RGB to CIELAB conversion

Question

import java.awt.*;
import java.awt.image.*;
import java.io.*;
import javax.imageio.ImageIO;


public class ConvertRGBtoLAB {


    public static void main(String[] args) {

      //get input image
     String fileName = "IMG_7990.jpg";
     //read input image
     BufferedImage image = null;
        try
        {
            image = ImageIO.read(new File(fileName));
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }


        //setup result image
        int sizeX = image.getWidth();
        int sizeY = image.getHeight();


                float r, g, b, X, Y, Z, fx, fy, fz, xr, yr, zr;
  float ls, as, bs;
  float eps = 216.f/24389.f;
  float k = 24389.f/27.f;

  float Xr = 0.964221f;  // reference white D50
  float Yr = 1.0f;
  float Zr = 0.825211f;

                for (int y = 0; y < image.getHeight(); y++) {
            for (int x = 0; x < image.getWidth(); x++) {
                int c = image.getRGB(x,y);
                int  R= (c & 0x00ff0000) >> 16;
                int  G = (c & 0x0000ff00) >> 8;
                int  B = c & 0x000000ff;

                 r = R/255.f; //R 0..1
  g = G/255.f; //G 0..1
  b = B/255.f; //B 0..1

  // assuming sRGB (D65)
  if (r <= 0.04045)
   r = r/12;
  else
   r = (float) Math.pow((r+0.055)/1.055,2.4);

  if (g <= 0.04045)
   g = g/12;
  else
   g = (float) Math.pow((g+0.055)/1.055,2.4);

  if (b <= 0.04045)
   b = b/12;
  else
   b = (float) Math.pow((b+0.055)/1.055,2.4);


  X =  0.436052025f*r     + 0.385081593f*g + 0.143087414f *b;
  Y =  0.222491598f*r     + 0.71688606f *g + 0.060621486f *b;
  Z =  0.013929122f*r     + 0.097097002f*g + 0.71418547f  *b;

  // XYZ to Lab
  xr = X/Xr;
  yr = Y/Yr;
  zr = Z/Zr;

  if ( xr > eps )
   fx =  (float) Math.pow(xr, 1/3.);
  else
   fx = (float) ((k * xr + 16.) / 116.);

  if ( yr > eps )
   fy =  (float) Math.pow(yr, 1/3.);
  else
  fy = (float) ((k * yr + 16.) / 116.);

  if ( zr > eps )
   fz =  (float) Math.pow(zr, 1/3.);
  else
   fz = (float) ((k * zr + 16.) / 116);

  ls = ( 116 * fy ) - 16;
  as = 500*(fx-fy);
  bs = 200*(fy-fz);


                int Ls = (int) (2.55* ls + .5);
                int As = (int) (as + .5);
                int Bs = (int) (bs + .5);

               int lab = 0xFF000000 + (Ls << 16) + (As << 8) + Bs; // and reassign

                image.setRGB(x, y, lab);

          }
        }


        //write new image
         File outputfile = new File("lab.png");
         try {
         // png is an image format (like gif or jpg)
         ImageIO.write(image, "png", outputfile);
         } catch (IOException ex) {
         ex.printStackTrace();
         }


    }


}

Hello,

I am trying to turn a RGB image into CIELAB colour space(LAB) I get an output but I have no idea what it is supposed to look like.

Can anyone point me to a already existing image converter or confirm that I have done this correctly?

Thanks!

There is no such color space as "RGB". RGB is an entire category of color spaces. — Dietrich Epp, Oct 05 '11 at 09:15

score 8 · Answer 1 · answered Nov 18 '10 at 19:08

8

I personally use this site as quick reference on conversion formulas between common color spaces.

answered Nov 18 '10 at 19:08

Agnius Vasiliauskas

10,935
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score 1 · Answer 2 · edited May 23 '17 at 10:29

1

OpenCV have functions for conversions between different color spaces. Look at my other answer here. This is in C, bout you can easily check you code.

edited May 23 '17 at 10:29

Community

1
1

answered Nov 18 '10 at 07:52

Ross

2,079
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score 1 · Answer 3 · answered Mar 22 '11 at 15:31

1

You will want to ensure that Ls, As, and Bs are clamped to the range 0 to 255. The statement you have to combine them into a single int will allow an out-of-bounds value to affect the other values.

answered Mar 22 '11 at 15:31

Mark Ransom

299,747
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a and b can be negative. They usually have a range of -128..+128 or -100..+100 – finnw Apr 26 '15 at 16:10
@finnw then they need to be offset. The value being constructed is unsigned. – Mark Ransom Apr 26 '15 at 16:19

RGB to CIELAB conversion

3 Answers3

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