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2 //
3 // Copyright (c) 2003, Industrial Light & Magic, a division of Lucas
4 // Digital Ltd. LLC
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33 ///////////////////////////////////////////////////////////////////////////
34
35
36 //-----------------------------------------------------------------------------
37 //
38 // CIE (x,y) chromaticities, and conversions between
39 // RGB tiples and CIE XYZ tristimulus values.
40 //
41 //-----------------------------------------------------------------------------
42
43 #include <ImfChromaticities.h>
44
45 namespace Imf {
46
47
Chromaticities(const Imath::V2f & red,const Imath::V2f & green,const Imath::V2f & blue,const Imath::V2f & white)48 Chromaticities::Chromaticities (const Imath::V2f &red,
49 const Imath::V2f &green,
50 const Imath::V2f &blue,
51 const Imath::V2f &white)
52 :
53 red (red),
54 green (green),
55 blue (blue),
56 white (white)
57 {
58 // empty
59 }
60
61
62 Imath::M44f
RGBtoXYZ(const Chromaticities chroma,float Y)63 RGBtoXYZ (const Chromaticities chroma, float Y)
64 {
65 //
66 // For an explanation of how the color conversion matrix is derived,
67 // see Roy Hall, "Illumination and Color in Computer Generated Imagery",
68 // Springer-Verlag, 1989, chapter 3, "Perceptual Response"; and
69 // Charles A. Poynton, "A Technical Introduction to Digital Video",
70 // John Wiley & Sons, 1996, chapter 7, "Color science for video".
71 //
72
73 //
74 // X and Z values of RGB value (1, 1, 1), or "white"
75 //
76
77 float X = chroma.white.x * Y / chroma.white.y;
78 float Z = (1 - chroma.white.x - chroma.white.y) * Y / chroma.white.y;
79
80 //
81 // Scale factors for matrix rows
82 //
83
84 float d = chroma.red.x * (chroma.blue.y - chroma.green.y) +
85 chroma.blue.x * (chroma.green.y - chroma.red.y) +
86 chroma.green.x * (chroma.red.y - chroma.blue.y);
87
88 float Sr = (X * (chroma.blue.y - chroma.green.y) -
89 chroma.green.x * (Y * (chroma.blue.y - 1) +
90 chroma.blue.y * (X + Z)) +
91 chroma.blue.x * (Y * (chroma.green.y - 1) +
92 chroma.green.y * (X + Z))) / d;
93
94 float Sg = (X * (chroma.red.y - chroma.blue.y) +
95 chroma.red.x * (Y * (chroma.blue.y - 1) +
96 chroma.blue.y * (X + Z)) -
97 chroma.blue.x * (Y * (chroma.red.y - 1) +
98 chroma.red.y * (X + Z))) / d;
99
100 float Sb = (X * (chroma.green.y - chroma.red.y) -
101 chroma.red.x * (Y * (chroma.green.y - 1) +
102 chroma.green.y * (X + Z)) +
103 chroma.green.x * (Y * (chroma.red.y - 1) +
104 chroma.red.y * (X + Z))) / d;
105
106 //
107 // Assemble the matrix
108 //
109
110 Imath::M44f M;
111
112 M[0][0] = Sr * chroma.red.x;
113 M[0][1] = Sr * chroma.red.y;
114 M[0][2] = Sr * (1 - chroma.red.x - chroma.red.y);
115
116 M[1][0] = Sg * chroma.green.x;
117 M[1][1] = Sg * chroma.green.y;
118 M[1][2] = Sg * (1 - chroma.green.x - chroma.green.y);
119
120 M[2][0] = Sb * chroma.blue.x;
121 M[2][1] = Sb * chroma.blue.y;
122 M[2][2] = Sb * (1 - chroma.blue.x - chroma.blue.y);
123
124 return M;
125 }
126
127
128 Imath::M44f
XYZtoRGB(const Chromaticities chroma,float Y)129 XYZtoRGB (const Chromaticities chroma, float Y)
130 {
131 return RGBtoXYZ (chroma, Y).inverse();
132 }
133
134
135 } // namespace Imf
136