Options to Color Rendering of Spectra

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Options to Color Rendering of Spectra

• a discussion to aid the T1
• by
• Marcelo Gattass

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Problem definition
Given
http//cvision.ucsd.edu/
Determine RGB values to render this colors in a
ITU-R BT.709monitor with D65 illuminant
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CIExyz coordinate for a pure color
Spectrum of a pure color of ? ?0
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Question What value of Y should we use?

• Two basic options
• Y(?)V (?)
• Y(?)Ymax (say Y(?)1 or 100)

John Walker option? http//www.fourmilab.ch/docume
nts/specrend/
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A simple C function
void YxytoRGB(float Y, float x, float y,
float R, float G, float B) float X
x ( Y / y ) float Z ( 1 - x - y ) ( Y /
y ) R 3.2406fX - 1.5372fY - 0.4986fZ
G -0.9689fX 1.8758fY 0.0415fZ B
0.0557fX - 0.2040fY 1.0570fZ
Derived in class for D65 andITU-R BT.709
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Spectra colors are outside the RGB gamut!
Values less than zero and greater than one!
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Possible solution add white to eliminate
negative and scale to fit in 0,1
void scale01(float R, float G, float B)
float min,max minMin(R,G,B) if (minlt0)
float w-min /white to be added to make
visible/ R w G w B
w maxMax(R,G,B) if (maxgt1) R
/max G /max B /max
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Should we correct for gamma as suggested in SRGB?
static float gammaCorrectRGB(float RGB) if (
RGB gt 0.0031308f ) return (float)
(1.055(pow(RGB,( 1 / 2.4 )))-0.055) else
return (float) (12.92 RGB) void
corrGammaRGB(float R, float G, float B) R
gammaCorrectRGB(R) G gammaCorrectRGB(G)
B gammaCorrectRGB(B)
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Red as function of Lambda
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Green as function of Lambda
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Blue as function of Lambda
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Rendering of spectrum colors