothly over an area while the halftone image has
to make a transition from one halftone dot (gray level) to
another. This transition can clearly be seen as a band running
through smooth image parts.
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