Question

I have the following:

#define PAD (  4 - ( (WIDTH*BPP)%4 )  )
#if PAD == 4
#define PAD 0
#endif

and PAD is redefined even though it is equal to 3 after the first definition. However if I explicitly define it as 3 then it isn’t redefined. Therefore I assume there is a problem with the way I have written the expression, but I’m not sure what.

Answer 1

What you want is

(PAD + (WIDTH * BPP)) % 4 == 0

right? (Of course 0 <= PAD < 4)

Then you can define PAD in this way:

#define PAD (3 - ((WIDTH * BPP + 3) % 4))

Example Python session:

>>> def f(x): return 3 - (x+3)%4
...
>>> [ (x, f(x), x + f(x)) for x in xrange(100,108) ]
[(100, 0, 100), (101, 3, 104), (102, 2, 104), (103, 1, 104), (104, 0, 104), (105, 3, 108), (106, 2, 108), (107, 1, 108)]

In general,

#define PAD ((N-1) - (X + (N-1)) % N))

makes PAD + X a multiple of N under a constraint of 0 <= PAD < N (Though I didn’t check negative cases…)