<page xmlns="http://projectmallard.org/1.0/"
      xmlns:its="http://www.w3.org/2005/11/its"
      type="topic" style="question"
      id="color-whatisspace">

  <info>
    <link type="guide" xref="color#profiles"/>
    <link type="seealso" xref="color-whatisprofile"/>
    <desc>A color space is a defined range of colors.</desc>

    <credit type="author">
      <name>Richard Hughes</name>
      <email>[email protected]</email>
    </credit>
    <include href="legal.xml" xmlns="http://www.w3.org/2001/XInclude"/>
  </info>

  <title>What is a color space?</title>

  <p>
    A color space is a defined range of colors.
    Well known color spaces include sRGB, AdobeRGB and ProPhotoRGB.
  </p>

  <p>
    The human visual system is not a simple RGB sensor, but we can
    approximate how the eye responds with a CIE 1931 chromaticity diagram
    that shows the human visual response as a horse-shoe shape.
    You can see that in human vision there are many more shades of green
    detected than blue or red.
    With a trichromatic color space like RGB we represent the colors
    on the computer using three values, which restricts up to encoding
    a <em>triangle</em> of colors.
  </p>

  <note>
    <p>
      Using models such as a CIE 1931 chromaticity diagram is a huge
      simplification of the human visual system, and real gamuts are
      expressed as 3D hulls, rather than 2D projections.
      A 2D projection of a 3D shape can sometimes be misleading, so if
      you want to see the 3D hull, use the <code>gcm-viewer</code>
      application.
    </p>
  </note>

  <figure>
    <desc>sRGB, AdobeRGB and ProPhotoRGB represented by white triangles</desc>
    <media its:translate="no" type="image" mime="image/png" src="figures/color-space.png"/>
  </figure>

  <p>
    First, looking at sRGB, which is the smallest space and can encode
    the least number of colors.
    It is an approximation of a 10 year old CRT display, and so most
    modern monitors can easily display more colors than this.
    sRGB is a <em>least-common-denominator</em> standard and is used
    in a large number of applications (including the Internet).
  </p>
  <p>
    AdobeRGB is frequently used as an <em>editing space</em>.
    It can encode more colors than sRGB, which means you can change
    colors in a photograph without worrying too much that the most vivid
    colors are being clipped or the blacks crushed.
  </p>
  <p>
    ProPhoto is the largest space available and is frequently used for
    document archival.
    It can encode nearly the whole range of colors detected by the human
    eye, and even encode colors that the eye cannot detect!
  </p>

  <p>
    Now, if ProPhoto is clearly better, why don’t we use it for everything?
    The answer is to do with <em>quantization</em>.
    If you only have 8 bits (256 levels) to encode each channel, then a
    larger range is going to have bigger steps between each value.
  </p>
  <p>
    Bigger steps mean a larger error between the captured color and the
    stored color, and for some colors this is a big problem.
    It turns out that key colors, like skin colors are very important,
    and even small errors will make untrained viewers notice that something
    in a photograph looks wrong.
  </p>
  <p>
    Of course, using a 16 bit image is going to leave many more steps and
    a much smaller quantization error, but this doubles the size of each
    image file.
    Most content in existence today is 8bpp, i.e. 8 bits-per-pixel.
  </p>
  <p>
    Color management is a process for converting from one color space to
    another, where a color space can be a well known defined space like
    sRGB, or a custom space such as your monitor or printer profile.
  </p>

</page>