java - Understanding BufferedImage.getRGB output values

Question

I'm getting an integer value for the pixel in an image using this method:

int colour = img.getRGB(x, y);

Then I'm printing out the values and I see that black pixels correspond to a value like "-16777216", a kind of blue to something like "-16755216", etc. Can someone please explain me the logic behind this value?

Answer 1

The RGB int color contains the Red, Green, Blue components of the color in its bits. You have to look at its binary or hexadecimal representation and not look at it as a whole integer number (not look at its decimal representation).

An int has 32 bits, 3x8 = 24 is used to store the RGB components (8 bits for each) in the following format:

               2          1          0
bitpos      32109876 54321098 76543210
------   --+--------+--------+--------+
bits     ..|RRRRRRRR|GGGGGGGG|BBBBBBBB|

You can extract or set the components using bitmasks:

int color = img.getRGB(x, y);

// Components will be in the range of 0..255:
int blue = color & 0xff;
int green = (color & 0xff00) >> 8;
int red = (color & 0xff0000) >> 16;

If the color also has an alpha component (transparency) ARGB, it gets the last remaining 8 bits.

           3          2          1          0
bitpos    10987654 32109876 54321098 76543210
------   +--------+--------+--------+--------+
bits     |AAAAAAAA|RRRRRRRR|GGGGGGGG|BBBBBBBB|

And the value:

int alpha = (color & 0xff000000) >>> 24; // Note the >>> shift
                                         // required due to sign bit

An alpha value of 255 means that a color is completely opaque and a value of 0 means that the color is completely transparent.

Your color:

Your color is color = -16755216 which has:

blue : 240         // Strong blue
green:  85         // A little green mixed in
red  :   0         // No red component at all
alpha: 255         // Completely opaque