使用ImageIO写入功能的GIF文件中的透明度问题

Question

Preamble: I was trying to save swt ImageData to a gif file but it was throwing "unsupported color depth". I decided to convert the org.eclipse.swt.graphics.ImageData to java.awt.image.BufferedImage and it works fine until I started using transparency.

Issue: Transparent pixels are not being handled correctly instead they are colored with last foreground color used when painting the GC. I noticed this undesired effect was down to this line in my code.

ImageIO.write(bufferedImage, "gif", file);

After doing some web searches I found out that the issue has been resolved in JDK 7 http://ubuntuforums.org/archive/index.php/t-1060128.html

The problem is I have to use Java 6 not 7. Can anyone kindly tell me how this can be achieved?

Answer 1

I had a similar problem. I have ImageData and I am trying to write to GIF or PNG with transparency. I used the following conversion function and it worked for me.

static BufferedImage convertToAWT(ImageData data) {
    ColorModel colorModel = null;
    PaletteData palette = data.palette;
    if (palette.isDirect) {
        int alphaMask = 0xff;
        colorModel = new DirectColorModel(data.depth, palette.redMask, palette.greenMask, palette.blueMask, alphaMask);

        BufferedImage bufferedImage = new BufferedImage(colorModel, colorModel.createCompatibleWritableRaster(data.width, data.height), false, null);
        for (int y = 0; y < data.height; y++) {
            for (int x = 0; x < data.width; x++) {
                int pixel = data.getPixel(x, y);
                RGB rgb = palette.getRGB(pixel);

                // ASSUME that pixel==0 is transparent!!!!

                int alpha = (pixel == 0) ? 0 : alphaMask; 

                bufferedImage.setRGB(x, y, alpha << 24 | rgb.red << 16 | rgb.green << 8 | rgb.blue);
            }
        }
        return bufferedImage;
    } else {
        return null;   // not supported
    }
}


private static void writeToFile(Image img, String name)
{
    BufferedImage ii = convertToAWT(img.getImageData());

    try
    {
        // You can change this to write GIF or JPG also

        ImageIO.write(ii, "PNG", new File(name + ".png"));
    }
    catch (IOException e)
    {
        e.printStackTrace();
    }
}        

Notice that this assumes that pixel zero is transparent. That maybe not the case for you. Perhaps you can try other values, like -1 or 255 etc, and see what works. Good luck.