86 lines
4.4 KiB
HTML
Executable File
86 lines
4.4 KiB
HTML
Executable File
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<title>Calibration vs. Characterization</title>
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<meta http-equiv="content-type" content="text/html;
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charset=windows-1252">
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</head>
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<body>
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<h2 style="text-decoration: underline; font-weight: bold;">Calibration
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vs.
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Characterization<br>
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</h2>
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Some of the terminology can be confusing. Many people are initially
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confused about the difference between <span style="font-weight:
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bold;">Calibration</span> and <span style="font-weight: bold;">Characterization/Profiling</span>.<br>
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<h3>What is Calibration ?</h3>
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"Calibration" is a short hand Graphic Arts term for adjusting a
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devices behavior to meet calibration targets.<br>
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Calibration is the process of modifying the color behavior of a
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device. This is typically done using two mechanisms:<br>
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1) Changing controls or internal settings that it
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has.<br>
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2) Applying curves to its color channels.<br>
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<br>
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The idea of calibration is to put a device in a defined state with
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regard to its color response. Often this is used as a day to day
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means of maintaining reproducible behavior. Calibration is often the
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most practical way of setting parameters such as white point and
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brightness of displays. Typically calibration will be stored in
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device or systems specific file formats that record the device
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settings and/or per channel calibration curves.<br>
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<br>
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For some specific applications (such as Video playback), calibration
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may be used to allow a video signal with a specific encoding (i.e.
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colorspace) to display correctly on a display without color
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management software or hardware being needed to transform between
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color spaces. For each source colorspace, a specific calibration
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needs to be made for the display, and the display can only show one
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source correctly at a time. If you have N source color spaces, and M
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displays, then N x M calibrations would need to be created to
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display any source on any display.
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<h3>What is Characterization/Profiling ?</h3>
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Characterization (or <span style="font-weight: bold;">profiling</span>)
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is <span style="text-decoration: underline;">recording</span> the
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way a device reproduces or responds to color. Typically the result
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is stored in a device <span style="font-weight: bold;">ICC</span>
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profile. Such a profile does not in itself modify color in any way.
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What it does is allow a system such as a CMM (Color Management
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Module) or color aware application to modify color when combined
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with another device profile. Only by knowing the characteristics of
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two devices or colorspaces, can a way of transferring color from one
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device representation to another be achieved.<br>
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<br>
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Given a source profile and a destination (i.e. display) profile, a
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direct transform can be computed between the two. In ICC terminology
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this is a <b>Device Link</b>.<br>
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The benefit of managing color this way is that such transforms can
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be created on the fly between any source colorspace and the display,
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and all the results rendered into the same output. Many sources with
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different color spaces, or color spaces that are unknown at the time
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of profiling the display can be rendered correctly. If you have N
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source colorspaces and M displays, then only N + M profiles need to
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be created to display any source on any display.<br>
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<br>
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Note that a characterization (profile) will only be valid for a
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device if it is in the same state of calibration as it was when it
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was characterized/profiled.<br>
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<h3>What about display calibration and profiles ?</h3>
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In the case of display profiles there is some additional confusion
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because often the <span style="font-weight: bold;">calibration</span>
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information is stored in the <span style="font-weight: bold;">profile</span>
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for convenience. By convention it is stored in a tag called the
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'vcgt' tag. Although it is stored in the profile, none of the normal
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<span style="font-weight: bold;">ICC</span> based tools or
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applications are aware of it, or do anything with it, it is just
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"along for the ride". Similarly, typical display calibration tools
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and applications will not be aware of, or do anything with the ICC
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characterization (profile) information.<br>
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<br>
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<br>
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</body>
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</html>
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