iw7-mod/deps/stb/tests/test_png_paeth.c
2024-08-13 05:15:34 -04:00

48 lines
1.0 KiB
C

#include <stdio.h>
#include <stdlib.h>
// Reference Paeth filter as per PNG spec
static int ref_paeth(int a, int b, int c)
{
int p = a + b - c;
int pa = abs(p-a);
int pb = abs(p-b);
int pc = abs(p-c);
if (pa <= pb && pa <= pc) return a;
if (pb <= pc) return b;
return c;
}
// Optimized Paeth filter
static int opt_paeth(int a, int b, int c)
{
int thresh = c*3 - (a + b);
int lo = a < b ? a : b;
int hi = a < b ? b : a;
int t0 = (hi <= thresh) ? lo : c;
int t1 = (thresh <= lo) ? hi : t0;
return t1;
}
int main()
{
// Exhaustively test the functions match for all byte inputs a, b,c in [0,255]
for (int i = 0; i < (1 << 24); ++i) {
int a = i & 0xff;
int b = (i >> 8) & 0xff;
int c = (i >> 16) & 0xff;
int ref = ref_paeth(a, b, c);
int opt = opt_paeth(a, b, c);
if (ref != opt) {
fprintf(stderr, "mismatch at a=%3d b=%3d c=%3d: ref=%3d opt=%3d\n", a, b, c, ref, opt);
return 1;
}
}
printf("all ok!\n");
return 0;
}
// vim:sw=3:sts=3:et