1. ffmpeg
  2. Commit 3ba69a15c680596feeef1d213ce2b85823dde985
Browse code

Implement av_bmg_next(), a Box-Muller Gaussian random generator.

See the thread:
"[FFmpeg-devel] [PATCH] Box-Muller gaussian generator".

Originally committed as revision 20808 to svn://svn.ffmpeg.org/ffmpeg/trunk

Stefano Sabatini authored on 2009/12/13 01:24:37
Showing 3 changed files
libavutil/avutil.h
History View file @ 3ba69a1
... ... 
@@ -35,8 +35,8 @@
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 #define AV_VERSION(a, b, c) AV_VERSION_DOT(a, b, c)
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 #define LIBAVUTIL_VERSION_MAJOR 50
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-#define LIBAVUTIL_VERSION_MINOR  5
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-#define LIBAVUTIL_VERSION_MICRO  1
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+#define LIBAVUTIL_VERSION_MINOR  6
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+#define LIBAVUTIL_VERSION_MICRO  0
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 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \
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                                                LIBAVUTIL_VERSION_MINOR, \
libavutil/lfg.c
History View file @ 3ba69a1
... ... 
@@ -39,6 +39,21 @@ void av_cold av_lfg_init(AVLFG *c, unsigned int seed){
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     c->index=0;
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 }
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+void av_bmg_get(AVLFG *lfg, double out[2])
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+{
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+    double x1, x2, w;
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+
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+    do {
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+        x1 = 2.0/UINT_MAX*av_lfg_get(lfg) - 1.0;
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+        x2 = 2.0/UINT_MAX*av_lfg_get(lfg) - 1.0;
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+        w = x1*x1 + x2*x2;
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+    } while (w >= 1.0);
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+
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+    w = sqrt((-2.0 * log(w)) / w);
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+    out[0] = x1 * w;
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+    out[1] = x2 * w;
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+}
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+
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 #ifdef TEST
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 #include "log.h"
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 #include "common.h"
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@@ -59,6 +74,24 @@ int main(void)
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         STOP_TIMER("624 calls of av_lfg_get");
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     }
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     av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
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+
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+    /* BMG usage example */
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+    {
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+        double mean   = 1000;
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+        double stddev = 53;
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+
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+        av_lfg_init(&state, 42);
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+
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+        for (i = 0; i < 1000; i += 2) {
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+            double bmg_out[2];
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+            av_bmg_get(&state, bmg_out);
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+            av_log(NULL, AV_LOG_INFO,
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+                   "%f\n%f\n",
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+                   bmg_out[0] * stddev + mean,
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+                   bmg_out[1] * stddev + mean);
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+        }
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+    }
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+
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     return 0;
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 }
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 #endif
libavutil/lfg.h
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@@ -51,4 +51,12 @@ static inline unsigned int av_mlfg_get(AVLFG *c){
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     return c->state[c->index++ & 63] = 2*a*b+a+b;
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 }
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+/**
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+ * Gets the next two numbers generated by a Box-Muller Gaussian
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+ * generator using the random numbers issued by lfg.
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+ *
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+ * @param out[2] array where are placed the two generated numbers
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+ */
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+void av_bmg_get(AVLFG *lfg, double out[2]);
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+
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 #endif /* AVUTIL_LFG_H */