Save buffered image with transparent background

问题:

I'm saving the image of a signature as a .jpg picture. I use graphic2d to paint on the image every pixel of the signature (gotten with a signature tablet) and it works perfectly but I'm always gettin a white background. If I want to put the signature on a PDF document, the borders of the white square of the jpg image covers some of the words of the PDF.

What I want to get is to save the jpg image with a transparent background, so when I put it on the PDF there are no words covered with the white image background but just the signature lines.

This is the code that saves the buffered image. It does it with the white background.

 // This method refers to the signature image to save
private RenderedImage getImage() {

    int width = tabletWidth;
    int height = tabletHeight;

    // Create a buffered image in which to draw
    BufferedImage bufferedImage = new BufferedImage(width, height,
            BufferedImage.TYPE_INT_RGB);

    // Create a graphics contents on the buffered image
    Graphics2D g2d = bufferedImage.createGraphics();

    // Draw graphics
    g2d.setColor(Color.WHITE);
    g2d.fillRect(0, 0, width, height);

    drawPoints(Tablet.getPenPoints(), g2d, Color.BLACK);

    // Graphics context no longer needed so dispose it
    g2d.dispose();

    return bufferedImage;
}

I tried to set it transparent but with no success, so I posted this working part.

回答1:

Use BufferedImage.TYPE_INT_ARGB instead of BufferedImage.TYPE_INT_RGB. And save it to PNG image, JPEG does not support the transparency.

UPD:

For set the background transparent, use it:

g2d.setComposite(AlphaComposite.Clear);
g2d.fillRect(0, 0, w, h);

And for draw your image:

g2d.setComposite(AlphaComposite.Src);
drawPoints(Tablet.getPenPoints(), g2d, Color.BLACK);

回答2:

As others have mentioned, you can't save JPEGs with transparency.

However, it's possible to store your file as you do (in JPEG, although I suggest using a grayscale JPEG in this case) and later interpret the white parts as transparent, and the black parts as non-transparent (ie: use the grayscale image as an alpha-mask). Then you could simply color the non-transparent parts either black or blue, to look like pen-ink.

Think of the white area as the paper, and the black parts as covered by ink. Note that this technique will only work for the use case where all white pixels should be transparent. Other answers in this thread will work better in the general case.

回答3:

JPEG does not support transperency. You have to use a different target format like png for example.

回答4:

You are setting the Buffered Image have a type of just RGB which has no Alpha component, you will have to use one that does have alpha to maintain transparency.