[XviD-devel] Hadamard Transform

[Christoph Lampert]

On 6 Sep 2002, pascal massimino wrote:
> 	Arg.. you shouldn't be teasing me. I failed miserably in 
> 	the trap :)
> 	Here are some Hadamard stuff. I let you imagine what
> 	the ASM version (or 8bits) would look like :)

#define BUTF(a, b, tmp) \
    (tmp) = (a)+(b);      \
    (a)   = (a)-(b);      \
    (b)   = (tmp)


If I'm not wrong, you can do (a,b) -> (a-b,a+b) operation (and
every permutation, sign change etc.) without temp storage 
doing:

       1 -= 2          2 += 2            2 += 1    
(a, b) -----> (a-b ,b) -----> (a-b, 2*b) -----> (a-b, b+a)

I don't know if this is faster (extra operation?), but it 
might help not needing a 9th register.

gruel 

(a computer science professor always proudly told his students 
(e.g. me) that he keeps betting he can "exchange the value of
two variables without need for a temporary third". He always wins,
using a method similar to the one above ;-)