Bits and Bytes: the forgotten relationship
Let’s have a look at the number 256:
256 in hexadecimal is 0×100
Binary is 1 0000 0000 (8 zeros)
256-1 is 0xFF which is 1111 1111
256/2 = 128/2 = 64/2 = 32/2 = 16/2 = 8/2 = 4/2 = 2/2 = 1
2 pow 8 = 1<<8 (logical shift) = 256
256 pow 4 = 4294967296 – 1 = the maximum 32
So if we understand 255 (256-1) being 0xFF, that means if you stack 4 bytes together you get 0xFFFFFFFF (4294967295, ARGB, 32 bits set). Let’s say we want to break the number down into ARGB ( pixel, alpha, red, green, blue) components, we do it in this way:
var argb:uint = 0xFFFF8040; // full alpha, full red, half green, quarter blue
var alpha:uint = argb >> 24;
var red:uint = (argb >> 16) & 0xFF;
var green:uint = (argb >> 8) & 0xFF;
var blue:uint = argb & 0xFF;
Let’s break this down, what is argb >> 24? Basically it is moving the number down 24 bits:
1111 1111 1111 1111 1000 0000 0100 0000 (0xFFFF8040) >> 24
0000 0000 0000 0000 0000 0000 1111 1111 (0x000000FF) // result
And doing the same with the red >> 16:
1111 1111 1111 1111 1000 0000 0100 0000 (0xFFFF8040) >> 16
0000 0000 0000 0000 1111 1111 1111 1111 (0x0000FFFF) //result
Trouble with this is that the A part (Alpha) is still lingering above the red, this is where the & (bitwise And) comes in:
Hex:
0x0000FFFF & 0x000000FF
Binary:
0000 0000 0000 0000 1111 1111 1111 1111
0000 0000 0000 0000 0000 0000 1111 1111
---------------------------------------
0000 0000 0000 0000 0000 0000 1111 1111
Why? Simply walk along the numbers if the bit is set in either then the result is 1, if one value is 0 then the answer is 0. Perhaps you need more reading up on this http://en.wikipedia.org/wiki/Bitwise_operation.
0×000000FF >> 8 = 0×0000FF00 (each digit is 4 bits, a nibble). Some crazy dude thought it would be interesting to use eating words for binary data, bit (1), nibble (1111) and byte (1111111).
So how about some real usage, well I use these bits for collision filtering (a bit primitive, but essential to AS3 for performance):
var bPerson:uint = 1<<0; // 1 (00001)
var bBuilding:uint = 1<<1; // 2 (00010)
var bRock:uint = 1<<2; // 4 (00100)
var bBarrel:uint = 1<<3; // 8 (01000)
var bWall:uint = 1<<4; // 16 (10000)
Let’s say each object has one of these variables assigned to something called flags, so a barrel.flags = bBarrel:
flags = bBarrel; // 1<<8 16 0x08 1000
if (bBarrel|bWall) & flags then <do something>
So instead of saying is it a barrel or is it a wall or is it a person etc. It’s a single line check. And you can pretty much use this everywhere you want to do this kind of comparison.
I hope this helps in your future game dev… good luck!
stormborns
jacobvanloon
blakekathryn
germansingergirl
janainaart
gummygunk
vaporwave
izzy806