improve colour docs
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John Cupitt
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@ -57,20 +57,33 @@
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* spaces, calculate colour differences, and move
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* to and from device spaces.
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*
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* All operations process colour from the first few bands and pass other bands
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* through unaltered. This means you can operate on images with alpha channels
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* safely. If you move to or from 16-bit RGB, any alpha channels are rescaled
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* for you.
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*
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* Radiance images have four 8-bits bands and store 8 bits of R, G and B and
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* another 8 bits of exponent, common to all channels. They are widely used in
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* the HDR imaging community.
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*
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*
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* The colour functions can be divided into three main groups. First,
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* functions to transform images between the different colour spaces supported
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* by VIPS: <emphasis>sRGB</emphasis>,
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* <emphasis>scRGB</emphasis>,
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* <emphasis>XYZ</emphasis>, <emphasis>Yxy</emphasis>,
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* <emphasis>Lab</emphasis>, <emphasis>LabQ</emphasis>,
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* <emphasis>LabS</emphasis>, <emphasis>LCh</emphasis> and
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* <emphasis>CMC</emphasis>). Use vips_colourspace() to move an image to a
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* by VIPS: #VIPS_INTERPRETATION_sRGB, #VIPS_INTERPRETATION_scRGB,
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* #VIPS_INTERPRETATION_B_W,
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* #VIPS_INTERPRETATION_XYZ, #VIPS_INTERPRETATION_YXY,
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* #VIPS_INTERPRETATION_LAB,
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* #VIPS_INTERPRETATION_LCH, and
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* #VIPS_INTERPRETATION_CMC.
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*
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* There are also a set of minor colourspaces which are one of the above in a
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* slightly different format:
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* #VIPS_INTERPRETATION_LAB, #VIPS_INTERPRETATION_LABQ,
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* #VIPS_INTERPRETATION_LABS, #VIPS_INTERPRETATION_LCH,
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* #VIPS_INTERPRETATION_RGB16, and #VIPS_INTERPRETATION_GREY16.
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*
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* Use vips_colourspace() to move an image to a
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* target colourspace using the best sequence of colour transform operations.
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*
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* Secondly, there are a set of operations for
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* calculating colour difference metrics. Finally, VIPS wraps LittleCMS and
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* uses it to provide a set of operations for reading and writing images with
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@ -84,100 +97,47 @@
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*
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* The colour spaces supported by VIPS are:
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*
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* <itemizedlist>
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* <listitem>
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* <para>
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* <emphasis><code>LabQ</code></emphasis>
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* * #VIPS_INTERPRETATION_LAB -- CIELAB '76 colourspace with a D65 white. This
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* uses three floats for each band, and bands have the obvious range.
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*
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* This is the principal VIPS colorimetric storage format.
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* LabQ images have four 8-bit bands and store 10 bits of L and 11 bits
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* of a and b.
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* There are two
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* variants, #VIPS_INTERPRETATION_LABQ and #VIPS_INTERPRETATION_LABS, which
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* use ints to store values. These are less precise, but can be quicker to
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* store and process.
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*
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* You cannot perform calculations on <code>LabQ</code> images (they are
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* tagged with %VIPS_CODING_LABQ), though a few operations such as
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* vips_extract_area() will work directly with them.
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* </para>
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* </listitem>
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* <listitem>
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* <para>
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* <emphasis><code>LabS</code></emphasis>
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* #VIPS_INTERPRETATION_LCH is the same, but with a*b* as polar coordinates.
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* Hue is expressed in degrees.
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*
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* This format represents coordinates in CIELAB space as a
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* three-band #VIPS_FORMAT_SHORT image, scaled to fit the full range of
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* bits. It is the best format for computation, being relatively
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* compact, quick, and accurate. Colour values expressed in this way
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* are hard to visualise.
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* </para>
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* </listitem>
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* <listitem>
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* <para>
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* <emphasis><code>Lab</code></emphasis>
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* * #VIPS_INTERPRETATION_XYZ -- CIE XYZ. This uses three floats.
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* See #VIPS_D75_X0 and friends for values for the ranges
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* under various illuminants.
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*
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* Lab colourspace represents CIELAB colour values with a three-band
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* #VIPS_FORMAT_FLOAT image. This is the simplest format for general
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* work: adding the constant 50 to the L channel, for example, has the
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* expected result.
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* #VIPS_INTERPRETATION_YXY is the same, but with little x and y.
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*
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* VIPS uses D65 LAB, but you can use other colour temperatures with a
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* little effort, see vips_XYZ2Lab().
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* </para>
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* </listitem>
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* <listitem>
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* <para>
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* <emphasis><code>XYZ</code></emphasis>
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* * #VIPS_INTERPRETATION_scRGB -- a linear colourspace with the sRGB
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* primaries. This is useful if you need linear light and don't care
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* much what the primaries are.
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*
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* CIE XYZ colour space represented as a three-band #VIPS_FORMAT_FLOAT
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* image.
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* </para>
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* </listitem>
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* <listitem>
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* <para>
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* <emphasis><code>Yxy</code></emphasis>
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* Linearization is performed with the usual sRGB equations, see below.
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*
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* CIE Yxy colour space represented as a three-band #VIPS_FORMAT_FLOAT
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* image.
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* </para>
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* </listitem>
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* * #VIPS_INTERPRETATION_sRGB -- the standard sRGB colourspace, see:
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* [wikipedia sRGB](http://en.wikipedia.org/wiki/SRGB).
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*
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* <listitem>
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* <para>
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* <emphasis><code>sRGB</code></emphasis>
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* This uses three 8-bit values for each of RGB.
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*
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* VIPS converts XYZ to and from sRGB using the usual formula:
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* #VIPS_INTERPRETATION_RGB16 is the same, but using three 16-bit values for
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* RGB.
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*
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* http://en.wikipedia.org/wiki/SRGB
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* * #VIPS_INTERPRETATION_B_W -- a monochrome image, roughly G from sRGB.
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* The grey value is
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* calculated in #VIPS_INTERPRETATION_scRGB space with the usual 0.2, 0.7, 0.1
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* RGB ratios.
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*
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* You can also use vips_icc_transform() and friends to go to and from
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* device space with a generic profile.
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* </para>
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* </listitem>
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* <listitem>
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* <para>
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* <emphasis><code>scRGB</code></emphasis>
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* #VIPS_INTERPRETATION_GREY16 is the same, but using 16-bits.
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*
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* scRGB is sRGB bit with linear primaries. It uses floats in the range
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* 0 - 1. It's convenient if you need a linear colourspace but don't
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* care too much what the primaries are.
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* </para>
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* </listitem>
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*
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* <listitem>
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* <para>
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* <emphasis><code>LCh</code></emphasis>
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*
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* Like <code>Lab</code>, but rectangular <code>ab</code> coordinates
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* are replaced with
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* polar <code>Ch</code> (Chroma and hue) coordinates.
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* Hue angles are expressed in degrees.
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* </para>
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* </listitem>
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* <listitem>
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* <para>
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* <emphasis><code>CMC</code></emphasis>
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*
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* A colour space based on the CMC(1:1) colour difference measurement.
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* This is a highly uniform colour space, much better than CIELAB for
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* expressing small differences.
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* * #VIPS_INTERPRETATION_CMC -- a colour space based on the CMC(1:1)
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* colour difference measurement. This is a highly uniform colour space,
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* much better than CIELAB for expressing small differences.
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*
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* The CMC colourspace is described in "Uniform Colour Space Based on the
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* CMC(l:c) Colour-difference Formula", M R Luo and B Rigg, Journal of the
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@ -187,9 +147,7 @@
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*
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* You can calculate metrics like CMC(2:1) by scaling the spaces before
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* finding differences.
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* </para>
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* </listitem>
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* </itemizedlist>
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*
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*/
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/* Areas under curves for Dxx. 2 degree observer.
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