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<?xml version="1.0" encoding="utf-8"?>
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<page xmlns="http://projectmallard.org/1.0/" type="topic" style="question" id="color-whatisspace" xml:lang="it">
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<link type="guide" xref="color#profiles"/>
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<link type="seealso" xref="color-whatisprofile"/>
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<desc>A color space is a defined range of colors.</desc>
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<name>Richard Hughes</name>
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<email>richard@hughsie.com</email>
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<include xmlns="http://www.w3.org/2001/XInclude" href="legal.xml"/>
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<mal:credit xmlns:mal="http://projectmallard.org/1.0/" type="translator copyright">
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<mal:name>Luca Ferretti</mal:name>
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<mal:email>lferrett@gnome.org</mal:email>
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<mal:years>2011</mal:years>
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<title>What is a color space?</title>
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A color space is a defined range of colors.
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Well known color spaces include sRGB, AdobeRGB and ProPhotoRGB.
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The human visual system is not a simple RGB sensor, but we can
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approximate how the eye responds with a CIE 1931 chromaticity diagram
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that shows the human visual response as a horse-shoe shape.
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You can see that in human vision there are many more shades of green
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detected than blue or red.
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With a trichromatic color space like RGB we represent the colors
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on the computer using three values, which restricts up to encoding
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a <em>triangle</em> of colors.
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Using models such as a CIE 1931 chromaticity diagram is a huge
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simplification of the human visual system, and real gamuts are
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expressed as 3D hulls, rather than 2D projections.
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A 2D projection of a 3D shape can sometimes be misleading, so if
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you want to see the 3D hull, use the <code>gcm-viewer</code>
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<desc>sRGB, AdobeRGB and ProPhotoRGB represented by white triangles</desc>
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<media type="image" mime="image/png" src="figures/color-space.png"/>
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First, looking at sRGB, which is the smallest space and can encode
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the least number of colors.
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It is an approximation of a 10 year old CRT display, and so most
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modern monitors can easily display more colors than this.
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sRGB is a <em>least-common-denominator</em> standard and is used
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in a large number of applications (including the Internet).
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AdobeRGB is frequently used as an <em>editing space</em>.
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It can encode more colors than sRGB, which means you can change
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colors in a photograph without worrying too much that the most vivid
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colors are being clipped or the blacks crushed.
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PhoPhoto is the largest space available and is frequently used for
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It can encode nearly the whole range of colors detected by the human
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eye, and even encode colors that the eye cannot detect!
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Now, if PhoPhoto is clearly better, why don't we use it for everything?
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The answer is to do with <em>quantization</em>.
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If you only have 8 bits (256 levels) to encode each channel, then a
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larger range is going to have bigger steps between each value.
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Bigger steps mean a larger error between the captured color and the
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stored color, and for some colors this is a big problem.
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It turns out that key colors, like skin colors are very important,
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and even small errors will make untrained viewers notice that something
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in a photograph looks wrong.
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Of course, using a 16 bit image is going to leave many more steps and
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a much smaller quantization error, but this doubles the size of each
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Most content in existance today is 8bpp, i.e. 8 bits-per-pixel.
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Color management is a process for converting from one color space to
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another, where a color space can be a well known defined space like
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sRGB, or a custom space such as your monitor or printer profile.
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