Working with Color in CG
Displaying colors on various devices
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Color gamuts - Color
gamuts represent the universe of colors that can be created or
displayed by a given color system or technology.
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In the above diagram, color gamuts are depicted for
each of three systems:
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Colors perceivable by the human visual system
would fall within the boundaries of the horseshoe shape derived from
the CIE-XYZ color space diagram.
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Within that space, colors that can be displayed
on an RGB monitor would fall within the red triangle that connects the
RGB primary dots.
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Colors that can be created using CMYK four-color
process lithography would fall within the light blue boundary that
connects the CMY primary dots.
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Obviously, the full range of humanly perceptible
color is not available with either the RGB monitor or via process color
printing, and neither the RGB monitor nor the four-color printing press
can reproduce all of the colors that are available with the other
technology.
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This latter deficiency is most apparent when moving
from RGB to CMYK, which has an important ramification for desktop
publishers: many colors that can be displayed on your monitor
cannot be created with printing inks.
Display technologies influence how colors are
represented and hence perceived
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Physical differences between inks/subtractive
primaries versus phosphors/additive primaries
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Dot gain - ink can tend to spread slightly depending
on how it lays on or is absorbed by the paper being used, effectively
filling in the spaces around dots and darkening halftones or filling in
fine light details that are surrounded by darker portions of an image
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Phosphor bloom - very light or highly saturated
colors (high applied voltages/highly excited phospors) can similarly
"spread" - light lines on a dark background may appear thicker
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LCD versus RGB displays - different technologies,
different color gamuts
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NTSC versus RGB displays - resolution, interlacing,
color encoding
Color management systems
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An intermediate, device-independent, color model
(e.g. CIE XYZ, CIELAB, CIELUV) is used as a common interchange format,
in conjunction with color calibration profiles and corresponding color
look up tables for devices supported by the system, to help assure that
"what you see is what you get" as you move from device to
device within an enterprise
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For each device to be added to a system, only
transforms to/from the intermediate color space need to be defined
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The goal is to get a given color to "look the
same" from one device to another, but obviously results will be
subject to the gamut limitations of each device and technology
employed accross the enterpirse
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The WWW is not an enterprise
Color reduction
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Translating images from a higher definition color
space to a lower definition color space (i.e. 24 bit to 8 bit color, or
24 bit image displayed on an 8 bit display or making an 8 bit GIF file
from a 24 bit image) can create image artifacts


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Dithering - Image
colors that were defined in the higher definition color space, but that
are not available in the lower definition color space, are approximated
by a dot pattern which arranges different colors from the lower
definition palette in a pixel array to create a perceptual
approximation of the unavailable color.


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Contouring - When
continuous smooth-shaded ramps of color are translated from a higher
definition color space to a lower definition color space, the colors
from the higher definition space are sampled, resulting in the
compression of several adjacent colors from the continuos ramp into a
single color value. The resulting reduction in color increments to make
the transition introduces perceptible bands of discrete color into what
was formerly a perceptually continuous gradation.
Color on the WWW
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Some end-user displays are not true-color displays
and are only capable of displaying 256 simutaneous colors (though the
displayed colors may be chosen from a much larger pallette)
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GIF standard is 256 colors.
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Macs and PCs use slightly different palettes for
applications (e.g. Windows reserves some colors for window borders,
etc.)
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216 color "non-dither" WWW palette (see
WWW resources) for index color images
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Use JPEG for transmitting photos
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can handle 24 bit color space, and photos will
dither OK to 8 bit space
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but gamma still can be a problem when displaying
on various systems
Gamma correction
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Perceived intensity has a non-linear relationship to
applied voltages
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Voltages derive from numerical values in
frame-buffer memory
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Gamma correction adjusts values (and hence voltages)
to provide a perceptually uniform response
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Gamma correction is not uniformly implemented across
all platforms and applications
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Problems with gamma show up as dark or washed-out or
contrasty images when moving from one system or application to another
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In practice within applications, gamma values above
1.0 will increase the brightness of pixels, whereas gamma values
greater than 0.0 but less than 1.0 decrease the brightness of pixels
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Gamma is not the same as brightness (in image
editing applications, for example), which tends to brighten all pixels
by a similar amount. Gamma correction will shift tones around more
proportionally to their existing brightness levels, achieving a very
different result through the tonal range of the image
PNG looks very promising for web interchange of
images, but has not been widely adopted:
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Tags images with gamma creation information so that
browsers can correct gamma for the current display system
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Has alpha channels for variable transparency
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Is simpler than TIFF
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Supports true-color and indexed color modes
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Generally compresses better than GIF
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Compression is lossless
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Browser and application support is substantial and
growing
Two disadvantages of PNG compared with GIF or JPEG are:
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Does not support animated images
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Does not compress as well as JPEG
Key Points
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Color perception is influenced by a combination of
technology, psycho-physiology, and environmental factors
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In an ideal world we could rely on color management
systems to move predictacbly from system to system, device to device
and environment to environment
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In the real world, one may need to do iterative
testing to determine a successful workflow
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Gamma continues to be a problem with the WWW
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By understaning the potential causes of apparent
inconsistencies, we can be better prepared to recognize and deal with
them as they arise
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