How does this unsafe code work?

Question

I read the "C.Sharp 3.0 in a Nutshell" book and met the next piece of code, that interested me.

  unsafe void RedFilter(int[,] bitmap)
    {
      int length = bitmap.Length;
      fixed (int* b = bitmap)
      {
        int* p = b;
        for(int i = 0; i < length; i++)
          *p++ &= 0xFF;
      }
    }

Could anyone explain me how does this "*p++ &= 0xFF" work?

Answer 1

The function is presumably meant to take a bitmap image, and filter out all the colors except red.

This assumes that it's a 32-bit bitmap, where each pixel is represented by an int. You're dereferencing the memory location currently pointed to by p (which is an int), and ANDing it with 0xFF, which effectively leaves only the red component of the pixel (assuming that the lowest byte is the red component). You're also automatically incrementing the pointer to the next int (with ++). Does that answer it?

Answer 2

It's the same as this (IMO the original *p++ &= 0xFF; trick is a little nasty -- it's one line of code that's doing two things):

*p = *p & 0xFF;
p++;

An expression like a = a & 0xFF sets all but the bottom 8 bits of variable a to zero.