无法从单个BufferedImage像素获取正确的Alpha值

Question

I need to get RGBA values, modify RGB, then set RGB (and A) values again.

However, it seems to return 255 for all the pixels, even though I know the image I'm loading has transparent pixels in it.

Here's an SSCCE

import javax.imageio.ImageIO;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

public class TestImage {

    public static void main(String args[]) {
        try {
            BufferedImage image = ImageIO.read(new File("/home/affy/Pictures/salamenceavatar.png"));

            BufferedImage buff = new BufferedImage(
                    image.getWidth(),
                    image.getHeight(),
                    BufferedImage.TYPE_INT_ARGB
            );

            Graphics2D g = buff.createGraphics();
            g.drawImage(image, 0, 0, null);

            for(int x = 0; x < image.getWidth(); x++) {
                for(int y = 0; y < image.getHeight(); y++) {
                    Color c = new Color(buff.getRGB(x, y));
                    int alpha = c.getAlpha();
                    System.out.println(alpha);
                }
            }
        }catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Answer 1

Use Color(int, boolean) instead, passing it true to tell it you want to extract the alpha value from the supplied packed int

Color c = new Color(buff.getRGB(x, y), true);

From the JavaDocs...

Creates an sRGB color with the specified combined RGBA value consisting of the alpha component in bits 24-31, the red component in bits 16-23, the green component in bits 8-15, and the blue component in bits 0-7. If the hasalpha argument is false, alpha is defaulted to 255.



rgba - the combined RGBA components
hasalpha - true if the alpha bits are valid; false otherwise