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Another method, called dithering, is used to improve image display with a small amount of
colors available. Dithering creates additional colors and shades from an existing palette by interspersing
pixels of different colors, as colors cannot be mixed directly like in painting. The
different colors can either be distributed randomly or regularly. The higher the resolution of the
display, the smoother the dithered color will appear to the eye.
2.3 Colors
If we had a controlled color system in which a color could be specified and would be displayed
on all devices in the same way we would not have to care about color. Unfortunately this is not
the case. Color will be different everywhere: on the screen, in print, and when scanning.
Monitors, printers, scanners, exposers, etc. have different characteristics. But let us start at the
beginning.
Colored images on the computer screen are generated using a red, green, and blue (RGB)
component. Due to this it is straight forward to store colors in this so called RGB system. If x
is the number of pixels used per pixel, usually x/3 bits per pixel are used for each of the components.
If x/3 is not an integer, some colors are assigned more bits than the others. For input
the RGB system is also very convenient as charged coupled device (CCD) chips are commonly
used with RGB filters. CCD chips are present in video cameras, digital cameras, and nners. sca
Unfortunately this is different for printing applications because these use a subtractive color
representation, whereas RGB is additive. Additive means that white is represented by the tuple
containing the highest available values and black is represented by (0, 0, 0). In printing it is
exactly the other way round, there the tuple (0, 0, 0, (0)) for the Cyan, Magenta, Yellow,
(Black) (CMY(K)) represents plain white.
There are other color systems that represent colors, for example, with Hue, Saturation, Brightness
(HSB), or Hue, Saturation, Value (HSV) for human convenience, and many others. Very
important are the systems of the Commission Internationale de l'Eclairage (CIE), for example
CIE L*a*b and CIE x*y*z that are based on an analysis of the human visual system. They allow
exact specifications of colors, whereas RGB or CMYK do not. Unfortunately the color
systems cannot directly be mapped to each other. There are colors, e.g. in CMYK, that cannot
be represented in RGB (and vice versa). This implies that an image in print may look different
from the RGB image on the screen. However, even a CMYK image on the screen may look
different to the same CMYK image in print as different devices have different capabilities to
represent colors.
And it is even worse.
Note: Two monitors may display the same image using the same color system
and it may look completely different.
This may be due to the gamma values and the different colors of white (white point) and red,
green and blue (chromacities). There are other minor factors as well.
A non linear correction has to be applied when displaying images, because the brightness
produced by Cathode Ray Tubes (CRTs) is not proportional to the voltage, but to the
voltage gamma . Different monitors have different gamma values. To get the gamma values of a
device, dithered colors (RGB) can be contrasted with not dithered colors. The gamma value
can be determined by comparing them. For example, if a not dithered color looks as the same
color dithered, the monitor's gamma has been corrected to a gamma value of one. To do
gamma adjustment for a value not equal to one a precorrected test image may be used.
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