mirror of https://github.com/GNOME/gimp.git
274 lines
10 KiB
Plaintext
274 lines
10 KiB
Plaintext
===================================
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Compositing and layer modes in GIMP
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===================================
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This document describes the process of compositing layers and the layer modes
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in GIMP.
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License
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-------
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This is free documentation; you can modify and/or redistribute
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it according to the terms of the GNU General Public License
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as published by the Free Software Foundation, either version
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2 of the license, or (at your option) any later version.
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About this document
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-------------------
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This document was originally written by Henning Makholm and part of the
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XCF file format specification. Because the topics here are more general
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in the context of GIMP they have been moved into a separate document.
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9. COMPOSITING AND LAYER MODES
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===============================
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This section describes the "flattening" process that GIMP applies
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when a multi-layer image is displayed in the editor or exported to
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other image file formats. It is present for reference only; an XCF
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consumer may well elect to do something different with pixel data from
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the layers than flattening them.
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Most XCF consumers will need to react to the layer mode property of
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each layer; such a reaction must be informed by knowledge of how the
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different layer modes affect the flattening process. In some
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applications it might be acceptable for an XCF consumer to refuse
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processing images with layer modes other than "Normal", but such an
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application will probably not be considered fully XCF capable by its
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users.
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In this section we consider primary color (or grayscale) intensities
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and alpha values for pixels to be real numbers ranging from 0.0 to
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1.0. This makes many of the formulas easier; the reader is asked to
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keep in mind that a (linear) conversion from the integral 0..255 scale
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of the actual XCF scale is implied whenever data from the XCF file is
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mentioned.
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Any practical implementation of the computations specified below may
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suffer rounding errors; this specification do not detail how these are
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to be handled. GIMP itself rounds values to an integral number of
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255ths at many points in the computation. This specification does not
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specify exactly which these points are, and authors of XCF renderers
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that aim to reproduce the effects of GIMP's flattening down to the
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least significant bits are referred to studying its source code.
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In the description below, the variable letter "a" is used for alpha
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values. The variable letter "r", "g", "b" are used for primary
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intensities, "y" is used for grayscale intensities, and "i" is used
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for color map indexed. The letter "c" is used for the complete
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color information for a pixel; depending on the color mode of the
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image that is either an (r,g,b) triple, a y, or a c.
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The flattening process works independently for each pixel in the
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canvas area. The description of some layer modes in the GIMP manual
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may give the impression that they involve filters that let pixels
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influence neighbor pixels, but that is not true.
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This description does not attempt to preserve the color information
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for completely transparent pixels in a layer. If an application uses
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this color information, it should document explicitly how it behaves
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when transparent pixels from several different layers cover the same
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point of the canvas.
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Flattening overview
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-------------------
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This is how to compute the flattened result for a single pixel
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position (in theory, that is - efficient implementations will of
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course follow this procedure or an equivalent one for many pixels in
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parallel):
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1. Initialize a "working pixel" (a1,c1) to be completely transparent
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(that is, a1=0.0 and the value of c1 is immaterial).
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2. Do the following for each visible layer in the image, starting with
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the one that comes LAST in the master layer list:
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3. Ignore the layer if it is the floating selection, or if it
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does not overlap the pixel position in question.
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4. Let (a2,c2) be the pixel data for the layer at the pixel
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position in question. If the layer does not have an alpha
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channel, then set a1 to 1.0.
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5. If the layer is the one that the floating selection is attached
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to and the floating selection overlaps the pixel position in
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question, then do the following:
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6. Let (a3,c3) be the pixel data for the floating selection
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layer at the pixel position in question.
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7. If there is a selection channel, then let x be its value
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at the pixel position in question, and set a3 to a3*x.
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8. Let m3 be the layer mode of the floating selection.
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9. Set (a2,c2) to COMPOSITE(a2,c2, a3,c3,m3).
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The COMPOSITE function is defined below.
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10. If the layer has a layer mask and it is enabled, then let x be
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the value of the layer mask at the pixel position in question,
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and set a2 to a2*x.
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11. Let m2 be the layer mode of the layer.
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12. If the layer is the bottommost visible layer (i.e., if it is
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the last visible layer in the master layer list) and m2 is not
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"Normal" or "Dissolve", then set m2 to "Normal".
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13. Set (a1,c1) to COMPOSITE(a1,c1, a2,c2,m2).
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The COMPOSITE function is defined below.
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14. If the flattened image is to be shown against a background of
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color c0, then actually visible pixel is
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COMPOSITE(1.0,c0, a1,c1,Normal).
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Note that unless all layers have mode Normal, it would give the
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wrong result to start by initializing (a1,c1) to (1.0,c0).
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Helper functions
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----------------
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The following auxiliary functions are used in the definition of
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COMPOSITE below:
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MIN(x1,...,xn) is the least value of x1...xn
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MAX(x1,...,xn) is the largest value of x1..xn
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MID(x1,...,xn) = (MIN(x1,...,xn)+MAX(x1,...,xn))/2
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CLAMP(x) = if x < 0 then 0.0 else if x > 1 then 1.0 else x
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BLEND(a1,x1, a2,x2) = (1-k)*x1 + k*x2
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where k = a2/(1-(1-a1)*(1-a2))
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Layer modes
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-----------
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This and the following sections define the COMPOSITE function used in
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the general flattening algorithm.
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"Normal" mode for RGB or grayscale images is the usual mode of
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compositing in computer graphics with alpha channels. In indexed
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mode, the alpha value gets rounded to either 1.0 or 0.0 such that
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no colors outside the color map get produced:
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COMPOSITE(a1,y1, a2,y2,Normal)
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= ( 1-(1-a1)*(1-a2), BLEND(a1,y1, a2,y2) )
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COMPOSITE(a1,r1,g1,b1, a2,r2,g2,b2,Normal)
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= ( 1-(1-a1)*(1-a2), BLEND(a1,r1, a2,r2),
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BLEND(a1,g1, a2,g2),
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BLEND(a1,b1, a2,b2) )
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COMPOSITE(a1,i1, a2,i2,Normal) = if a2 > 0.5 then (1.0,i2) else (a1,i1)
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"Dissolve" mode corresponds to randomly dithering the alpha channel to
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the set {0.0, 1.0}:
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COMPOSITE(a1,c1, a2,c2,Dissolve) = chose pseudo-randomly between
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(1.0,c2) with probability a2
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(a1,c1) with probability 1-a2
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These two modes are the only ones that make sense for all of the RGB,
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grayscale and indexed color models. In the indexed color model, all
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layer modes except Dissolve are treated as Normal.
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Most layer modes belong to the following group, which makes sense for
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RGB and grayscale images, but not for indexed ones:
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COMPOSITE(a1,y2, a2,y2,m)
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= ( a1, BLEND(a1,y1, MIN(a1,a2),f(y1,y2, m)) )
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COMPOSITE(a1,r1,g1,b1, a2,r2,g2,b2,m)
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= ( a1, BLEND(a1,r2, MIN(a1,a2),f(r1,r2, m)),
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BLEND(a1,g1, MIN(a1,a2),f(g1,g2, m)),
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BLEND(a1,b1, MIN(a1,a2),f(b1,g2, m)) )
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when 3 <= m <= 10 or 15 <= m <= 21.
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The following table defines f(x1,x2,m):
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Multiply: f(x1,x2, 3) = x1*x2
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Screen: f(x1,x2, 4) = 1-(1-x1)*(1-x2)
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Overlay: f(x1,x2, 5) = (1-x2)*x1^2 + x2*(1-(1-x2)^2)
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Difference: f(x1,x2, 6) = if x1 > x2 then x1-x2 else x2-x1
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Addition: f(x1,x2, 7) = CLAMP(x1+x2)
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Subtract: f(x1,x2, 8) = CLAMP(x1-x2)
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Darken Only: f(x1,x2, 9) = MIN(x1,x2)
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Lighten Only: f(x1,x2, 10) = MAX(x1,x2)
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Divide: f(x1,x2, 15) = CLAMP(x1/x2)
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Dodge: f(x1,x2, 16) = CLAMP(x1/(1-x2))
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Burn f(x1,x2, 17) = CLAMP(1-(1-x1)/x2)
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Hard Light: f(x1,x2, 18) = if x2 < 0.5 then 2*x1*x2 else 1-2*(1-x1)(1-x2)
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Soft Light: f(x1,x2, 19) = (1-x2)*x1^2 + x2*(1-(1-x2)^2)
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Grain Extract: f(x1,x2, 20) = CLAMP(x1-x2+0.5)
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Grain Merge: f(x1,x2, 21) = CLAMP(x1+x2-0.5)
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Note that the "Overlay" and "Soft Light" modes have identical effects.
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In the "Divide", "Dodge", and "Burn" modes, division by zero should
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be considered to produce a number so large that CLAMP(x/0) = 1 unless
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x=0, in which case CLAMP(0/0) = 0.
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The remaining four layer modes only make sense in the RGB color model;
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if the color mode of the image is grayscale or indexed they will be
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interpreted as Normal.
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COMPOSITE(a1,r1,g1,b1, a2,r2,g2,b2,m)
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= ( a1, BLEND(a1,r2, MIN(a1,a2),r0),
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BLEND(a1,g1, MIN(a1,a2),g0),
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BLEND(a1,b1, MIN(a1,a2),b0) )
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where (r0,g0,b0) = h(r1,g1,b1, r2,g2,b2, m)
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when 11 <= m <= 14.
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For defining these modes, we say that
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(r,g,b) has the _hue_ of (r',g',b')
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if r' = g' = b' and r >= g = b
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or there exist p and q such that p>=0 and r=p*r'+q and b=p*b'+q and g=p*g'+q
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(r,g,b) has the _value_ of (r',g',b')
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if MAX(r,g,b) = MAX(r',g',b')
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(r,g,b) has the _HSV-saturation_ of (r',g',b')
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if r' = g' = b' = 0 and r = g = b
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or MIN(r,g,b) = MAX(r,g,b)*MIN(r',g',b')/MAX(r',g',b')
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(r,g,b) has the _luminosity_ of (r',g',b')
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if MID(r,g,b) = MID(r',g',b')
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(r,g,b) has the _HSL-saturation_ of (r',g',b')
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if r' = g' = b' and r = g = b
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or MAX(r,g,b)-MIN(r,g,b) = MIN(MID(r,g,b),1-MID(r,g,b)) *
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(MAX(r',g',b')-MIN(r',g',b'))/MIN(MID(r',g',b'),1-MID(r',g',b'))
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Mode 11: Hue (H of HSV)
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h(r1,g1,b1, r2,g2,b2, 11) is
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if r2=g2=b2 then (r1,g1,b1) unchanged
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otherwise: the color that has
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the hue of (r1,g2,b2)
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the value of (r1,g1,b1)
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the HSV-saturation of (r1,g1,b1)
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Mode 12: Saturation (S of HSV)
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h(r1,g1,b1, r2,g2,b2, 12) is the color that has
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the hue of (r1,g1,b1)
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the value of (r1,g1,b1)
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the HSV-saturation of (r2,g2,b2)
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Mode 13: Color (H and S of HSL)
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h(r1,g1,b1, r2,g2,b2, 13) is the color that has
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the hue of (r2,g2,b2)
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the luminosity of (r1,g1,b1)
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the HSL-saturation of (r2,g2,b2)
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Mode 14: Value (V of HSV)
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h(r1,g1,b1, r2,g2,b2, 14) is the color that has
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the hue of (r1,g1,b1)
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the value of (r2,g2,b2)
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the HSV-saturation of (r1,g1,b1) |