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ImfChromaticities.h

///////////////////////////////////////////////////////////////////////////
//
// Copyright (c) 2003, Industrial Light & Magic, a division of Lucas
// Digital Ltd. LLC
// 
// All rights reserved.
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// its contributors may be used to endorse or promote products derived
// from this software without specific prior written permission. 
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#ifndef INCLUDED_IMF_CHROMATICITIES_H
#define INCLUDED_IMF_CHROMATICITIES_H

//-----------------------------------------------------------------------------
//
//    CIE (x,y) chromaticities, and conversions between
//    RGB tiples and CIE XYZ tristimulus values.
//
//-----------------------------------------------------------------------------

#include <ImathVec.h>
#include <ImathMatrix.h>

namespace Imf {

   
struct Chromaticities
{
    Imath::V2f    red;
    Imath::V2f    green;
    Imath::V2f    blue;
    Imath::V2f    white;

    Chromaticities (const Imath::V2f &red   = Imath::V2f (0.6400f, 0.3300f),
                const Imath::V2f &green = Imath::V2f (0.3000f, 0.6000f),
                const Imath::V2f &blue  = Imath::V2f (0.1500f, 0.0600f),
                const Imath::V2f &white = Imath::V2f (0.3127f, 0.3290f));
};


//
// Conversions between RGB and CIE XYZ
//
// RGB to XYZ:
//
//    Given a set of chromaticities, c, and the luminance, Y, of the RGB
//    triple (1,1,1), or "white", RGBtoXYZ(c,Y) computes a matrix, M, so
//    that multiplying an RGB value, v, with M produces an equivalent
//    XYZ value, w.  (w == v * M)
// 
//    If we define that
// 
//       (Xr, Yr, Zr) == (1, 0, 0) * M
//       (Xg, Yg, Zg) == (0, 1, 0) * M
//       (Xb, Yb, Zb) == (0, 0, 1) * M
//       (Xw, Yw, Zw) == (1, 1, 1) * M,
// 
//    then the following statements are true:
// 
//       Xr / (Xr + Yr + Zr) == c.red.x
//       Yr / (Xr + Yr + Zr) == c.red.y
// 
//       Xg / (Xg + Yg + Zg) == c.red.x
//       Yg / (Xg + Yg + Zg) == c.red.y
// 
//       Xb / (Xb + Yb + Zb) == c.red.x
//       Yb / (Xb + Yb + Zb) == c.red.y
// 
//       Xw / (Xw + Yw + Zw) == c.red.x
//       Yw / (Xw + Yw + Zw) == c.red.y
// 
//       Yw == Y.
// 
// XYZ to RGB:
// 
//    YYZtoRGB(c,Y) returns RGBtoXYZ(c,Y).inverse().
// 
// Warning:
// 
//    It would seem that RGBtoXYZ() and XYZtoRGB() are all you need
//    to convert RGB values with one set of primary and white point
//    chromaticities into perceptually equivalent RGB values with
//    different primary and white point chromaticities:
// 
//        M44f M = RGBtoXYZ (chromaticities1, Y1) *
//               XYZtoRGB (chromaticities2, Y2);
// 
//    However, this simple conversion does not account for white point
//    adaptation, and produces undesirable results.  The proper thing
//    to do is to perform a Bradford or a von Kries transform, which
//    moves the white point of the original color space to the white
//    point of the destination color space, dragging other colors with
//    it in a sensible fashion.  
//

Imath::M44f RGBtoXYZ (const Chromaticities chroma, float Y);
Imath::M44f XYZtoRGB (const Chromaticities chroma, float Y);


} // namespace Imf

#endif

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