Change color of non-transparent parts of png in Java

Question

I am trying to automatically change the color for a set of icons. Every icon has a white filled layer and the other part is transparent. Here is an example: (in this case it's green, just to make it visible)

I tried to do the following:

private static BufferedImage colorImage(BufferedImage image) {
        int width = image.getWidth();
        int height = image.getHeight();

        for (int xx = 0; xx < width; xx++) {
            for (int yy = 0; yy < height; yy++) {
                Color originalColor = new Color(image.getRGB(xx, yy));
                System.out.println(xx + "|" + yy + " color: " + originalColor.toString() + "alpha: "
                        + originalColor.getAlpha());
                if (originalColor.equals(Color.WHITE) && originalColor.getAlpha() == 255) {
                    image.setRGB(xx, yy, Color.BLUE.getRGB());
                }
            }
        }
        return image;
    }

The problem I have is that every pixel I get has the same value:

32|18 color: java.awt.Color[r=255,g=255,b=255]alpha: 255

So my result is just a colored square. How can I achieve to change the color of the non-transparent parts only? And why is it, that all pixels have even the same alpha value? I guess that's my main problem: That the alpha value isn't read correctly.

Answer 1

Why it doesn't work, I don't know, this will.

This changes all the pixles to blue, maintaining their alpha values...

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

public class TestColorReplace {

    public static void main(String[] args) {
        try {
            BufferedImage img = colorImage(ImageIO.read(new File("NWvnS.png")));
            ImageIO.write(img, "png", new File("Test.png"));
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    private static BufferedImage colorImage(BufferedImage image) {
        int width = image.getWidth();
        int height = image.getHeight();
        WritableRaster raster = image.getRaster();

        for (int xx = 0; xx < width; xx++) {
            for (int yy = 0; yy < height; yy++) {
                int[] pixels = raster.getPixel(xx, yy, (int[]) null);
                pixels[0] = 0;
                pixels[1] = 0;
                pixels[2] = 255;
                raster.setPixel(xx, yy, pixels);
            }
        }
        return image;
    }
}

Answer 2

The problem is, that

Color originalColor = new Color(image.getRGB(xx, yy));

discards all the alpha values. Instead you have to use

 Color originalColor = new Color(image.getRGB(xx, yy), true);

to keep alpha values available.

Answer 3

If your bitmap is already set in an ImageView, just do :

imageView.setColorFilter(Color.RED);

to set all non transparent pixels to red.

Answer 4

Since we will always be replacing only the first three bands of the RGB pixel, more effective way to achieve the same without unnecessary allocation of new arrays would be:

private static void colorImageAndPreserveAlpha(BufferedImage img, Color c) {
    WritableRaster raster = img.getRaster();
    int[] pixel = new int[] {c.getRed(),c.getGreen(),c.getBlue()};
    for (int x = 0; x < raster.getWidth(); x++) 
        for (int y = 0; y < raster.getHeight(); y++)
            for (int b = 0; b < pixel.length; b++)
                raster.setSample(x,y,b,pixel[b]);    
}