Why is JPG altered on retrieval? - Java

Question

I am trying to retrieve an jpg image as a BufferedImage then decompose it into a 3D array [RED][GREEN][BLUE] and then turn it back into a BufferedImage and store it under a different file-name. All looks fine to me BUT, when I am trying to reload the 3D array using the new file created I get different values for RGB although the new image looks fine to the naked eye. I did the following.

BufferedImage bi = ImageIO.read(new File("old.jpg"));
int[][][] one = getArray(bi);
save("kite.jpg", one);

BufferedImage bi2 = ImageIO.read(new File("new.jpg"));
int[][][] two = getArray(bi2);

private void save(String destination, int[][][] in) {
    try {
    BufferedImage out = new BufferedImage(in.length, in[0].length, BufferedImage.TYPE_3BYTE_BGR);

        for (int x=0; x<out.getWidth(); x++) {
            for (int y = 0; y < out.getHeight(); y++) {
               out.setRGB(x, y, new Color(in[x][y][0], in[x][y][1], in[x][y][2]).getRGB());
            }
        }

        File f = new File("name");
        ImageIO.write(out, "JPEG", f);
    } catch (IOException e) {
        System.out.println(e.getMessage());
    }

}

so in the example above the values that arrays one and two are holding are different.

I am guessing there is something to do with the different types of retrieving and restoring images? I am trying to figure out what is going on all day but with no luck. Any help appreciated.

It sounds like JPEG compression just slightly adjusted the values when you stored it. — Louis Wasserman, Feb 07 '16 at 00:28
jpeg is a lossy format, it loses some of its color information on save. Use it only as a write-once format — Ferrybig, Feb 07 '16 at 00:29
http://stackoverflow.com/q/17108234/869736 has details on how to control that parameter, FWIW. — Louis Wasserman, Feb 07 '16 at 00:29
I tried both the answers on that question and didn't help much... I am not sure why. It seems like I do get less pixels altered when raising compression quality but never gets to lossless — Rakim, Feb 07 '16 at 00:50
I am afraid there is no way to get a JPG 100% lossless. Just decrease the loss of data — Rakim, Feb 07 '16 at 01:06

score 0 · Answer 1 · answered Feb 07 '16 at 00:34

Pretty simple:

JPEG is a commonly used method of lossy compression for digital images

(from wikipedia). Each time the image is compressed, the image is altered to reduce file-size. In fact repeating the steps of decompression and compression for several hundred times alters the image to the point where the entire image turns into a plain gray area in most cases. There are a few compressions that work lossless, but most operation-modes will alter the image.

Why is JPG altered on retrieval? - Java

1 Answers1