Thread: Shifting Bits

If I use the code:
#define RGB32BIT(a, r, g, b) ((b) + (g << 8) + (r << 16) + (a << 24)

UINT pixel = RGB32BIT(0, 0, 255, 0);

How do I get back the alpha, red, green, and blue values if I just have the pixel?

alpha = (pixel >> 24) & 0xff;
red = (pixel >> 16) & 0xff;
green = (pixel >> 8) & 0xff;
blue = pixel & 0xff;

Congratulations! You have proven yourself to be among the ranks of the programmers that know too much. I knew the answer. I just wanted to see if anyone had a clue about it. I guess you do. I prefer:

Code:

UCHAR alpha = pixel >> 24;
UCHAR red = pixel >> 16;
UCHAR green = pixel >> 8;
UCHAR blue = pixel;

The

Code:

UCHAR

automaticaly drops off the extra so you don't need the

Code:

& 0xFF

. I put that up at the game programmers message board and I got all kinds of junk.

How about:

Code:

b = pixel & 0x000000ff;
g = (pixel >> 8) & 0x000000ff;
r = (pixel >> 16) & 0x000000ff;
a = pixel >> 24;

Originally Posted by Brad0407

Congratulations! You have proven yourself to be among the ranks of the programmers that know too much. I knew the answer. I just wanted to see if anyone had a clue about it. I guess you do. I prefer:

Code:

UCHAR alpha = pixel >> 24;
UCHAR red = pixel >> 16;
UCHAR green = pixel >> 8;
UCHAR blue = pixel;

The

Code:

UCHAR

automaticaly drops off the extra so you don't need the

Code:

& 0xFF

. I put that up at the game programmers message board and I got all kinds of junk.

But in your original code a, g, b, and r aren't UCHAR's. So now you're changing their original types.

Asked and answered in your previous post (which you claim you answered)
http://cboard.cprogramming.com/showthread.php?t=65502

How about something like:

Code:

union pixel_data
{
  uint32_t pixel;
  struct
  {
    uint8_t alpha;
    uint8_t red;
    uint8_t green;
    uint8_t blue;
  } part;
};

Code:

{
  union pixel_data pix1;

  pix1.pixel = 0x12345678;

  printf("red: %d\n", pix1.part.red);
  printf("green: %d\n", pix1.part.green);
  printf("blue: %d\n", pix1.part.blue);
}

Maybe? Not sure if word-alignment or padding would screw it up.

Code:

enum { cAlpha, cRed, cGreen, cBlue };

union foo
{
    uint32_t pixel;
    uint8_t part[ sizeof( uint32_t ) ];
};

union foo bar;

printf("Alpha: %d\n", bar.part[ cAlpha ] );

Or an array. Unions are definately fun.

Quzah.