CODEWITHSUNDEEP
javaimagepngarraysbufferedimage
initWithStyle
initWithStyle

Reputation: 337

Trying to extract the bytes from a PNG image

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 4x1 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?

Upvotes: 1

Views: 1312

Answers (2)

Tesseract
Tesseract

Reputation: 8139

What is the type of the BufferedImage? I suspect it's TYPE_4BYTE_ABGR. Since a byte with value 255 would become -1 in Java, the output seems correct. -1, 0, 0, -1 = alpha: 255, blue: 0, green: 0, red 255.

Upvotes: 2

Patashu
Patashu

Reputation: 21793

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.

Upvotes: 3

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