CIE2RGB

Name

cie2rgb

Purpose (one Line Only)

Convert from CIE Chromaticity coordinates to RGB color-space values

Description

Conversion from CIE Chromaticity (D65) to sRGB according to
IEC 61966-2-1:1999. Note that I'm calling RGB the same as sRGB,
after converting to 0-255 byte range instead of (0.0,1.0)
Also, since I really don't care for gamma (contrast) I have implemented
a simpler form than found in the IEC standard so that I can change
gamma to any value and the calculation is easy to perform. The standard
formula includes a piecewise continuous function that is logrithmic
everywhere except for very low brightness. At low brightness it uses
a linear ramp down to zero. Instead, I just use the following for gamma:
out = in^(1.0/gamma)
when converting from bigy (Y) to RGB (before converting from 0-1 to 0-255)
The default in this program is a gamma=1.0 and this formula is bypassed.
The standard gamma in the CIE prescription is "approximately 2.2".
The difference between my implementation and the prescription is unlikely
to be seen in the midst of large variations in monitors and printers
at the very low intensity range (bottom few percent).

Calling Sequence

cie2rgb,x,y,bigy,r,g,b

Inputs

x,y - scalar, vector, or arrays of Chromaticity (see spec2xyz)
must all be the same length
bigy - intensity
Note: white is x=0.3127, y=0.3290, bigy=Y=1.0
(z=0.3583 because x+y+z=1)

Optional Input Parameters

GAMMA - default=1

Keyword Input Parameters

D65 - Flag, if set the output will be based on the D65 illuminant
Otherwise, it will be corrected for Illuminant C
NORMALIZED - Flag, if set the output will be normalized floating point
values (0-1) for RGB rather than byte values

Outputs

r,g,b - same size and rank as input, RGB byte values

Keyword Output Parameters

Common Blocks

Side Effects

Restrictions

Procedure

Modification History

Written by Marc W. Buie, Southwest Research Institute, 2009/01/05
2009/01/15, MWB, added NORMALIZED keyword
2009/01/16, MWB, added D65 keyword