@node Dither, Do, Device, Commands

@iftex
@syntax Dither.
@end iftex
@example
Syntax: DITHER x y min max fac
@end example
@pindex Dither
@findex 2-dimensional graphics, dither

Generates a dithered represention of an image read by the IMAGE command
(@pxref{Image}). The vectors x and y are set to points whose density
is proportional to the image's intensity (only values between min and
max are considered).

Any pixels whose value is @code{NaN} are ignored, as are any whose value
equals the SM variable @code{$missing_data}; by default this is some
very large number.

The dithering algorithm used is Floyd-Steinberg, and each pixel of
the image is represented by (up to) fac*fac points, where (of course)
all fac*fac points are only used for intensities greater than equal to
max. Even the case fac == 1 can produce nice output for suitably sampled
images.

The resulting vectors are usually plotted as @code{ptype 1 1}.

An example of dithering an image would be:
@example
IMAGE image_file
DITHER x y 0 3000 4
PTYPE 1 1
POINTS x y
@end example
@noindent
Of course, you can be more sophisticated; for example:
@example
define min 3000  define max 60000  define gamma 2
set ii = image[*,*]
set ii=(ii < $min ? 0 : ii > $max ? 1 : (ii-$min)/($max - $min)
set image[*,*] = (1 + $gamma)*ii/(1 + $gamma*ii)
dither x y 0 1 4
define v local foreach v (x y) @{
   set $v=$v + 0.5*random(dimen($v))
@}
POINTS x y
@end example
@noindent
or
@example
DITHER x y 0 3000 1
expand 0.3 + IMAGE(x,y)/3000
PTYPE 4 3
POINTS x y
@end example

See also IMAGE for reading images, CONTOUR for contouring them, IMAGE CURSOR
for using cursors to get values from images, MINMAX for finding the minimum
and maximum of images, and Arithmetic for extracting cross-sections of images.