In one of the projects I am working on at the moment, I needed to convert the background colour of an image to be transparent so the image looks better on a non-white background.

Looking on the Web, I found the following article from Dustin Marx:
Making White Image Backgrounds Transparent with Java 2D/Groovy
If the link is broken, please download his code from the following link: ImageTransparency.java

The method which makes the background colour transparent is called makeColorTransparent. This method works pretty well, except in some cases as shown in the example below:

Original image	Converted image using Dustin’s method

This is actually quite normal. Indeed, his code is converting a specific colour (#FFFFFF in our case) to be transparent. But what if the background is not homogeneous?

This is the reason why I had to modify his method to add a new parameter called tolerance:

private Image makeColorTransparent(final BufferedImage im, final Color color, int tolerance) {
    int temp = 0;
    if (tolerance < 0 || tolerance > 100) {
        System.err.println("The tolerance is a percentage, so the value has to be between 0 and 100.");
        temp = 0;
    } else {
        temp = tolerance * (0xFF000000 | 0xFF000000) / 100;
    }
    final int toleranceRGB = Math.abs(temp);
		
    final ImageFilter filter = new RGBImageFilter() {
        // The color we are looking for (white)... Alpha bits are set to opaque
        public int markerRGBFrom = (color.getRGB() | 0xFF000000) - toleranceRGB;
        public int markerRGBTo = (color.getRGB() | 0xFF000000) + toleranceRGB;

        public final int filterRGB(final int x, final int y, final int rgb) {
            if ((rgb | 0xFF000000) >= markerRGBFrom && (rgb | 0xFF000000) <= markerRGBTo) {
                // Mark the alpha bits as zero - transparent
                return 0x00FFFFFF & rgb;
            } else {
                // Nothing to do
                return rgb;
            }
        }
    };

    final ImageProducer ip = new FilteredImageSource(im.getSource(), filter);
    return Toolkit.getDefaultToolkit().createImage(ip);
}

Such as Photoshop, the tolerance is a percentage value between 0 and 100. The higher the tolerance is, the bigger the range of colours will be.

Let’s take our previous example and apply a 50% tolerance:

That looks much better, isn’t it? 🙂