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copied!<p>I did that recently, to answer a question of my project manager.<br> The function transforming gray to transparency is:</p> <pre><code> private Image TransformGrayToTransparency(BufferedImage image) { ImageFilter filter = new RGBImageFilter() { public final int filterRGB(int x, int y, int rgb) { return (rgb << 8) & 0xFF000000; } }; ImageProducer ip = new FilteredImageSource(image.getSource(), filter); return Toolkit.getDefaultToolkit().createImage(ip); } </code></pre> <p>Actually, it acts on a gray-level image, so I just copy a RGB component (the R one) to alpha, discarding the others which are identical in my case.<br> You can adapt it to filter a specific color, eg. with a test of equality or range, etc.<br> Of course, the BufferedImage must be of BufferedImage.TYPE_INT_ARGB type.</p> <p>I don't address the question of saving, as it is pretty trivial, but I can add this code page too.</p> <p>[EDIT] To convert Image to BufferedImage:</p> <pre><code>BufferedImage dest = new BufferedImage( imageWidth, imageHeight, BufferedImage.TYPE_INT_ARGB); Graphics2D g2 = dest.createGraphics(); g2.drawImage(image, 0, 0, null); g2.dispose(); </code></pre> <p>[EDIT 2] I come after Christoffer posted his complete solution, but here is mine, I show how to make a range of colors transparent. Can be improved, eg. using HSB components instead.</p> <pre><code>import java.awt.*; import java.awt.image.BufferedImage; import java.awt.image.FilteredImageSource; import java.awt.image.ImageFilter; import java.awt.image.ImageProducer; import java.awt.image.RGBImageFilter; import java.io.*; import javax.imageio.ImageIO; public class AddTransparency { AddTransparency() throws IOException { String imagePath = "E:/Documents/images/"; File inFile = new File(imagePath, "map.png"); BufferedImage image = ImageIO.read(inFile); Image transpImg1 = TransformGrayToTransparency(image); BufferedImage resultImage1 = ImageToBufferedImage(transpImg1, image.getWidth(), image.getHeight()); File outFile1 = new File(imagePath, "map_with_transparency1.png"); ImageIO.write(resultImage1, "PNG", outFile1); Image transpImg2 = TransformColorToTransparency(image, new Color(0, 50, 77), new Color(200, 200, 255)); BufferedImage resultImage2 = ImageToBufferedImage(transpImg2, image.getWidth(), image.getHeight()); File outFile2 = new File(imagePath, "map_with_transparency2.png"); ImageIO.write(resultImage2, "PNG", outFile2); } private Image TransformGrayToTransparency(BufferedImage image) { ImageFilter filter = new RGBImageFilter() { public final int filterRGB(int x, int y, int rgb) { return (rgb << 8) & 0xFF000000; } }; ImageProducer ip = new FilteredImageSource(image.getSource(), filter); return Toolkit.getDefaultToolkit().createImage(ip); } private Image TransformColorToTransparency(BufferedImage image, Color c1, Color c2) { // Primitive test, just an example final int r1 = c1.getRed(); final int g1 = c1.getGreen(); final int b1 = c1.getBlue(); final int r2 = c2.getRed(); final int g2 = c2.getGreen(); final int b2 = c2.getBlue(); ImageFilter filter = new RGBImageFilter() { public final int filterRGB(int x, int y, int rgb) { int r = (rgb & 0xFF0000) >> 16; int g = (rgb & 0xFF00) >> 8; int b = rgb & 0xFF; if (r >= r1 && r <= r2 && g >= g1 && g <= g2 && b >= b1 && b <= b2) { // Set fully transparent but keep color return rgb & 0xFFFFFF; } return rgb; } }; ImageProducer ip = new FilteredImageSource(image.getSource(), filter); return Toolkit.getDefaultToolkit().createImage(ip); } private BufferedImage ImageToBufferedImage(Image image, int width, int height) { BufferedImage dest = new BufferedImage( width, height, BufferedImage.TYPE_INT_ARGB); Graphics2D g2 = dest.createGraphics(); g2.drawImage(image, 0, 0, null); g2.dispose(); return dest; } public static void main(String[] args) throws IOException { AddTransparency at = new AddTransparency(); } } </code></pre>

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