Displaying colors on various devices

• Color gamuts - Color gamuts represent the universe of colors that can be created or displayed by a given color system or technology.
  
  
  
  
• In the above diagram, color gamuts are depicted for each of three systems:
  
  
  • Colors perceivable by the human visual system would fall within the boundaries of the horseshoe shape derived from the CIE-XYZ color space diagram.
    
    
  • Within that space, colors that can be displayed on an RGB monitor would fall within the red triangle that connects the RGB primary dots.
    
    
  • Colors that can be created using CMYK four-color process lithography would fall within the light blue boundary that connects the CMY primary dots.

  
  

• 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.
  
  
• 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

• Physical differences between inks/subtractive primaries versus phosphors/additive primaries
  
  
• 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
  
  
• 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
  
  
• LCD versus RGB displays - different technologies, different color gamuts
  
  
• NTSC versus RGB displays - resolution, interlacing, color encoding
  
  

Color management systems

• 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
  
  
• For each device to be added to a system, only transforms to/from the intermediate color space need to be defined
  
  
• 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
  
  
• The WWW is not an enterprise

Color reduction

• 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

• 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.

• 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

• 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)
  
  
• GIF standard is 256 colors.
  
  
• Macs and PCs use slightly different palettes for applications (e.g. Windows reserves some colors for window borders, etc.)
  
  
• 216 color "non-dither" WWW palette (see WWW resources) for index color images
  
  
• Use JPEG for transmitting photos
  
  
  • can handle 24 bit color space, and photos will dither OK to 8 bit space
  • but gamma still can be a problem when displaying on various systems
    
    

Gamma correction

• Perceived intensity has a non-linear relationship to applied voltages
  
  
• Voltages derive from numerical values in frame-buffer memory
  
  
• Gamma correction adjusts values (and hence voltages) to provide a perceptually uniform response
  
  
• Gamma correction is not uniformly implemented across all platforms and applications
  
  
• Problems with gamma show up as dark or washed-out or contrasty images when moving from one system or application to another
  
  
• 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
  
  
• 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:

• Tags images with gamma creation information so that browsers can correct gamma for the current display system
  
  
• Has alpha channels for variable transparency
  
  
• Is simpler than TIFF
  
  
• Supports true-color and indexed color modes
  
  
• Generally compresses better than GIF
  
  
• Compression is lossless
  
  
• Browser and application support is substantial and growing
  
  
  

Two disadvantages of PNG compared with GIF or JPEG are:

• Does not support animated images
  
  
• Does not compress as well as JPEG
  
  

Key Points

• Color perception is influenced by a combination of technology, psycho-physiology, and environmental factors
  
  
• 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
  
  
• In the real world, one may need to do iterative testing to determine a successful workflow
  
  
• Gamma continues to be a problem with the WWW
  
  
• 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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