使用RGBA中的Alpha来使图像不透明

Question

I tried to make a simple program which load an image, make it blue effect, and also make it half transperant. I'm doing it by running on the image pixels and change the RGB's blue value and alpha value. I successed to make a nice blue effect to the image. But I couldn't manage to change the image opacity. It seems that no matter how I change the alpha value of the pixels, it doesn't effect the image.

Here is my code:

try {
    image1 = ImageIO.read(new File("image1.png"));
} catch(IOException e) {e.printStackTrace();}


for(int x=0;x<image1.getWidth();x++) {
    for(int y=0;y<image1.getHeight();y++) {
        int rgb = image1.getRGB(x, y);
        Color color = new Color(rgb);
        int r = color.getRed();
        int g = color.getGreen();
        int b = color.getBlue();
        int a = color.getAlpha();

        System.out.println(a);
        a= 100;
        if(b<155)
            b+=100;
        else
            b=255;

        color = new Color(r,g,b,a);

        image1.setRGB(x, y, color.getRGB());
    }
}

UPDATE: I also tried this. still doesn't work:

for(int x=0;x<image1.getWidth();x++) {
    for(int y=0;y<image1.getHeight();y++) {
        int rgb = image1.getRGB(x, y);
        Color color = new Color(rgb,true);
        int r = color.getRed();
        int g = color.getGreen();
        int b = color.getBlue();
        int a = color.getAlpha();

        a= 100;
        if(b<155)
            b+=100;
        else
            b=255;


        rgb = rgb | b;
        rgb = rgb & 0x33ffffff;


        image1.setRGB(x, y, rgb);
    }
}

Answer 1

Use an AlphaComposite :

BufferedImage img = //some code here
BufferedImage imgClone = //some clone of img, but let its type be BufferedImage.TYPE_INT_ARGB
Graphics2D imgCloneG = imgClone.createGraphics();
imgCloneG.setComposite(AlphaComposite.getInstance(AlphaComposite.DST_IN, 0.5f));
imgCloneG.drawImage(img, 0, 0, null);
//imgClone is now img at half alpha

imgClone can be made like this:

...
imgClone = new BufferedImage(img.getWidth(), img.getHeight(), 
                             BufferedImage.TYPE_INT_ARGB);
Graphics2D imgCloneG = imgClone.createGraphics();
imgCloneG.drawImage(img, 0, 0, null);
imgCloneG.setComposite(AlphaComposite.getInstance(AlphaComposite.DST_IN, 0.5f));
...

Answer 2

The Color constructor that takes only an int creates an opaque color (with alpha 255). When you take out the alpha again with getAlpha() , it's always 255 here.

Relevant Java Color code, from line 413 (at least in Java 1.7):

public Color(int rgb) {
    value = 0xff000000 | rgb;
}

The most significant 8 bytes are set, overwriting whatever alpha component was there.

To preserve the alpha component, you need to use the Color constructor that takes an int for the values and a boolean for whether it has an alpha component.

Color color = new Color(rgb, true);