I'm trying to convert sRGB values (in Photoshop) to L*ab values (also for use in Photoshop) I'm in the ballpark, but not getting the same results as EasyRGB which is where I got the conversion formulas from. I expect there's rounding errors (not at the end, where I round to three places). Only I can't see them.
To make it easier I've lifted out any Photoshop code, so it's just JavaScript
var msg = "";
// RGB
// var Red = foregroundColor.rgb.red;
// var Green = foregroundColor.rgb.green;
// var Blue = foregroundColor.rgb.blue;
var Red = 255;
var Green = 0;
var Blue = 255;
msg = "RGB: " + Red + ", " + Green + ", " + Blue + "<br>";
printy(msg);
// user colour converted to XYZ space
var myColXYZ = RGB_to_XYZ(Red, Green, Blue)
var colX = myColXYZ[0];
var colY = myColXYZ[1];
var colZ = myColXYZ[2];
// using CIE L-ab* colour space
var myLab = XYZ_to_LAB(colX, colY, colZ)
//msg = "L*ab: " + myLab[0] + ", " + myLab[1] + ", " + myLab[2] + "<br>";
// b4 rounding: 60.319933664076004, 98.25421868616108, -60.84298422386232
// round to three places
for (var i = 0; i < 3; i++) {
myLab[i] = round_nicely(myLab[i], 3);
}
msg = "L*ab: " + myLab[0] + ", " + myLab[1] + ", " + myLab[2] + "<br>";
printy(msg);
// results
// RGB: 255, 0, 255
// L*ab: 60.32, 98.254, -60.843
// should be
// CIE-L*ab = 60.324 98.234 -60.825
// function RGB TO XYZ (R, G, B)
// --------------------------------------------------------
// http://www.easyrgb.com/index.php?X=MATH&H=02#text2
function RGB_to_XYZ(r, g, b) {
var_R = parseFloat(r / 255); //r from 0 to 255
var_G = parseFloat(g / 255); //g from 0 to 255
var_B = parseFloat(b / 255); //b from 0 to 255
if (var_R > 0.04045) var_R = Math.pow(((var_R + 0.055) / 1.055), 2.4)
else var_R = var_R / 12.92
if (var_G > 0.04045) var_G = Math.pow(((var_G + 0.055) / 1.055), 2.4)
else var_G = var_G / 12.92
if (var_B > 0.04045) var_B = Math.pow(((var_B + 0.055) / 1.055), 2.4)
else var_B = var_B / 12.92
var_R = var_R * 100;
var_G = var_G * 100;
var_B = var_B * 100;
// Observer. = 2°, Illuminant = D65
X = var_R * 0.4124 + var_G * 0.3576 + var_B * 0.1805;
Y = var_R * 0.2126 + var_G * 0.7152 + var_B * 0.0722;
Z = var_R * 0.0193 + var_G * 0.1192 + var_B * 0.9505;
return [X, Y, Z]
}
// function XYZ TO LAB (x, y, z)
// --------------------------------------------------------
// http://www.easyrgb.com/index.php?X=MATH&H=16#text16
function XYZ_to_LAB(x, y, z) {
var ref_X = 95.047;
var ref_Y = 100.000;
var ref_Z = 108.883;
var_X = x / ref_X; //ref_X = 95.047 Observer= 2°, Illuminant= D65
var_Y = y / ref_Y; //ref_Y = 100.000
var_Z = z / ref_Z; //ref_Z = 108.883
if (var_X > 0.008856) var_X = Math.pow(var_X, (1 / 3))
else var_X = (7.787 * var_X) + (16 / 116)
if (var_Y > 0.008856) var_Y = Math.pow(var_Y, (1 / 3))
else var_Y = (7.787 * var_Y) + (16 / 116)
if (var_Z > 0.008856) var_Z = Math.pow(var_Z, (1 / 3))
else var_Z = (7.787 * var_Z) + (16 / 116)
CIE_L = (116 * var_Y) - 16;
CIE_a = 500 * (var_X - var_Y);
CIE_b = 200 * (var_Y - var_Z);
return [CIE_L, CIE_a, CIE_b]
}
// function ROUND NICELY (num number, number of decimal places)
// --------------------------------------------------------
function round_nicely(num, places) {
// if(! places) places = 3;
var p = Math.pow(10, places)
return Math.round(num * p) / p
}
// function PRINTY (str)
// --------------------------------------------------------
function printy(message) {
var outputDiv = document.getElementById('myPage');
outputDiv.innerHTML += message;
}<!-- page content -->
<p id="myPage"></p>Consider utilizing Photoshop's ExtendScript/JS API instead of EasyRGB for an accurate color transformation.
The custom rgbToLab function in the gist below demonstrates how the SolidColor class can be utilized to firstly create a new RGB color (using the given color components; R, G, B) and how to subsequently obtain the corresponding Lab values.
#target photoshop
/**
* Converts RGB color values to the Lab color model.
* @param {Number} red - The red component value (0-255).
* @param {Number} green - The green component value (0-255).
* @param {Number} blue - The blue component value (0-255).
* @returns {Array} The Lab values.
*/
function rgbToLab(red, green, blue) {
var color = new SolidColor;
color.rgb.red = red;
color.rgb.green = green;
color.rgb.blue = blue;
return [
color.lab.l,
color.lab.a,
color.lab.b
]
}
var labValues = rgbToLab(255, 0, 255);
$.writeln('L*ab: '+ labValues) // --> L*ab: 60.16845703125,92.6736755371094,-60.763671875