Question

I’m having problems generating thumbnails of images with an Alpha channel (transparency). The code I use is this:

public void saveThumbnail(File file, String imageType) {
    if (bufferedThumb == null) {
        return;
    }

    if(bufferedImage.getColorModel().hasAlpha()) {
        logger.debug("Original image has Alpha channel");
    }

    BufferedImage bi = new BufferedImage(bufferedThumb.getWidth(null), bufferedThumb.getHeight(null), BufferedImage.TYPE_INT_ARGB);
    Graphics g = bi.getGraphics();
    g.drawImage(bufferedThumb, 0, 0, null);
    try {
        ImageIO.write(bi, imageType, file);
    } catch (IOException ioe) {
        ioe.printStackTrace();
        System.out.println("Error occured saving thumbnail");
    }
}

However, if I supply for example a GIF image with a transparent background I always end up with a black or colored background.

EDIT:

This is how it’s called from the class using the thumbnail, I missed the two-parameter version of the getThuimbnail()-method the last time:

Thumbnail th = new Thumbnail(file.getPath());
th.getThumbnail(100);

Added methods used for getting the images:

public Thumbnail(String fileName) {
    try {
        this.bufferedImage = ImageIO.read(new File(fileName));
    } catch (IOException ex) {
        logger.error("Failed to read image file: " + ex.getMessage());
    }
}

public Image getThumbnail(int size) {
    int dir = VERTICAL;
    if (bufferedImage.getHeight() < bufferedImage.getWidth()) {
        dir = HORIZONTAL;
    }
    return getThumbnail(size, dir);
}

/**
 * Creates image with specifed max sized to a specified direction.
 * Will use Image.SCALE_SMOOTH for scaling.
 * @param size Maximum size
 * @param dir Direction of maximum size - 0 = vertical, 1 = height.
 * @return Resized image.
 */
public Image getThumbnail(int size, int dir) {
    return getThumbnail(size, dir, Image.SCALE_SMOOTH);
}

/**
 * Creates image with specified size.
 * @param size Maximum size
 * @param dir Direction of maximum size - 0 = vertical, 1 = height.
 * @param scale Image.Scale to use for conversion.
 * @return Resized image.
 */
public Image getThumbnail(int size, int dir, int scale) {
    if (dir == HORIZONTAL) {
        bufferedThumb = bufferedImage.getScaledInstance(size, -1, scale);
    } else {
        bufferedThumb = bufferedImage.getScaledInstance(-1, size, scale);
    }
    return bufferedThumb;
}

Thanks!

Answer 1

Solved it by using an AffineTransform scale operation:

AffineTransform af = new AffineTransform();
af.scale(scaleFactor, scaleFactor);

AffineTransformOp operation = new AffineTransformOp(af, AffineTransformOp.TYPE_NEAREST_NEIGHBOR);
bufferedThumb = operation.filter(bufferedImage, null);
return bufferedThumb;

This seems to preserve any transparency in the image.