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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 32bit 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
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