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You might want to take that up with Adobe, because they have no problem spitting out Lab numbers if you input RGB into their color picker.
By @scotwllm
It’s not clear why you see that as a counter-argument, because as D Fosse is explaining, Lab is the device-independent reference for all Photoshop colors in any color mode.
In other words, of course Adobe has “no problem spitting out Lab numbers if you input RGB into their color picker” because that’s what it does every time anyway, for any color. It will always run the color values through Lab to get to any other color space. (The only exception might be a Device Link profile conversion, but I don’t have enough experience with those.)
I need to calculate the Delta-E between multiple colors, and plugging them into a website form one by one takes too much time. I'm too old to waste it on that kind of stuff, know what I mean?
By @scotwllm
Which dE: de76/dEab, dE94 etc?
If the formula can be translated into JavaScript, there may be a way to do this...
I'm sure you know of the following:
The following three scripts provide a rough dE evaluation between the foreground and background colour picker values.
/*
Calculates the CIE76 (dE76) color difference between the foreground and background colors in Photoshop
v1.1
https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-does-photoshop-calculate-lab-values/m-p/15028840
*/
#target photoshop
function calculateDE76(lab1, lab2) {
var deltaL = lab1[0] - lab2[0];
var deltaA = lab1[1] - lab2[1];
var deltaB = lab1[2] - lab2[2];
return Math.sqrt(deltaL * deltaL + deltaA * deltaA + deltaB * deltaB);
}
function main() {
/*
if (!app.documents.length) {
alert("No document open.");
return;
}
*/
// Get foreground and background colors
var fgColor = app.foregroundColor.lab;
var bgColor = app.backgroundColor.lab;
var fgLab = [Math.round(fgColor.l), Math.round(fgColor.a), Math.round(fgColor.b)];
var bgLab = [Math.round(bgColor.l), Math.round(bgColor.a), Math.round(bgColor.b)];
// Calculate dE76
var dE76 = calculateDE76(fgLab, bgLab);
alert("Foreground vs. Background Color Picker Difference\n" +
"Foreground color (L:" + fgLab[0] + " a:" + fgLab[1] + " b:" + fgLab[2] + ")\n" +
"Background color (L:" + bgLab[0] + " a:" + bgLab[1] + " b:" + bgLab[2] + ")\n" +
"The dE76 color difference is: " + dE76.toFixed(2));
}
main();
/*
Calculates the CIE94 (dE94) color difference between the foreground and background colors in Photoshop
v1.1
https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-does-photoshop-calculate-lab-values/m-p/15028840
*/
#target photoshop
function calculateCIE94(lab1, lab2) {
// Weighting factors for CIE94
var kL = 1;
var kC = 1;
var kH = 1;
var K1 = 0.045;
var K2 = 0.015;
// Differences in Lab
var deltaL = lab1[0] - lab2[0];
var deltaA = lab1[1] - lab2[1];
var deltaB = lab1[2] - lab2[2];
// Calculate chroma
var c1 = Math.sqrt(lab1[1] * lab1[1] + lab1[2] * lab1[2]);
var c2 = Math.sqrt(lab2[1] * lab2[1] + lab2[2] * lab2[2]);
var deltaC = c1 - c2;
// Calculate deltaH (corrected)
var deltaH = deltaA * deltaA + deltaB * deltaB - deltaC * deltaC;
// Scaled components
var sl = 1; // Lightness scale
var sc = 1 + K1 * c1; // Chroma scale
var sh = 1 + K2 * c1; // Hue scale
// CIE94 formula
var dE94 = Math.sqrt(
Math.pow(deltaL / (kL * sl), 2) +
Math.pow(deltaC / (kC * sc), 2) +
Math.pow(deltaH / (kH * sh), 2)
);
return dE94;
}
function main() {
/*
if (!app.documents.length) {
alert("No document open.");
return;
}
*/
// Get foreground and background colors
var fgColor = app.foregroundColor.lab;
var bgColor = app.backgroundColor.lab;
var fgLab = [Math.round(fgColor.l), Math.round(fgColor.a), Math.round(fgColor.b)];
var bgLab = [Math.round(bgColor.l), Math.round(bgColor.a), Math.round(bgColor.b)];
// Calculate dE94
var dE94 = calculateCIE94(fgLab, bgLab);
alert("Foreground vs. Background Color Picker Difference\n" +
"Foreground color (L:" + fgLab[0] + " a:" + fgLab[1] + " b:" + fgLab[2] + ")\n" +
"Background color (L:" + bgLab[0] + " a:" + bgLab[1] + " b:" + bgLab[2] + ")\n" +
"The CIE94 color difference is: " + dE94.toFixed(2));
}
main();
/*
Calculates the CIEDE2000 (dE00) color difference between the foreground and background colors in Photoshop
v1.1
https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-does-photoshop-calculate-lab-values/m-p/15028840
*/
#target photoshop
function calculateCIEDE2000(lab1, lab2) {
// Weighting factors
var kL = 1;
var kC = 1;
var kH = 1;
// Helper functions
function degToRad(deg) {
return (deg * Math.PI) / 180;
}
function radToDeg(rad) {
return (rad * 180) / Math.PI;
}
// Calculate chroma
var c1 = Math.sqrt(lab1[1] * lab1[1] + lab1[2] * lab1[2]);
var c2 = Math.sqrt(lab2[1] * lab2[1] + lab2[2] * lab2[2]);
// Mean chroma
var cBar = (c1 + c2) / 2;
// Calculate G
var g = 0.5 * (1 - Math.sqrt(Math.pow(cBar, 7) / (Math.pow(cBar, 7) + Math.pow(25, 7))));
// Adjusted a values
var a1Prime = lab1[1] * (1 + g);
var a2Prime = lab2[1] * (1 + g);
// Recalculate chroma with adjusted a values
var c1Prime = Math.sqrt(a1Prime * a1Prime + lab1[2] * lab1[2]);
var c2Prime = Math.sqrt(a2Prime * a2Prime + lab2[2] * lab2[2]);
// Mean chroma prime
var cBarPrime = (c1Prime + c2Prime) / 2;
// Calculate h primes
var h1Prime = Math.atan2(lab1[2], a1Prime);
var h2Prime = Math.atan2(lab2[2], a2Prime);
if (h1Prime < 0) h1Prime += 2 * Math.PI;
if (h2Prime < 0) h2Prime += 2 * Math.PI;
var hBarPrime = Math.abs(h1Prime - h2Prime) > Math.PI
? (h1Prime + h2Prime + 2 * Math.PI) / 2
: (h1Prime + h2Prime) / 2;
// Delta h prime
var deltaHPrimeRaw = Math.abs(h1Prime - h2Prime) > Math.PI
? h2Prime - h1Prime + 2 * Math.PI * (h2Prime <= h1Prime ? 1 : -1)
: h2Prime - h1Prime;
// Delta L, C, and H primes
var deltaLPrime = lab2[0] - lab1[0];
var deltaCPrime = c2Prime - c1Prime;
var deltaHPrime = 2 * Math.sqrt(c1Prime * c2Prime) * Math.sin(deltaHPrimeRaw / 2);
// Weighting functions
var lBar = (lab1[0] + lab2[0]) / 2;
var t = 1
- 0.17 * Math.cos(hBarPrime - degToRad(30))
+ 0.24 * Math.cos(2 * hBarPrime)
+ 0.32 * Math.cos(3 * hBarPrime + degToRad(6))
- 0.20 * Math.cos(4 * hBarPrime - degToRad(63));
var sl = 1 + ((0.015 * Math.pow(lBar - 50, 2)) / Math.sqrt(20 + Math.pow(lBar - 50, 2)));
var sc = 1 + 0.045 * cBarPrime;
var sh = 1 + 0.015 * cBarPrime * t;
var deltaTheta = degToRad(30) * Math.exp(-Math.pow((hBarPrime - degToRad(275)) / degToRad(25), 2));
var rc = 2 * Math.sqrt(Math.pow(cBarPrime, 7) / (Math.pow(cBarPrime, 7) + Math.pow(25, 7)));
var rt = -rc * Math.sin(2 * deltaTheta);
// CIEDE2000 formula
var dE00 = Math.sqrt(
Math.pow(deltaLPrime / (kL * sl), 2) +
Math.pow(deltaCPrime / (kC * sc), 2) +
Math.pow(deltaHPrime / (kH * sh), 2) +
rt * (deltaCPrime / (kC * sc)) * (deltaHPrime / (kH * sh))
);
return dE00;
}
function main() {
/*
if (!app.documents.length) {
alert("No document open.");
return;
}
*/
// Get foreground and background colors
var fgColor = app.foregroundColor.lab;
var bgColor = app.backgroundColor.lab;
var fgLab = [Math.round(fgColor.l), Math.round(fgColor.a), Math.round(fgColor.b)];
var bgLab = [Math.round(bgColor.l), Math.round(bgColor.a), Math.round(bgColor.b)];
// Calculate dE00
var dE00 = calculateCIEDE2000(fgLab, bgLab);
alert("Foreground vs. Background Color Picker Difference\n" +
"Foreground color (L:" + fgLab[0] + " a:" + fgLab[1] + " b:" + fgLab[2] + ")\n" +
"Background color (L:" + bgLab[0] + " a:" + bgLab[1] + " b:" + bgLab[2] + ")\n" +
"The CIEDE2000 color difference is: " + dE00.toFixed(2));
}
main();
https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html
That's too bad. I was looking forward to reading it. I've noticed that lots of sites with color info are blocked.
By @scotwllm
All of the information is there and there is nothing nefarious IMHO. Use different browsers or not, it's up to you.
The color picker doesn't ask about sRGB or Adobe RGB or ProPhoto RGB, etc. It just says RGB.
That’s not a relevant observation, the Color Space is clear based on the image or (if the image is unprofiled) Photoshop’s Color Settings.
There is - or should always be! - an embedded color profile. This is what Photoshop uses for the calculation:
All numbers primarily refer to the color space of the open document. Secondarily, if the calculation is across different modes so that there isn't an open document to refer to, Photoshop goes to your working spaces as the reference for the numbers. It also does that if the document is missing an embedded profile.
Another, simpler way of saying this is that the embedded color profile overrides the working space.
But Lab is always constant, as the underlying reference for everything. The whole color management architecture is built upon this.
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