Improved Paeth-transformation

paeth.c

#include <stdlib.h>

int naive_paeth(int a, int b, int c) {
	int
		p = a + b - c,
		pa = abs(p - a),
		pb = abs(p - b),
		pc = abs(p - c);

	if (pa <= pb && pa <= pc) return a;
	if (pb <= pc) return b;
	return c;
}

int libpng_paeth(int a, int b, int c) {
     int pa, pb, pc, p;

      p = b - c;
      pc = a - c;

      pa = abs(p);
      pb = abs(pc);
      pc = abs(p + pc);

      /* Find the best predictor, the least of pa, pb, pc favoring the earlier
       * ones in the case of a tie.
       */
      if (pb < pa) pa = pb, a = b;
      if (pc < pa) a = c;

      return a;
}