How to Convert from Cmyk to Rgb in Java Correctly

java CMYK to RGB values converter

Yes, using the CMYK to RGB formulae:

double red = 255*(1-cyan)*(1-black);
double green = 255*(1-magenta)*(1-black);
double blue = 255*(1-yellow)*(1-black);

Where CMYK values range from 0 to 1 and RGB values range from 0 to 255.

JMagick - How to convert a picture from CMYK to RGB?

Update: You are using GIF images. They don't support "CMYK" so the transform won't work for you (see this forum post at imagemagick's web site)!

Use MagicImage.rgbTransformImage(ColorspaceType.CMYKColorspace). From the API:

public boolean rgbTransformImage(int colorspace) throws MagickException

Converts the reference image from RGB to an alternate colorspace. The transformation matrices are not the standard ones: the weights are rescaled to normalized the range of the transformed values to be [0..MaxRGB].

Example:

try {
    MagickImage image = new MagickImage(new ImageInfo(baseName + fileName));

    if (!image.rgbTransformImage(ColorspaceType.CMYKColorspace))
         throw new Exception("Couldn't convert image color space");

    ...
} catch (MagickException e) {
    ...
}

how to read an image from a url and convert it from CMYK to RGB in java?

You don't need to change a lot in my answer. The method ImageIO.createImageInputStream can take an InputStream instead of a file.

So in the simplest case, you change the code like this:

public BufferedImage readImage(String imageUrl) throws IOException, ImageReadException {

    /* some code omitted */

    URL url = new URL(imageUrl;
    ImageInputStream stream = ImageIO.createImageInputStream(url.openStream());

Update

As the input data is required several times, it makes sense to first read it into a byte array and then continue to work with the byte array:

byte[] readIntoByteArray(URL url) throws IOException {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    InputStream is = url.openStream ();

    byte[] buffer = new byte[4096];
    int n;
    while ( (n = is.read(buffer)) > 0 ) {
        baos.write(bufer, 0, n);
    }

    return boas. toByteArray();
}

public BufferedImage readImage(String imageUrl) throws IOException, ImageReadException {

    /* code omitted */

    byte[] imageData = readIntoByteArray(imageUrl);
    ImageInputStream stream = ImageIO.createImageInputStream(new ByteArrayInputStream(imageData));

    /* code omitted */
}

public void checkAdobeMarker(byte[] imageData) throws IOException, ImageReadException {
    JpegImageParser parser = new JpegImageParser();
    ByteSource byteSource = new ByteSourceArray(imageData);

     /* code omitted */
}

RGB to CMYK and back algorithm

As Lea Verou said you should make use of color space information because there isn't an algorithm to map from RGB to CMYK. Adobe has some ICC color profiles available for download1, but I'm not sure how they are licensed.

Once you have the color profiles something like the following would do the job:

import java.awt.color.ColorSpace;
import java.awt.color.ICC_ColorSpace;
import java.awt.color.ICC_Profile;
import java.io.IOException;
import java.util.Arrays;

public class ColorConv {
    final static String pathToCMYKProfile = "C:\\UncoatedFOGRA29.icc";

    public static float[] rgbToCmyk(float... rgb) throws IOException {
        if (rgb.length != 3) {
            throw new IllegalArgumentException();
        }
        ColorSpace instance = new ICC_ColorSpace(ICC_Profile.getInstance(pathToCMYKProfile));
        float[] fromRGB = instance.fromRGB(rgb);
        return fromRGB;
    }
    public static float[] cmykToRgb(float... cmyk) throws IOException {
        if (cmyk.length != 4) {
            throw new IllegalArgumentException();
        }
        ColorSpace instance = new ICC_ColorSpace(ICC_Profile.getInstance(pathToCMYKProfile));
        float[] fromRGB = instance.toRGB(cmyk);
        return fromRGB;
    }

    public static void main(String... args) {
        try {
            float[] rgbToCmyk = rgbToCmyk(1.0f, 1.0f, 1.0f);
            System.out.println(Arrays.toString(rgbToCmyk));
            System.out.println(Arrays.toString(cmykToRgb(rgbToCmyk[0], rgbToCmyk[1], rgbToCmyk[2], rgbToCmyk[3])));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Java CMYK to RGB with profile. Output is too dark

Like I said, the idea was to convert CMYK pictures to RGB, and use them in my application.

But for some reason ConvertOp doesn't do any CMYK to RGB conversion. It reduces numBand numbers to 3 and that's it. And I decided to try CMYKtoRGB algorithms.

i.e. Get an image, recognize its ColorSpace and read it or convert it.

Also another problem was Photoshop. This quote I found on the internet.

In the case of adobe it includes the CMYK profile in the metadata, but then saves the raw image data as inverted YCbCrK colors.

Finally I could achieve my goal with this algorithm below.
I don't use icc_profiles so far, the output looks a little bit darker.. I got proper RGB images which looks fine.

pseudocode

BufferedImage result = null;
Raster r = reader.readRaster()
if (r.getNumBands != 4){
    result = reader.read(0);
} else {

   if (isPhotoshopYCCK(reader)){
       result = YCCKtoCMYKtoRGB(r);
   }else{
      result = CMYKtoRGB(r);
   }
}

private boolean isPhotoshopYCCK(reader){
    // read IIOMetadata from reader and according to
    // http://download.oracle.com/javase/6/docs/api/javax/imageio/metadata/doc-files/jpeg_metadata.html decide which ColorSpace is used
    // or maybe there is another way to do it
    int transform = ... // 2 or 0 or something else
    return transform;
}

I doesn't make any sense to show YCCKtoCMYKtoRGB or CMYKtoRGB algorithms.
It is easy to find on the internet.

Draw CMYK Color from RGB

It looks to me like the java color class, has a constructor for making a color object in cmyk

https://docs.oracle.com/javase/7/docs/api/java/awt/Color.html#Color(java.awt.color.ColorSpace,%20float[],%20float)

and

https://docs.oracle.com/javase/7/docs/api/java/awt/color/ColorSpace.html

So you would end up with something like

Color cmykColorValue = new Color(TYPE_CMYK, [cValue, mValue, yValue, kValue], alpha)

Where alpha is form 0 to 1, and cValue, mValue, yValue, kValue are the corresponding cmyk values.

That should make a new CMYK color object that can be used anywhere a color object can be used.

How do I convert images between CMYK and RGB in ColdFusion (Java)?

I use the Java ImageIO libraries (https://jai-imageio.dev.java.net). They aren't perfect, but can be simple and get the job done. As far as converting from CMYK to RGB, here is the best I have been able to come up with.

Download and install the ImageIO JARs and native libraries for your platform. The native libraries are essential. Without them the ImageIO JAR files will not be able to detect the CMYK images. Originally, I was under the impression that the native libraries would improve performance but was not required for any functionality. I was wrong.

The only other thing that I noticed is that the converted RGB images are sometimes much lighter than the CMYK images. If anyone knows how to solve that problem, I would be appreciative.

Below is some code to convert a CMYK image into an RGB image of any supported format.

Thank you,

Randy Stegbauer

package cmyk;

import java.awt.color.ColorSpace;
import java.awt.image.BufferedImage;
import java.awt.image.ColorConvertOp;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;

import org.apache.commons.lang.StringUtils;

public class Main
{

    /**
     * Creates new RGB images from all the CMYK images passed
     * in on the command line.
     * The new filename generated is, for example "GIF_original_filename.gif".
     *
     */
    public static void main(String[] args)
    {
        for (int ii = 0; ii < args.length; ii++)
        {
            String filename = args[ii];
            boolean cmyk = isCMYK(filename);
            System.out.println(cmyk + ": " + filename);
            if (cmyk)
            {
                try
                {
                    String rgbFile = cmyk2rgb(filename);
                    System.out.println(isCMYK(rgbFile) + ": " + rgbFile);
                }
                catch (IOException e)
                {
                    System.out.println(e.getMessage());
                }
            }
        }
    }

    /**
     * If 'filename' is a CMYK file, then convert the image into RGB,
     * store it into a JPEG file, and return the new filename.
     *
     * @param filename
     */
    private static String cmyk2rgb(String filename) throws IOException
    {
        // Change this format into any ImageIO supported format.
        String format = "gif";
        File imageFile = new File(filename);
        String rgbFilename = filename;
        BufferedImage image = ImageIO.read(imageFile);
        if (image != null)
        {
            int colorSpaceType = image.getColorModel().getColorSpace().getType();
            if (colorSpaceType == ColorSpace.TYPE_CMYK)
            {
                BufferedImage rgbImage =
                    new BufferedImage(
                        image.getWidth(), image.getHeight(), BufferedImage.TYPE_3BYTE_BGR);
                ColorConvertOp op = new ColorConvertOp(null);
                op.filter(image, rgbImage);

                rgbFilename = changeExtension(imageFile.getName(), format);
                rgbFilename = new File(imageFile.getParent(), format + "_" + rgbFilename).getPath();
                ImageIO.write(rgbImage, format, new File(rgbFilename));
            }
        }
        return rgbFilename;
    }

    /**
     * Change the extension of 'filename' to 'newExtension'.
     *
     * @param filename
     * @param newExtension
     * @return filename with new extension
     */
    private static String changeExtension(String filename, String newExtension)
    {
        String result = filename;
        if (filename != null && newExtension != null && newExtension.length() != 0);
        {
            int dot = filename.lastIndexOf('.');
            if (dot != -1)
            {
                result = filename.substring(0, dot) + '.' + newExtension;
            }
        }
        return result;
    }

    private static boolean isCMYK(String filename)
    {
        boolean result = false;
        BufferedImage img = null;
        try
        {
            img = ImageIO.read(new File(filename));
        }
        catch (IOException e)
        {
            System.out.println(e.getMessage() + ": " + filename);
        }
        if (img != null)
        {
            int colorSpaceType = img.getColorModel().getColorSpace().getType();
            result = colorSpaceType == ColorSpace.TYPE_CMYK;
        }

        return result;
    }
}