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