how can I convert an image to grayscale without losing transparency?

Question

I am having problems converting a colored image with some transparent pixels to grayscale. I have searched and found related questions on this website but nothing I have be

Answer 1

As only BITMASK support transparency, first convert the image to greyScale using TYPE_BYTE_GRAY, then use BITMARK to convert the image to have transparency. Hope this helps. I had the same problem I used this approach.

//grey scale image 
BufferedImage b = new BufferedImage(
        colorImage.getWidth(),
        colorImage.getHeight(),
        BufferedImage.TYPE_BYTE_GRAY);
File lostFGFile = new File(lostFgFileName);
Graphics g = b.createGraphics();

g.drawImage(colorImage, 0, 0, null);
g.setColor(Color.gray);
ImageIO.write(b, "png", lostFGFile);
g.dispose();

//convert the lost image as transparent
BufferedImage greyImage = ImageIO.read(new FileInputStream(lostFgFileName));
b = new BufferedImage(
        colorImage.getWidth(),
        colorImage.getHeight(),
        BufferedImage.BITMASK);
g = b.createGraphics();

g.drawImage(greyImage, 0, 0, null);
ImageIO.write(b, "png", lostFGFile);
g.dispose();