Working Color Spaces


Working color spaces are designed for ease of image editing. For instance, a neutral gray value is always achieved by having equal values of red, green and blue channels (e.g. rgb(127, 127, 127) ). The 3 primary working color spaces<ref>ColorMatch is another alternative that is often encountered as a working color space option.</ref>, in order of increasing gamut, are:

  1. sRGB
  2. Adobe RGB
  3. ProPhoto RGB

A good deal of controversy exists over which color space is the best in which to work. In reality each has its own set of benefits and drawbacks. There is an important tradeoff between how wide a space's gamut is and how smooth the gradations it can produce. This is the case because there are only a finite set of numbers available to represent different colors. The wider the gamut the more spread out these values are and therefore the more pronounced the step is from one value to the next. To illustrate this, consider two color images with only 4-bits per channel of color representation (i.e. only 16 possible values for each channel).



Because of its wider gamut, the first image representation is able to capture more vivid colors. But also notice that the bands of blocky tones visible in the sky, balloons, and wall. This effect is referred to, aptly, as banding or posterization. Wider gamuts do not come without a cost.

The above example was designed to illustrate the tradeoff between gamut width and gradation smoothness. Typically color images are displayed with a higher color depth of 8-bits or 16-bits per channel - also referred to as the sum of the the 3 channels: 24-bit and 48-bit color, respectively. With 8-bits per channel there are 256 possible values per channel - 16 times as much as 4-bit, so the wide gamut image would maintain its vividness but also be able to capture smooth gradations.

Bringing this back to a practical discussion, ProPhoto RGB is such a wide gamut color space that it necessitates 16-bits per channel; otherwise there is a good risk of banding. ProPhoto RGB is the color space used for Photoshop's Camera RAW processing.

Jumping from 8-bit to 16-bit mode means image files that are twice the size and will also take longer to process. Especially for very large files with many layers, there is a definite advantage to sticking with 8-bit mode. At this point the decision is between sRGB and AdobeRGB. When shooting jpegs with digital cameras, the photos are usually recorded in either either sRGB or AdobRGB and tagged with appropriate profiles. sRGB is almost always the default, but some digital SLR's allow images to be taken in AdobRGB as well.

Of the two, AdobeRGB has the wider gamut, so vivid colors are less likely to be out-of-gamut and therefor clipped. In the exaggerated illustration, AdobeRGB would be like the first image. So the drawback, as with the example, is a greater likelihood of banding. On the other hand, sRGB is more likely to clip saturated colors but is more likely to have smooth gradations of in-gamut values. When colors are clipped, there is also a loss of smooth gradations (this drawback is not captured in the example images above).