TYPE_INT_RGB and TYPE_BYTE_GRAY weirdness

[shypen42]

Hi all,

I've got a very simple question.

I create three color "bands": black, grey and white, with the
following code then save the BufferedImage to a .png file
(ie picture is saved without any color loss).

(don't pay attention to the method duplication, that's not the point)

BufferedImage dest = new BufferedImage( 100, 100,
BufferedImage.TYPE_INT_RGB);
// we draw 33 black lines
for (int y = 0; y < 33; y++) {
for (int x = 0; x < dest.getWidth(); x++) {
int level = 0x00;
int black = (level << 16) | (level << 8) | level;
dest.setRGB(x, y, black);
}
}
// we draw 33 grey lines
for (int y = 33; y < 66; y++) {
for (int x = 0; x < dest.getWidth(); x++) {
int level = 0x80;
int gray = (level << 16) | (level << 8) | level;
dest.setRGB(x, y, gray);
}
}
// we draw 34 white lines
for (int y = 66; y < 100; y++) {
for (int x = 0; x < dest.getWidth(); x++) {
int level = 0xff;
int white = (level << 16) | (level << 8) | level;
dest.setRGB(x, y, white);
}
}
ImageIO.write( dest, "PNG", new
File(System.getProperty("user.home") + "/weird.png") );

Now if I open weird.png with, say, a picture editor/image
manipulation program (Gimp, Photoshop, whatever...) I can
see that the middle color band is grey and has the RGB
value "128,128,128" (0x80, 0x80, 0x80).

This is exactly what one can expect when working with
24 bit RGB pictures (or 32 bit ARGB pictures).



Now I change a single line in this program, by asking for
TYPE_BYTE_GRAY
instead of TYPE_INT_RGB and the weirdness begins...

TYPE_BYTE_GRAY description says:

* Represents a unsigned byte grayscale image, non-indexed.
* This image has a <code>ComponentColorModel</code> with
* a CS_GRAY {@link ColorSpace}

So this is a 8-bit grayscale image, so far so good.

And the doc for CS_GRAY color space says:

* The built-in linear gray scale color space.

"linear gray scale", so far so good...

When the program is run again, it creates the weird.png picture, with
three color bands: black, grey and white.

So "0" corresponds to black, "255" (0xff) to white... So far, so good.

But the grey band is *not* having the RGB value 128, 128, 128 but
is actually having the RGB value 55, 55, 55.

So black stays black, white stays white, but grey gets modified.

What is the explanation for this?