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47 lines
1.0 KiB
47 lines
1.0 KiB
#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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