Question

I’m trying to extract the bytes from a PNG and am getting some weird looking results.

Here is my extract byte method:

public static byte[] extractBytes (String ImageName) throws IOException {
     // open image
     File imgPath = new File(ImageName);
     BufferedImage bufferedImage = ImageIO.read(imgPath);

     // get DataBufferBytes from Raster
     WritableRaster raster = bufferedImage.getRaster();
     DataBufferByte data   = (DataBufferByte) raster.getDataBuffer();

     return ( data.getData() );
}

And here is where I call the function and loop through the resulting byte[]:

byte[] bytes = extractBytes("colorstrip.png");

for (int x : bytes) {
    System.out.println(x);
}

I’ve been testing this code on a 4×1 image simply containing a red pixel, a blue pixel, a green pixel, and a purple pixel in that order. Here is the output:

-1
0
0
-1
-1
0
-1
0
-1
-1
0
0
-1
-1
0
-1

This output does not look correct to me. I believe the output should look something like this (I’ve left the alpha channels blank):

255
0
0

0
255
0

0
0
255

255
0
255

Any idea what the problem is?

Answer 1

Java bytes are signed, so when you’d expect 255 in a 0-255 range, java is using a -128 to 127 range and so prints unsigned 255 as signed -1.