·
The
process of generating a binary pattern of black and white dots from an image is
termed halftoning.
·
In
traditional newspaper and magazine production, this process is carried out
photographically by projection of a transparency through a 'halftone screen'
onto film.
·
The
screen is a glass plate with a grid etched into it.
·
Different
screens can be used to control the size and shape of the dots in the halftoned
image.
·
A
fine grid, with a 'screen frequency' of 200‑300 lines per inch, gives the image
quality necessary for magazine production.
·
A
screen frequency of 85 lines per inch is deemed acceptable for newspapers.
·
A
simple digital halftoning technique known as patterning involves
replacing each pixel by a pattern taken from a 'binary font'.
·
Figure
5. 1 shows such a font, made up of ten 3 x 3 matrices of pixels.
·
This
font can be used to print an image consisting of ten grey levels.
•
A
pixel with a grey level of 0 is replaced by a matrix containing no white
pixels; a pixel with a grey level of 1 is replaced by a matrix containing a
single white pixel; and so on.
•
Note
that, since we are replacing each pixel by a 3 x 3 block of pixels, both the
width and the height of the image increase by a factor of 3.
Another
technique for digital halftoning is dithering.
•
Dithering
can be accomplished by thresholding the image against a dither matrix.
•
The
first two dither matrices, rescaled for application to 8‑bit images, are
•
The
elements of a dither matrix are thresholds.
•
The
matrix is laid like a tile over the entire image and each pixel value is
compared with the corresponding threshold from the matrix.
•
The
pixel becomes white if its value exceeds the threshold or black otherwise.
•
This
approach produces an output image with the same dimensions as the input image,
but with less detail visible.
Algorithm
to halftone an image using a dither matrix.
for all x & y do
if f(x,y) > m(x,y) then
g(x,y) = white
else
g(x,y) = black
end if
End
for
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