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Message:

what does that exactly mean?

Posted by Matt Gerrans on October 10, 2001 at 5:24 PM

What this means is that the 32 bits of information in the pixel integer is diveded evenly into four chunks of 8 bits, like so:
aaaaaaaarrrrrrrrggggggggbbbbbbbb

The >> operator shifts the bits to the right by the specified amount (the number on the right side). The "& 0xff" is a mask (the & is a bitwise and operator), and it blanks (zeros) out everything except the lowest 8 bits. So the effect of the combined operation of shifting right by 8 (or 16, or 24) bits and then masking is to look at a specific chunk of 8 bits within the 32-bit field.

So if we did it in the opposite order that you specified, for illustative purposes, it would be something like this:

int p = _pixels[index];
// Look at the lower 8 bits:
// No shift, so:
// aaaaaaaarrrrrrrrggggggggbbbbbbbb -> aaaaaaaarrrrrrrrggggggggbbbbbbbb
// Result of masking:
// aaaaaaaarrrrrrrrggggggggbbbbbbbb -> 000000000000000000000000bbbbbbbb
_blue = (p) & 0xff; 
// Look next group 8 bits:
// After shift right 8:
// aaaaaaaarrrrrrrrggggggggbbbbbbbb -> ????????aaaaaaaarrrrrrrrgggggggg
// Result of masking:
// ????????aaaaaaaarrrrrrrrgggggggg -> 000000000000000000000000gggggggg
_green = (p >> 8) & 0xff;
// Look next group 8 bits:
// After shift right 16:
// aaaaaaaarrrrrrrrggggggggbbbbbbbb -> ????????????????aaaaaaaarrrrrrrr
// Result of masking:
// ????????????????aaaaaaaarrrrrrrr -> 000000000000000000000000rrrrrrrr
_red = (p >> 16) & 0xff;
// Look last group 8 bits:
// After shift right 24:
// aaaaaaaarrrrrrrrggggggggbbbbbbbb -> ????????????????????????aaaaaaaa
// Result of masking:
// ?????????????????????????aaaaaaaa -> 00000000000000000000000aaaaaaaa
_alpha = (p >> 24) & 0xff;

> hi!
> Is there anyone who can explain me what this exactly means...
> (i know the result...but cant understand how it comes to it)
> using the pixelgrabber class i grab a pixel...
> and extract the colors....hmm...i copied this from an example...and dont understand what the operators mean and what for is everything
> int p=_pixels[index];
> _alpha = (p >> 24) & 0xff;
> _red = (p >> 16) & 0xff;
> _green = (p >> 8) & 0xff;
> _blue = (p ) & 0xff;
> i get the right values for the colors... but how??
> what does the '>>' mean and what for is '&', how do they exactly work?
> thx
> -ilya