ios - 将 Lab 颜色转换为 RGB
<p><p>我正在尝试将 Lab 颜色转换为 RGB。作为此转换的示例，Photoshop
转换</p>
<pre><code>L= 100, a = 0, b = 127
</code></pre>
<p>进入</p>
<pre><code>R=255, G=252, B=0.
</code></pre>
<p>这个颜色是亮黄色的。</p>
<p>我编写了以下转换例程(C/Objective C)。然而，有些东西
是不正确的。代码将输出</p>
<pre><code>XYZ = 100.000000 100.000000 4.862713
RGB = 163.908401 90.755638 -13.093634
</code></pre>
<p>这里的 RGB 结果和 Photoshop 的 RGB 之间的不匹配似乎相当大。
负蓝色分量也是个问题。</p>
<p>任何关于如何更正以下代码的想法将不胜感激？或者，任何其他用于将 Lab 转换为 RGB 的算法/公式/伪代码？谢谢。</p>
<p>代码:</p>
<pre><code>/*
* See Wikipedia artice on Lab color space
*
* http://en.wikipedia.org/wiki/Lab_color_space
*
*/
CGFloat fInv (CGFloat t) {
CGFloat T0 = (CGFloat) 6.0f/29.0f;
if (t > T0) {
return t*t*t;
}
return 3.0 * T0 * T0 * (t - 4.0f/29.0f);
}
int test() {
//
//Bright yellow test
//
CGFloat Ls = 100.0f;
CGFloat as = 0.0f;
CGFloat bs = 127.0f;
//
//Reference white point (ad hoc)
//
CGFloat Xn = 100;
CGFloat Yn = 100;
CGFloat Zn = 100;
//
// Taken Wikipedia (see above)
//
CGFloat X =Xn * fInv(1/116.0f * (Ls + 16.0f) + 1/500.0f * as);
CGFloat Y =Yn * fInv(1/116.0f * (Ls + 16.0f));
CGFloat Z =Zn * fInv(1/116.0f * (Ls + 16.0f) - 1/200.0f * bs);
NSLog(@" XYZ = %f %f %f", X, Y, Z);
//
//
// XYZ to RGB transfomration using AppleRGB. Taken from
//
// http://brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
//
//
CGFloat r =2.9515373 * X- 1.2894116 * Y - 0.4738445 * Z;
CGFloat g = -1.0851093 * X+ 1.9908566 * Y + 0.0372026 * Z;
CGFloat b =0.0854934 * X- 0.2694964 * Y + 1.0912975 * Z;
NSLog(@" RGB = %f %f %f", r, g, b);
}
</code></pre></p>
<br><hr><h1><strong>Best Answer-推荐答案</ strong></h1><br>
<p><p>尝试实现 <a href="http://www.easyrgb.com/index.php?X=MATH&H=08#text8" rel="noreferrer noopener nofollow">this site</a> 中的公式.我刚刚实现了 RGB->Lab(通过 XYZ)，它工作正常。</p>
<p>C++ 代码:</p>
<pre><code>// using http://www.easyrgb.com/index.php?X=MATH&H=01#text1
void rgb2lab( float R, float G, float B, float & l_s, float &a_s, float &b_s )
{
float var_R = R/255.0;
float var_G = G/255.0;
float var_B = B/255.0;
if ( var_R > 0.04045 ) var_R = pow( (( var_R + 0.055 ) / 1.055 ), 2.4 );
else var_R = var_R / 12.92;
if ( var_G > 0.04045 ) var_G = pow( ( ( var_G + 0.055 ) / 1.055 ), 2.4);
else var_G = var_G / 12.92;
if ( var_B > 0.04045 ) var_B = 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
float X = var_R * 0.4124 + var_G * 0.3576 + var_B * 0.1805;
float Y = var_R * 0.2126 + var_G * 0.7152 + var_B * 0.0722;
float Z = var_R * 0.0193 + var_G * 0.1192 + var_B * 0.9505;
float var_X = X / 95.047 ; //ref_X =95.047 Observer= 2°, Illuminant= D65
float var_Y = Y / 100.000; //ref_Y = 100.000
float var_Z = Z / 108.883; //ref_Z = 108.883
if ( var_X > 0.008856 ) var_X = pow(var_X , ( 1./3. ) );
else var_X = ( 7.787 * var_X ) + ( 16. / 116. );
if ( var_Y > 0.008856 ) var_Y = pow(var_Y , ( 1./3. ));
else var_Y = ( 7.787 * var_Y ) + ( 16. / 116. );
if ( var_Z > 0.008856 ) var_Z = pow(var_Z , ( 1./3. ));
else var_Z = ( 7.787 * var_Z ) + ( 16. / 116. );
l_s = ( 116. * var_Y ) - 16.;
a_s = 500. * ( var_X - var_Y );
b_s = 200. * ( var_Y - var_Z );
}
//http://www.easyrgb.com/index.php?X=MATH&H=01#text1
void lab2rgb( float l_s, float a_s, float b_s, float& R, float& G, float& B )
{
float var_Y = ( l_s + 16. ) / 116.;
float var_X = a_s / 500. + var_Y;
float var_Z = var_Y - b_s / 200.;
if ( pow(var_Y,3) > 0.008856 ) var_Y = pow(var_Y,3);
else var_Y = ( var_Y - 16. / 116. ) / 7.787;
if ( pow(var_X,3) > 0.008856 ) var_X = pow(var_X,3);
else var_X = ( var_X - 16. / 116. ) / 7.787;
if ( pow(var_Z,3) > 0.008856 ) var_Z = pow(var_Z,3);
else var_Z = ( var_Z - 16. / 116. ) / 7.787;
float X = 95.047 * var_X ; //ref_X =95.047 Observer= 2°, Illuminant= D65
float Y = 100.000 * var_Y; //ref_Y = 100.000
float Z = 108.883 * var_Z ; //ref_Z = 108.883
var_X = X / 100. ; //X from 0 to95.047 (Observer = 2°, Illuminant = D65)
var_Y = Y / 100. ; //Y from 0 to 100.000
var_Z = Z / 100. ; //Z from 0 to 108.883
float var_R = var_X *3.2406 + var_Y * -1.5372 + var_Z * -0.4986;
float var_G = var_X * -0.9689 + var_Y *1.8758 + var_Z *0.0415;
float var_B = var_X *0.0557 + var_Y * -0.2040 + var_Z *1.0570;
if ( var_R > 0.0031308 ) var_R = 1.055 * pow(var_R , ( 1 / 2.4 ))- 0.055;
else var_R = 12.92 * var_R;
if ( var_G > 0.0031308 ) var_G = 1.055 * pow(var_G , ( 1 / 2.4 ) )- 0.055;
else var_G = 12.92 * var_G;
if ( var_B > 0.0031308 ) var_B = 1.055 * pow( var_B , ( 1 / 2.4 ) ) - 0.055;
else var_B = 12.92 * var_B;
R = var_R * 255.;
G = var_G * 255.;
B = var_B * 255.;
}
</code></pre></p>
<p style="font-size: 20px;">关于ios - 将 Lab 颜色转换为 RGB，我们在Stack Overflow上找到一个类似的问题：
<a href="https://stackoverflow.com/questions/7880264/" rel="noreferrer noopener nofollow" style="color: red;">
https://stackoverflow.com/questions/7880264/
</a>
</p>
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