swr: add a dither_scale parameter to tune the amplitude of the dither.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/dither.c

@@ -36,6 +36,8 @@ void swri_get_dither(SwrContext *s, void *dst, int len, unsigned seed, enum AVSa
     if(in_fmt == AV_SAMPLE_FMT_S32 && out_fmt == AV_SAMPLE_FMT_U8 ) scale = 1L<<24;
     if(in_fmt == AV_SAMPLE_FMT_S16 && out_fmt == AV_SAMPLE_FMT_U8 ) scale = 1L<<8;
 
+    scale *= s->dither_scale;
+
     for(i=0; i<len + TMP_EXTRA; i++){
         double v;
         seed = seed* 1664525 + 1013904223;

libswresample/swresample.c

@@ -53,7 +53,8 @@ static const AVOption options[]={
 {"rmvol", "rematrix volume"     , OFFSET(rematrix_volume), AV_OPT_TYPE_FLOAT, {.dbl=1.0}, -1000, 1000, 0},
 {"flags", NULL                  , OFFSET(flags)        , AV_OPT_TYPE_FLAGS, {.dbl=0}, 0,  UINT_MAX, 0, "flags"},
 {"res", "force resampling", 0, AV_OPT_TYPE_CONST, {.dbl=SWR_FLAG_RESAMPLE}, INT_MIN, INT_MAX, 0, "flags"},
-{"dither", "dither method"      , OFFSET(dither_method), AV_OPT_TYPE_INT, {.dbl=0}, 0, SWR_DITHER_NB-1, 0, "dither_method"},
+{"dither_scale" , "dither scale"       , OFFSET(dither_scale ), AV_OPT_TYPE_FLOAT, {.dbl=1}, 0, INT_MAX, 0},
+{"dither_method", "dither method"      , OFFSET(dither_method), AV_OPT_TYPE_INT  , {.dbl=0}, 0, SWR_DITHER_NB-1, 0, "dither_method"},
 {"rectangular", "rectangular dither", 0, AV_OPT_TYPE_CONST, {.dbl=SWR_DITHER_RECTANGULAR}, INT_MIN, INT_MAX, 0, "dither_method"},
 {"triangular" , "triangular dither" , 0, AV_OPT_TYPE_CONST, {.dbl=SWR_DITHER_TRIANGULAR }, INT_MIN, INT_MAX, 0, "dither_method"},
 {"triangular_hp" , "triangular dither with high pass" , 0, AV_OPT_TYPE_CONST, {.dbl=SWR_DITHER_TRIANGULAR_HIGHPASS }, INT_MIN, INT_MAX, 0, "dither_method"},

libswresample/swresample_internal.h

@@ -51,6 +51,7 @@ struct SwrContext {
     int used_ch_count;                              ///< number of used input channels (mapped channel count if channel_map, otherwise in.ch_count)
     enum SwrDitherType dither_method;
     int dither_pos;
+    float dither_scale;
 
     int int_bps;                                    ///< internal bytes per sample
     int resample_first;                             ///< 1 if resampling must come first, 0 if rematrixing