Colored image will become black and white

Question

For a project of mine, which is targeting java 6, I had been in the need to have different algorithms/methods to change a BufferdImage with a ColorSpace.TYPE_RGB to a plain ole black and white one.

To test these implementations I therefore created an interface

public interface BlackAndWhiteConverter {
    BufferedImage toBlackAndWhite(BufferedImage image);
    void setThreshold(int threshold);
}  

and the, for me, most successful implementation is up for review

import java.awt.color.ColorSpace;
import java.awt.image.BufferedImage;
import java.awt.image.DataBuffer;
import java.awt.image.DataBufferInt;
import java.awt.image.Raster;
import java.awt.image.WritableRaster;

public class BlackAndWhiteRedThresholdConverter implements BlackAndWhiteConverter {

    @Override
    public BufferedImage toBlackAndWhite(final BufferedImage image) {

        final int imageType = image.getColorModel().getColorSpace().getType();

        if (imageType != ColorSpace.TYPE_RGB) {
            throw new IllegalArgumentException();
        }

        final int width = image.getWidth();
        final int height = image.getHeight();
        final int upperLimit = width * height;

        final DataBuffer buffer = image.getRaster().getDataBuffer();
        final int[] source = ((DataBufferInt) buffer).getData();

        byte[] destination = new byte[upperLimit];

        for (int i = 0; i < upperLimit; i++) {

            int red = (source[i] >> 16) & 0x000000FF;

            if (red > threshold) {
                destination[i] = -1;
            }
        }

        WritableRaster raster = Raster.createPackedRaster(DataBuffer.TYPE_BYTE, width, height, 1, 2, null);
        raster.setDataElements(0, 0, width, height, destination);

        BufferedImage destinationImage = new BufferedImage(width, height, BufferedImage.TYPE_BYTE_BINARY);
        destinationImage.setData(raster);
        return destinationImage;
    }

    private int threshold = 110;

    @Override
    public void setThreshold(int threshold) {
        this.threshold = threshold;
    }


}

Please comment on anything that springs into your face. No matter what, I can't use a java version higher than 6.

Answer 1

Input validation

If the received image is not RGB, throwing an IllegalArgumentException without a custom message is not very friendly. Users would have to look at the code to understand what went wrong exactly.

The long chain image.getColorModel().getColorSpace().getType() is extracted to an imageType local variable that's only used for validation. I would be good to move the validation to a dedicated helper method, let's say validateImageType.

Java conventions

Member variables should be declared at the top, followed by constructors, followed by methods. Very roughly speaking. So move the declaration of threshold higher.

Naming

As the interface name already implies the conversion to black and white, the method name toBlackAndWhite is redundant. I'd rename it to simply convert.

Misc

0x000000FF is the same as 0xFF, so you can shorten it.

Answer 2

Taking into account other colors

In case your input images aren't just grayscale images, you may need to take into account colors other than red. The standard RGB to grayscale formula looks like this:

gray = (red * 0.3 + green * 0.59 + blue * 0.11)

You can approximate that without using floating point like this:

gray = (red*3 + green*6 + blue) / 10

So your loop would look like this:

    int grayThreshold = threshold * 10;

    for (int i = 0; i < upperLimit; i++) {

        int red   = (source[i] >> 16) & 0xFF;
        int green = (source[i] >>  8) & 0xFF;
        int blue  = (source[i])       & 0xFF;
        int gray  = red*3 + green*6 + blue;

        if (gray > grayThreshold) {
            destination[i] = -1;
        }
    }

Answer 3

My first thought is that this isn't very object-oriented. There's only one field and you can very easily refactor the class so that it doesn't need it:

public abstract class BlackAndWhiteRedThresholdConverter implements BlackAndWhiteConverter {

    public static BufferedImage toBlackAndWhite(final BufferedImage image, final int threshold) {
        // convert
    }

    @Override
    public static BufferedImage toBlackAndWhite(final BufferedImage image) {
        return toBlackAndWhite(image, 110);
    }
}

This class is slightly easier to use than yours. There's no need to instantiate, and I can still use either my own threshold or the default one. Basically though it's no different. However, it's easier to see from this class definition that it's not object-oriented. It's procedural. It ends with "-er" and that's often bad.

A better solution might involve extending BufferedImage. Now when I instantiate a BlackAndWhiteImage I get a proper object who is actually useful.

public class BlackAndWhiteImage extends BufferedImage
{
    private static final int DEFAULT_THRESHOLD = 110;

    public static BlackAndWhiteImage make(final BufferedImage image, final int threshold)
    {
        return toBlackAndWhite(image, threshold);
    }

    public static BlackAndWhiteImage make(final BufferedImage image)
    {
        return make(image, DEFAULT_THRESHOLD);
    }

    /* Private constructor to create underlying BufferedImage */ 
    private BlackAndWhiteImage(ColorModel colourModel, WritableRaster raster, boolean alphaPremult)
    {
        super(colourModel, raster, alphaPremult, null);
    }

    private static BlackAndWhiteImage toBlackAndWhite(final BufferedImage image, final int threshold)
    {
        /* Conversion stuff is all the same */
        return new BlackAndWhiteImage( destinationImage.getColorModel(),
                                       destinationImage.getRaster(),
                                       destinationImage.isAlphaPremultiplied() );
    }
}

The static factory methods (make) are unfortunate but I couldn't get around using them.

Example usage:

BufferedImage img = new BufferedImage(50, 50, BufferedImage.TYPE_INT_RGB);
BlackAndWhiteImage bw = BlackAndWhiteImage.make(img);

File outputfile = new File("");
ImageIO.write(bw, "jpg", outputfile);