How to convert color object from parsed PSD into usable RGBA

Question

I'm parsing a PSD getting normal RGBA values and recently I encountered this strange float color values:

{
  "red": 1.0343483686447144,
  "green": 0.9487225413322449,
  "blue": -0.12634865939617157,
  "alpha": 100,
  "rgba": "rgba(1.0343483686447144, 0.9487225413322449, -0.12634865939617157, 1)"
}

Here is my parsing function:

function getRGBColor(color, opacity = 100) {
   if ("Clr " in color) {
      color = color["Clr "];
   }

   var type = color.class.id;
   var red = color["Rd  "];
   var green = color["Grn "];
   var blue = color["Bl  "];

   if ("blueFloat" in color) {
      red = color.redFloat;
      green = color.greenFloat;
      blue = color.blueFloat;
   }

   var rgba = {
      red: red, 
      green: green,
      blue: blue,
      alpha: opacity,
      rgba: `rgba(${red}, ${green}, ${blue}, ${opacity/100})`
   }

   return rgba;
}

It's supposed to be yellow.

The object is coming from strokeColorContent.

     var vectorMask = node.get("vectorMask")?.export();
     var vectorStroke = node.get("vectorStroke")?.data;
     var vectorStrokeContent = node.get("vectorStrokeContent")?.data;
     var fillColor = getRGBColor(vectorStrokeContent);

Related question:
How to convert PSD vector path to SVG path data

Update:
I've updated the CSS to use the lab() color space as suggested and it seems to work:

lab(94 -12 103)

I haven't confirmed the alpha value yet but this seems to work:

lab(94 -12 103 / 1)

It seems that, in most browsers, lab() is being supported (not currently in Firefox but in Firefox 113).

To get the rgba() value there needs to be a formula from lab to rgba.

The negative blue values indicate these values are related to the LAB color space. L: 100, a:-12; b:94 (all values multiplied with 100) would also produce a yellow color. However it's most certainly more quirky — herrstrietzel, Mar 31 '23 at 00:40
@herrstrietzel I think that seemed to work. I've updated it to `lab(94 -12 103)` and it produces the same yellow. However lab doesn't seem to be supported in Firefox — 1.21 gigawatts, Apr 05 '23 at 20:29
*It is supported in Firefox behind a flag and enabled in version 113. — 1.21 gigawatts, Apr 05 '23 at 21:27

herrstrietzel · Accepted Answer · 2023-04-06T01:54:09.167

So apparently, the psd.js color output object confuses RGB with LAB values.

Unfortunately, you can only convert LAB to to a more generic RGB approximation.

You can't precisely convert LAB to RGB (without a target color space definition)

The whole idea behind the LAB color system is to provide a system that represents all colors distinguishable by the human eye as a reference color space for all kind of color conversions.

Workaround: generic RGB approximation:

Provided, your psd used a more generic RGB color profile e.g similar to sRGB, you can use antimatter15's rgb-lab library

let psdCols = {
  "red": 1.0343483686447144,
  "green": 0.9487225413322449,
  "blue": -0.12634865939617157,
  "alpha": 100,
}


let labRGBAVals =  [[psdCols.green,  psdCols.blue, psdCols.red].map(val=>+(val*100).toFixed(0)) ,psdCols.alpha / 100].flat();

console.log('LAB values: ', labRGBAVals.join(' '));

let rgba = lab2rgb(labRGBAVals).map(val=>+(val).toFixed(0) );

labToRgb.style.backgroundColor = `rgba( ${rgba.join(', ')} )`
console.log('RGB values: ', rgba)


/**
* based on 
* https://github.com/antimatter15/rgb-lab/blob/master/color.js
*/
function lab2rgb(lab){
  let y = (lab[0] + 16) / 116,
      x = lab[1] / 500 + y,
      z = y - lab[2] / 200,
      r, g, b;

  x = 0.95047 * ((x * x * x > 0.008856) ? x * x * x : (x - 16/116) / 7.787);
  y = 1.00000 * ((y * y * y > 0.008856) ? y * y * y : (y - 16/116) / 7.787);
  z = 1.08883 * ((z * z * z > 0.008856) ? z * z * z : (z - 16/116) / 7.787);

  r = x *  3.2406 + y * -1.5372 + z * -0.4986;
  g = x * -0.9689 + y *  1.8758 + z *  0.0415;
  b = x *  0.0557 + y * -0.2040 + z *  1.0570;

  r = (r > 0.0031308) ? (1.055 * Math.pow(r, 1/2.4) - 0.055) : 12.92 * r;
  g = (g > 0.0031308) ? (1.055 * Math.pow(g, 1/2.4) - 0.055) : 12.92 * g;
  b = (b > 0.0031308) ? (1.055 * Math.pow(b, 1/2.4) - 0.055) : 12.92 * b;

  return [Math.max(0, Math.min(1, r)) * 255, 
          Math.max(0, Math.min(1, g)) * 255, 
          Math.max(0, Math.min(1, b)) * 255]
}

.color {
  width: 200px;
  height: 200px;
  border: 1px solid #ccc;
}

#rgb {
  background: rgb(255, 242, 0)
}

<h3>rgba(255, 242, 0, 1)</h3>
<div class="color" id="rgb"></div>
<h3>LAB*a (95, -13, 103, 1) to RGB</h3>
<div class="color" id="labToRgb"></div>

Ideally, you could parse a "output-intent" (target) color profile

E.g an .icc file and then convert LAB colors to meet the desired target color space.

How to convert color object from parsed PSD into usable RGBA

1 Answers1

You can't precisely convert LAB to RGB (without a target color space definition)

Workaround: generic RGB approximation:

Ideally, you could parse a "output-intent" (target) color profile

Linked