halftoning notes

1. Halftoning

2. Halftoning
 Newspaper photographs simulate a greyscale, despite the fact that they
have been printed using only black ink.
• A newspaper picture is, in fact, made up of a pattern of tiny black dots of
varying size.
• The human visual system has a tendency to average brightness over small
areas, so the black dots and their white background merge and are
perceived as an intermediate shade of grey.

3. Halftoning
 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.

4. Halftoning
 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.

5. Halftoning

6. Halftoning – dot size

7. Halftoning – Moire Patterns
 Repeated use of same dot pattern
for particular shade results in
repeated pattern
 Perceived as a moire pattern
 Instead, randomize halftone pattern

8. Patterning
 A simple digital halftoning technique known as patterning involves
replacing each pixel by a pattern taken from a 'binary font'.
 We 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.

9. Fig : A 3 x 3 binary font for printing a greyscale.

10. Patterning
 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.
 Figure 5. 2 shows an example of halftoning using the binary font depicted
in Figure 5. 1.

11. Fig: Halftoning with a binary font. (a) 8-bit image (b) halftoned image

12. Dithering
 Halftoning for color images

13. Specifying Color
 Color perception usually involves three quantities:
 Hue: Distinguishes between colors like red, green, blue, etc
 Saturation: How far the color is from a gray of equal intensity
 Lightness: The perceived intensity of a reflecting object
 Sometimes lightness is called brightness if the object is
emitting light instead of reflecting it.
 In order to use color precisely in computer graphics, we
need to be able to specify and measure colors.

14. Combining Colors
Additive (RGB)
Shining colored lights
on a white ball
Subtractive (CMYK)
Mixing paint colors and
illuminating with white light

15. How Do Artists Do It?
 Artists often specify color as tints, shades, and tones of saturated (pure)
pigments
• Tint: Gotten by adding white to a pure pigment, decreasing saturation
• Shade: Gotten by adding
black to a pure pigment,
decreasing lightness
• Tone: Gotten by adding
white and black to a pure
pigment
White
Pure Color
Black
Grays Tones

16. 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

19. Dithering
 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.

20. 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

21. Fig: Halftoning with dither matrices. (a) Input image. (b) Halftoned
image using D1. (c) Halftoned image using D2

22. thanks to all