Thursday, February 18, 2010

1976 Uniform Chromaticity Scales: Yu'v'

A while back I came across Seyno Sluterman's short article "The NTSC Color Triangle is Obsolete, But No One Seems to Know". It's an interesting and short read but it also makes a call to use the 1976 Yu'v' Uniform Chromaticity Scales instead of the typical CIE 1931 chromaticity values when plotting display gamuts.

Which is a great reminder (1).



Plotting display gamuts using CIE 1931 chromaticity values appears to be pervasive but it is obsolete.

To compute Yu'v' values, start with 1931 chromaticity of xy values and then compute u' and v' as follows:

u' = (4 * x) / ((-2 * x) + (12 * y) + 3)

v' = (9 * y) / ((-2 * x) + (12 * y) + 3)

The Y or luminance value is the same. And below, for your interactive convenience, is a simple xyY to Yu'v' calculator:



To use the above Yu'v' calculator (2), simply enter the xyY chromaticity values on the left and hit the 'Convert' button to compute the corresponding 1976 Yu'v' values. The 'Clear' button can be used to empty the Yu'v' values and reset the xyY values to 0.3333, 0.3333 and 60.

Computing Yu'v' values is nothing complicated, but it is curious that in 2010 a majority of display gamuts image search results (shown above) are plotted using a representation from 1931 that was superseded in 1976.




Footnotes
1. After my Model Based Color Tolerances paper in 1999 I had Dr. R.W.G. Hunt patiently reminded me. I glad to say I haven't needed a second reminder.
2. In this case the source JavaScript looks like:

function computeYuvp() {
document.f1.theYout.value = document.f1.theY.value;
var one = parseFloat(document.f1.thex.value);
var two = parseFloat(document.f1.they.value);
var temp = (-2 * one) + (12 * two) + 3;
document.f1.theup.value = (4 * one) / temp;
document.f1.thevp.value = (9 * two) / temp;
}