creating alpha images in java

Question

I'm about to start work on a project which will generate some PNGs with alpha (gradients) and hoping to draw them programmatically within java.

For (a simple) example, I would like to draw a box, add a drop-shadow and then save this to a PNG file which can then be overlayed on some other graphic.

1. Is this possible with the standard JRE system libraries?
2. which libraries would make this kind of operation simple?

thanks.

Answer 1

Is this possible with the standard JRE system libraries?

Yes, it is possible and is pretty simple aswell. The code below produces this image (transparent png):

public static void main(String[] args) throws IOException {

    int x = 50, y = 50, w = 300, h = 200;

    // draw the "shadow"
    BufferedImage img = new BufferedImage(400, 300, BufferedImage.TYPE_INT_ARGB);
    Graphics g = img.getGraphics();
    g.setColor(Color.BLACK);
    g.fillRect(x + 10, y + 10, w, h);

    // blur the shadow
    BufferedImageOp op = getBlurredOp();
    img = op.filter(img, null);

    // draw the box
    g = img.getGraphics();
    g.setColor(Color.RED);
    g.fillRect(x, y, w, h);

    // write it to disk
    ImageIO.write(img, "png", new File("test.png"));
}

private static BufferedImageOp getBlurredOp() {

    float[] matrix = new float[400];
    for (int i = 0; i < 400; i++)
        matrix[i] = 1.0f/400.0f;

    return new ConvolveOp(new Kernel(20, 20, matrix),
                          ConvolveOp.EDGE_NO_OP, null);
}

Which libraries would make this kind of operation simple?

I would say that it depends on your other use cases. For simple shapes like boxes and ovals I would go for the solution above, no library is needed.