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From: Mike Day 
Sent: Friday, February 16, 2007 11:08 AM
To: Mike Day; tech
Subject: RE: curiously small code...

A while ago I sent out the email below regarding PS3 texture swizzling; part of what the code does is take an (x,y) position in a square texture and converts it into the physical address of the swizzled location.

Now suppose you wanted to do the reverse… specifically, say you want to step sequentially through the physical address locations of a square texture and generate the (x,y) coordinates that correspond to it in the swizzle pattern. The simplest version I could come up with was this:

  u32 x=0, y=0;
  for (u32 i=0; i<num_texels; i++)
  {
    u32 and0 = x & y;
    u32 and1 = and0 + 1;
    u32 xinc = and0 ^ and1;
    u32 yinc = x & xinc;
    x ^= xinc;
    y ^= yinc;
  }

I have no idea whether this is vaguely standard or not; does anybody know a better way?
How about a better way to do the inverse mapping? (i.e. the one in the original email below)

Mike