Greyscale JPG: Scaled image gets brighter

Hello,

After reading, rescaling, and writing a JPG, the brightness gets changed. If the rescaling is skipped, the brightness is correct.

This is one of our test-images: https://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Broadway_tower_grayscale.jpg/256px-Broadway_tower_grayscale.jpg (The other we cannot share)

This only seems to happen with the color-model grey.

Interestingly, it also happens when Image.getScaledInstance is used.

    public static void main(String ... args) throws Exception {
        Iterator<ImageReader> readers = ImageIO.getImageReaders(ImageIO.createImageInputStream(new FileInputStream(new java.io.File("/Users/x1fkirma/test2/original.jpg"))));
        int i = 0;
        while(readers.hasNext()) {
            i += 1;
            ImageReader reader = readers.next();
            System.out.println("Reader: " + reader);
            reader.setInput(ImageIO.createImageInputStream(new FileInputStream(new java.io.File("/Users/x1fkirma/test2/tower.jpg"))));
            BufferedImage input = reader.read(0);

            //twelvemonkey
            BufferedImage result = com.twelvemonkeys.image.ImageUtil.createScaled(input, 875, 583, Image.SCALE_DEFAULT);

            //java.awt.rescale
            //java.awt.Image img = input.getScaledInstance(875, 583, java.awt.Image.SCALE_SMOOTH);
            //BufferedImage result = new BufferedImage(875, 583, input.getType());
            //Graphics2D g2 = result.createGraphics();
            //g2.drawImage(img, 0, 0, null);
            //g2.dispose();


            javax.imageio.ImageIO.write(result, "jpg", new java.io.File("/Users/x1fkirma/test2/scaled_" + i + ".jpg"));
        }
        
    }

[FlorianKirmaier]

You are right, it’s my fault. It works properly with ResampleOp. Sorry for the confusion. I was properly confused because the image already looked “light greyish”. I assume ResampleOp is the method which is also recommended to rescale an image?

[FlorianKirmaier]

@haraldk I’ve looked more into the topic and noticed the behavior depends on the performance hints. I’ve found the following cases: ImageUtil:

ImageUtil.createScaled(input, 875, 583, Image.SCALE_DEFAULT); // wrong
ImageUtil.createScaled(input, 875, 583, Image.SCALE_FAST); // correct

ResampleOp with the following RenderingHints, and filter is called with the second argument as null:

// No settings // correct
map.put(RenderingHints.KEY_INTERPOLATION,RenderingHints.VALUE_INTERPOLATION_NEAREST_NEIGHBOR); // correct
map.put(RenderingHints.KEY_INTERPOLATION,RenderingHints.VALUE_INTERPOLATION_BICUBIC);  // wrong
map.put(RenderingHints.KEY_INTERPOLATION,RenderingHints.VALUE_INTERPOLATION_BILINEAR); // wrong

ResampleOp seems to work in all cases when the second argument for filter is a pre created BufferedImage, created like the following:

BufferedImage result = new BufferedImage(875, 583, input.getType());