RGB

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RGB stands for Red, Green, and Blue colors and describes a system of color that is the primary method of color generation used on electronic devices. Refer to the Color page for a background. This page will develop some more advanced color topics.

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[edit] History

The initial color electronics device was the TV which appeared in color during the 1950's. It used a basic analog CRT using 3 electron guns which lit color pixels on the the TV surface. It was well known that even in this color system the intensity of the brightness was not linear with the Voltage applied to the tube. Adjustments were built into the electronics to compensate for this difference. Eventual this adjustment would become what we now call the color profile. Note that the TV signal itself did not broadcast the RGB values. They were encoded in the TV signal along with the brightness (luminance) and syncing signals that were all decoded inside the TV set.

In the 70s color began to appear on computer displays. Initially it was only a few colors with a total of perhaps 4 colors, including black and white. Additional effort brought a few more colors up to 16, but real useful colors didn't appear until the VGA format was developed in the late 80s and early 90s. The VGA CRT screen was analog as well and worked similarly to the TV screen. It had a resolution of 640 x 480 pixels. There was an electronic card that translated the digital information into the analog screen. The card could support 4 bits for color which allowed a total of 16 colors on the screen. The colors were determined separately using a palette. The palette devoted 6 bits to each color R,G, and B which allowed 262,144 different colors but you could only pick 16 of them. A lower resolution 320 x 200 screen would support 256 separate colors. As resolution of screens increased the images developed to the point that paintings and photographs became possible. Having 256 separate colors and a palette with enough color choices, depth, such as 18 bits can provide excellent pictures. This technique is used in GIF images where the depth of the palette is set by the user. PNG can also support a palette approach.

In the 90s color displays took on a new dimension when LCD color displays appeared. All modern displays are a variation on these LCD techniques. For the first time, the digital value remained digital until the display itself converted it to colors. Typically these displays are RGB with 8 bits per color although newer devices are now offering wide color with 10 bits per color. The source data continues to be organized differently than the resultant display data. (See Graphics for some possible formats.) An exception is the BMP format where the RGB values are stored exactly the same as the displayed data.

[edit] Limitations

While theoretically you could define all the colors using the three primary colors there are more colors that are outside any triangle you might choose even though you could choose a starting point for each color that may or may not be visible such as in the infrared range.

[edit] brightness

A second limitation is that an image has more to it that just colors. The illumination or brightness of the image is also an important consideration. Often a screen display will offer a separate brightness control but this is an artificial display brightness, not the inherent brightness of the image itself.

RGB represents the brightness (illumination) level by assuming full color is as bright as it gets and then reducing all of the color values equally for lower levels of illumination. For example all values of 255, 255, 255 for each color is full white while all values of 128, 128, 128 for each color would look gray. This could be because the object is gray or it could be a white color in a dark area. For this reason there are other coding schemes that attempt to capture the brightness separately. JPG images treat color and brightness with different criteria but when it comes to the display of the final picture on a screen it is converted to RGB. There is also a coding called RGBW where the W is White which stores the brightness level. This can be particularly valuable when less color depth is used. For example the color E Ink Triton display uses 4 bits for each color but does have an additional 4 bits used to store the brightness.

In addition the color display is not linear in the human eye thus the increments from 0 to 255 are not strictly linear values. They use an exponential ratio (averaged about 2.2) to adjust for the eye and also for the medium being used. For example a CRT display has non-linear results for the different primary colors which will be different if you change the type of display. See HDR for more limitations related to brightness.

[edit] color profile

A color profile is a method of describing the information needed to standardize the definition of the color. This description allows various color models to display the same colors for the user. This metadata can be stored with the image itself. If there is no metadata the sRGB standard is assumed. ICC (International Color Consortium) profiles are designed so you can convert an RGB color scheme to a CMYK color scheme showing the same visual colors, or any other color scheme which has a defined profile. Several profiles are shown below (same picture as on color page.)

RGBgamutcomparison.png

[edit] For more information

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