Question about sRGB --> XYZ and back
lazarus / 39-Graphics

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stab · 2022-02-11 21:53

Have tried the equations given by EasyRGB to convert from sRGB to XYZ
and back again.
From sRGB to XYZ gave exactly the same result as EasyRGBs' Color
Calculator, but when converting back again gave wrong result.
Start with sR:64, sG:128, sB:255 gave
               X:27.877, Y:23.745, Z:97.702
back gave sR:64, sG:176, sB:249

Procedure for XYZ --> sRGB conversion:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
XYZTosRGB(X, Y, Z : double; var sR, sG, sB : byte);var  var_X, var_Y,
var_Z : double;   function Adjust(s : double) : double;  begin    if s
> 0.0031308 then      Result := 1.055 * Power(s, 1.0 / 2.4) - 0.055  
else      Result := 12.92 * s;  end; begin  //X, Y and Z input refer
to a D65/standard illuminant.  //sR, sG and sB (standard RGB) output2°
 range := 0 ÷ 255   var_X := X / 100;  var_Y := Y / 100;  var_Z := Z /
100;   var_X := var_X *  3.2406 + var_Y * -1.5372 + var_Z * -0.4986; 
var_Y := var_X * -0.9689 + var_Y *  1.8758 + var_Z *  0.0415;  var_Z
:= var_X *  0.0557 + var_Y * -0.2040 + var_Z *  1.0570;   var_X :=
Adjust(var_X);  var_Y := Adjust(var_Y);  var_Z := Adjust(var_Z);   sR
:= Round(var_X * 255);  sG := Round(var_Y * 255);  sB := Round(var_Z *
255);end;  
Thankful for any suggestions to what I'm doing wrong

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winni · 2022-02-12 00:53

Hi!


Don't reinvent the wheel.

Use BGRAbitmap.

Read https://wiki.freepascal.org/BGRABitmap

And look for:

ConvertToLinearRGB : to convert from sRGB to RGB.
ConvertFromLinearRGB : viceversa

Winni

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stab · 2022-02-12 14:00

Thanks for your response.

Noticed in Circulars response about  New version of BGRABitmap in
August 20, 2018 that it is possible to convert to many color spaces
with BGRABitmap. I'll try to read his colorspace demoapp carefully