Hi everyone,
	I'm trying to understand how the following define works (render.cpp):
#define rand2d(a, b) 
jitttab[(int)(hashTable[(int)(hashTable[(int)((a)&0xfff)]^(b))&0xfff])&0xff]

I got that jitttab is an array of 256 random float ranging from -0.5 to 0.5.

hashtable is an array of 8192 unsigned short, where the first 4096 
numbers are the the growing 0..4095 scattered randomly, and the last 
4096 are just the same:

   for (i = 0; i < 4096; i++)
   {
     hashTable[i] = i;
   }

   for (i = 4095; i >= 0; i--)
   {
     j = POV_RAND() % 4096;
     temp = hashTable[i];
     hashTable[i] = hashTable[j];
     hashTable[j] = temp;
   }

   for (i = 0; i < 4096; i++)
   {
     hashTable[4096 + i] = hashTable[i];
   }

My question is: what are the and and xor mask for?

Thanxs

Adam

Post a reply to this message

Adam wrote:

> Hi everyone,
> I'm trying to understand how the following define works (render.cpp):
> #define rand2d(a, b)
> jitttab[(int)(hashTable[(int)(hashTable[(int)((a)&0xfff)]^(b))&0xfff])&0xff]
> 
> I got that jitttab is an array of 256 random float ranging from -0.5 to 0.5.
> 
Re-write it to read it more easily: 

idx = a;
idx = hashTable[idx & 0xfff]^b;
idx = hashTable[idx & 0xfff];
jitttab[ idx & 0xff ];

The & 0xff and &0xfff are just fast ways of doing the modulo operation 
(i.e. make sure that we do not access beyond the end of the hash array). 

The XOR (^) is probably just there to combine the hashes since rand2d 
takes 2 arguments. 

Wolfgang

Post a reply to this message