Digital Images

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Digital Images
The fundamental properties of the digital
photographic image.   Monochrome Images
Color Images What is a halftone?  
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Display of Images
Images are normally viewed on a monitor employing
cathode-ray tube ( CRT) technology .
The phosphor dots are arranged into triangular
groups of three ( R,G and B dots). See Figure 1.
They are sufficiently small to be unresolvable by
a human eye, instead , what we perceive at each
point on the screen is a mixture of light from
the red, green and blue phosphors of a triplet.
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Cathode-ray Tube (CRT)

• Colors are represented using Red, Green, and Blue
  components.
• The CRT has a mechanism for displaying these
  three components Figure 1.
• Grayscale .The Table list the specifications for
  intensity codes for a four level grayscale
  system. With 3 bits per pixel, we can accommodate
  8 gray levels.

 
 
Intensity 0.5min(r,g,b) max(r,g,b)
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In the RGB color model, the frame buffer stores
the RGB components for every pixel
Figure 2
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In the indexed color model, the frame buffer
stores an index into a color-lookup table
Figure 3
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Gamma

• Intensity output is not a linear function of
  voltage input
• The shape of the curve may be represented by a
  power function y x1/?
• - x is the input voltage
• - ? is the gamma parameter - typically 2.2
• - y is the light intensity output
• http//www.libpng.org/pub/png/spec/PNG-GammaAppend
  ix.html
• http//www.inforamp.net/7Epoynton/index.html

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Gamma Correction
When we "compute" colors we generally assume that
they are linear quanities.
Unfortunately, most display devices are
nonlinear. The most common correction method is
called gamma correction.
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High resolution 24-bit images display 16.7
million colors.
Each pixel in a 24-bit image has one of 256
brightness values for red, green and blue.
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How many bits do we need to store an image?
For number of bits b, we need to store an image
of size N x N with 2m different grey levels is
bN x N x m So for a typical 512 x 512 image
with 256 grey levels (m8) we need 2,097,152
bits. That is why we often try to reduce m and N
without significant loss in the quality of the
picture.
The resolution of an image expresses how much
detail we can see in it and clearly depends on
both N and m. Keeping m constant and decreasing N
results in the checkerboard effect. Keeping N
constant and reducing m results in false
contouring.
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Common Values
Table 1 Common values of digital image
parameters Quite frequently we see cases of
MN2K where K 8,9,10. This can be motivated
by digital circuitry or by the use of certain
algorithms such as the (fast) Fourier transform .
The number of distinct gray levels is usually a
power of 2, that is, L2B where B is the number
of bits in the binary representation of the
brightness levels. When Bgt1 we speak of a
gray-level image when B1 we speak of a binary
image. In a binary image there are just two gray
levels which can be referred to, for example, as
"black" and "white" or "0" and "1".
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What resolution should I use for my images?
This is a frustrating question because it all
depends on your final print output, and more
specifically, what line screen frequency, better
known as lpi, your commercial printer uses. Since
most people pick a printer after their project
has been completed, this can be tricky. Use a
resolution that is too low, and you end up with
poor quality graphics. With a resolution that is
too high, you end up with longer process time and
little gain on the overall quality of the
halftone. There are, however, some typical line
screen frequencies that are used as general
guidelines Newspapers or publications printed
on newspaper stock use a 65 or 85 lpiBooks
printed on uncoated stock use a 120 or 133
lpiBooks and magazines printed on coated stock
use a 133 or 150 lpiHigh quality books with
lots of graphics (art books), calendars, etc. on
coated stock will use 150 or higher lpi
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What resolution should I use for my images?
When determining resolution, the basic rule is
that final image resolution should be twice that
of your intended commercial printer's lpi. In
general, a higher screen frequency produces a
smoother, more detailed image. However, for
certain types of paper a higher screen frequency
is not better. Typically newsprint uses an LPI of
85. 133 is the norm for glossy paper such as in
magazines. Check with your printer to determine
the best LPI settings to use for their equipment
and your paper. So, for example, if you are
creating a family history book with black and
white images, you can't go wrong with an image
resolution of 300 ppi. However, 267 ppi is the
most popular ppi for most book projects with
grayscale graphics.
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The limit to display 24-bit RGM images
There is a trade-off between spatial and color
resolution . If spatial resolution is increased
so that larger images can be displayed on-screen,
then color resolution may need to be reduced.
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What is a halftone?
Halftoning is the process of turning continuous
tone grayscale or color images into a series of
dots for printing that fool the eye. Halftones
are images made up of a series of dots in a
specific pattern that simulates the look of a
continuous tone image. Because printers cannot
print continuous tones -- whether it's the many
shades of gray in a grayscale image or the
millions of colors in a color photograph --
images must be converted to halftones. Another
term for halftoning is dithering
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White Paper, Black Ink
Some ideas can be conveyed effectively with only
the most basic materials. For example, the theme
of "Good vs. Evil" is easy to represent
graphically using only white paper and black ink.
Where we want to illustrate "good," we allow the
white paper to remain pure and unblemished. 
Where we want to illustrate evil, we smother the
paper with black ink.
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Halftone Screening 
The solution to our dilemma is to use "halftone
screening" a process that allows us to
approximate shades of gray while using only black
ink. If we want to see a light gray, we print a
bunch of small black dots, evenly spaced. If we
want to see a medium gray, we print larger black
dots. If we want a dark gray, we print black dots
that are large enough to overlap, covering most,
but not all of the surface with black ink. If we
want black, the spaces in between the dots
disappear, and the ink covers the surface
completely. The dots are printed small enough to
escape the individual attention of the human eye.
The eye mixes the color of the ink and the color
of the paper showing through together. We call
the mixture a "tint" of the original ink, a term
that comes down to us from the traditional
practice of mixing colored paint with white to
produce a lighter color
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Patterning
The previous arrangement uses cells of 4 pixels
each, and yields 5 possible gray-levels. The
disadvantage of patterning is the loss of spatial
resolution, making it acceptable only if the
resolution of the image is lower than the
resolution of the display. The use of patterning
with the previous cells, results in a new image,
with doubled dimensions. To obtain 256x256 new
images, we have to use 128x128 original images
Left original image 128x128, 5 gray levels
Right resultant 256x256 binary image
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Patterning
Left original image 128x128, 5 gray levels
Right resultant 256x256 binary image
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The most used halftoning techniques
The images we will work on are given below
Left 256 gray-levels bird's image Right some
gray values
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Thresholding
The simplest technique for improving visual
resolution is to use a threshold pattern. We
create this pattern and compare with the original
image. If the pixel in the original image exceeds
the correspondent pixel in the threshold pattern,
it is replaced by 1(white), otherwise, it is
replaced by 0 ( black).
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Constant Threshold
In this case, the treshold pattern is a constant.
For example, if T 127, we have the following
results
Left binary bird Right the levels are divided
in 2 black or white
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Gaussian Pattern
In this step, we change the constant pattern by
the gaussian pattern showed below
Pattern for threshold each cell in the image
represents a gaussian curve
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Gaussian Pattern
The results obtained with this pattern are showed
below
Left binary bird Right the gray-levels,
binarized by thresholding with the gaussian
pattern
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Noise pattern for thresholding
To better see the effect of the threshold's
pattern in the resultant binary image, we can,
for example, use a noise pattern
The results, as can be seen below, are noisy
binary images
Left binary bird Right the gray-levels,
binarized by thresholding with the noise pattern
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Order Dithering
This technique attempts to introduce a random
error into the image. This error is represented
by a dither matrix, which is compared with the
image in a repeating checkerboard pattern
(actually, we do the threshold with the pattern
given by the dither matrix). The smallest
ordered dither pattern is
A nn gray levels image can be reproduced from a
dither pattern Dn. Our examples are images of
256 gray levels so, to use D8, we must reduce
our number of gray levels to 64.
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Order Dithering
The elements of the 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
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Order Dithering
The pattern given by D8 is
 The results, as can be seen below, are better
than the results with the thresholding
techniques.
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Which graphic file format is best for halftones?
Macintosh or PC, you can't go wrong with TIFF
format, and it is the preferred and the most
widely supported printing format. PICT files are
sometimes used and have been known to cause
problems in QuarkXPress. Avoid PICT stick with
TIFF. Even if you are able to place a JPEG image
into a desktop publishing application like Quark
or Pagemaker, convert it to TIFF first, and
you'll save yourself some heartache later.
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See http//www.markschulze.net/halftone/index.ht
ml http//webdevfp.uwyo.edu/webdesign/graphics/04-
tech/dithering13.html http//www.webdesignref.com/
examples/dither.htm For color image
see http//www.techcolor.com/help/separations.htm
l