[FFmpeg-devel] [PATCH] avfilter: Add blue and violet noise generation filters

[george at spotify.com]

From: George Boyle <george at spotify.com>

For the blue and violet noise, I took the pink and brown noise
respectively and subtracted the offsets instead of adding them. When I
eyeball the frequency spectrum of the resulting outputs it looks correct
to me, i.e. the blue graph appears to be a mirror image of the pink, and
the same can be said of the violet and the brown. I did not do anything
else to confirm the correctness.
---
 doc/filters.texi             |  4 ++--
 libavfilter/asrc_anoisesrc.c | 41 +++++++++++++++++++++++++++++++++++------
 2 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/doc/filters.texi b/doc/filters.texi
index 2708fdb1df..dd88636bb9 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -4471,8 +4471,8 @@ Specify the duration of the generated audio stream. Not specifying this option
 results in noise with an infinite length.
 
 @item color, colour, c
-Specify the color of noise. Available noise colors are white, pink, and brown.
-Default color is white.
+Specify the color of noise. Available noise colors are white, pink, brown,
+blue and violet. Default color is white.
 
 @item seed, s
 Specify a value used to seed the PRNG.
diff --git a/libavfilter/asrc_anoisesrc.c b/libavfilter/asrc_anoisesrc.c
index be0d5ab6a5..b60fdfe331 100644
--- a/libavfilter/asrc_anoisesrc.c
+++ b/libavfilter/asrc_anoisesrc.c
@@ -51,12 +51,14 @@ static const AVOption anoisesrc_options[] = {
     { "a",            "set amplitude",    OFFSET(amplitude),    AV_OPT_TYPE_DOUBLE,    {.dbl = 1.},     0.,  1.,         FLAGS },
     { "duration",     "set duration",     OFFSET(duration),     AV_OPT_TYPE_DURATION,  {.i64 =  0},      0,  INT64_MAX,  FLAGS },
     { "d",            "set duration",     OFFSET(duration),     AV_OPT_TYPE_DURATION,  {.i64 =  0},      0,  INT64_MAX,  FLAGS },
-    { "color",        "set noise color",  OFFSET(color),        AV_OPT_TYPE_INT,       {.i64 =  1},      0,  2,          FLAGS, "color" },
-    { "colour",       "set noise color",  OFFSET(color),        AV_OPT_TYPE_INT,       {.i64 =  1},      0,  2,          FLAGS, "color" },
-    { "c",            "set noise color",  OFFSET(color),        AV_OPT_TYPE_INT,       {.i64 =  0},      0,  2,          FLAGS, "color" },
+    { "color",        "set noise color",  OFFSET(color),        AV_OPT_TYPE_INT,       {.i64 =  1},      0,  4,          FLAGS, "color" },
+    { "colour",       "set noise color",  OFFSET(color),        AV_OPT_TYPE_INT,       {.i64 =  1},      0,  4,          FLAGS, "color" },
+    { "c",            "set noise color",  OFFSET(color),        AV_OPT_TYPE_INT,       {.i64 =  0},      0,  4,          FLAGS, "color" },
     {     "white",    0,                  0,                    AV_OPT_TYPE_CONST,     {.i64 =  0},      0,  0,          FLAGS, "color" },
     {     "pink",     0,                  0,                    AV_OPT_TYPE_CONST,     {.i64 =  1},      0,  0,          FLAGS, "color" },
     {     "brown",    0,                  0,                    AV_OPT_TYPE_CONST,     {.i64 =  2},      0,  0,          FLAGS, "color" },
+    {     "blue",     0,                  0,                    AV_OPT_TYPE_CONST,     {.i64 =  3},      0,  0,          FLAGS, "color" },
+    {     "violet",   0,                  0,                    AV_OPT_TYPE_CONST,     {.i64 =  4},      0,  0,          FLAGS, "color" },
     { "seed",         "set random seed",  OFFSET(seed),         AV_OPT_TYPE_INT64,     {.i64 = -1},     -1,  UINT_MAX,   FLAGS },
     { "s",            "set random seed",  OFFSET(seed),         AV_OPT_TYPE_INT64,     {.i64 = -1},     -1,  UINT_MAX,   FLAGS },
     { "nb_samples",   "set the number of samples per requested frame", OFFSET(nb_samples), AV_OPT_TYPE_INT, {.i64 = 1024}, 1, INT_MAX, FLAGS },
@@ -121,6 +123,22 @@ static double pink_filter(double white, double *buf)
     return pink * 0.11;
 }
 
+static double blue_filter(double white, double *buf)
+{
+    double blue;
+
+    /* Same as pink_filter but subtract the offsets rather than add */
+    buf[0] = 0.0555179 * white - 0.99886 * buf[0];
+    buf[1] = 0.0750759 * white - 0.99332 * buf[1];
+    buf[2] = 0.1538520 * white - 0.96900 * buf[2];
+    buf[3] = 0.3104856 * white - 0.86650 * buf[3];
+    buf[4] = 0.5329522 * white - 0.55000 * buf[4];
+    buf[5] = -0.016898 * white + 0.76160 * buf[5];
+    blue = buf[0] + buf[1] + buf[2] + buf[3] + buf[4] + buf[5] + buf[6] + white * 0.5362;
+    buf[6] = white * 0.115926;
+    return blue * 0.11;
+}
+
 static double brown_filter(double white, double *buf)
 {
     double brown;
@@ -130,6 +148,15 @@ static double brown_filter(double white, double *buf)
     return brown * 3.5;
 }
 
+static double violet_filter(double white, double *buf)
+{
+    double violet;
+
+    violet = ((0.02 * white) - buf[0]) / 1.02;
+    buf[0] = violet;
+    return violet * 3.5;
+}
+
 static av_cold int config_props(AVFilterLink *outlink)
 {
     AVFilterContext *ctx = outlink->src;
@@ -144,9 +171,11 @@ static av_cold int config_props(AVFilterLink *outlink)
     s->duration = av_rescale(s->duration, s->sample_rate, AV_TIME_BASE);
 
     switch (s->color) {
-    case 0: s->filter = white_filter; break;
-    case 1: s->filter = pink_filter;  break;
-    case 2: s->filter = brown_filter; break;
+    case 0: s->filter = white_filter;  break;
+    case 1: s->filter = pink_filter;   break;
+    case 2: s->filter = brown_filter;  break;
+    case 3: s->filter = blue_filter;   break;
+    case 4: s->filter = violet_filter; break;
     }
 
     return 0;
-- 
2.13.3