avfilter/ebur128: add dualmono measurement option

doc/filters.texi

@@ -12685,6 +12685,15 @@ stream for better peak accuracy. It logs a message for true-peak.
 This mode requires a build with @code{libswresample}.
 @end table
 
+@item dualmono
+Treat mono input files as "dual mono". If a mono file is intended for playback
+on a stereo system, its EBU R128 measurement will be perceptually incorrect.
+If set to @code{true}, this option will compensate for this effect.
+Multi-channel input files are not effected by this option.
+
+@item panlaw
+Set a specific pan law to be used for the measurement of dual mono files.
+This parameter is optional, and has a default value of -3.01dB.
 @end table
 
 @subsection Examples

libavfilter/f_ebur128.c

@@ -139,6 +139,8 @@ typedef struct {
     /* misc */
     int loglevel;                   ///< log level for frame logging
     int metadata;                   ///< whether or not to inject loudness results in frames
+    int dual_mono;                  ///< whether or not to treat single channel input files as dual-mono
+    double pan_law;                 ///< pan law value used to calulate dual-mono measurements
 } EBUR128Context;
 
 enum {
@@ -163,6 +165,8 @@ static const AVOption ebur128_options[] = {
         { "none",   "disable any peak mode",   0, AV_OPT_TYPE_CONST, {.i64 = PEAK_MODE_NONE},          INT_MIN, INT_MAX, A|F, "mode" },
         { "sample", "enable peak-sample mode", 0, AV_OPT_TYPE_CONST, {.i64 = PEAK_MODE_SAMPLES_PEAKS}, INT_MIN, INT_MAX, A|F, "mode" },
         { "true",   "enable true-peak mode",   0, AV_OPT_TYPE_CONST, {.i64 = PEAK_MODE_TRUE_PEAKS},    INT_MIN, INT_MAX, A|F, "mode" },
+    { "dualmono", "treat mono input files as dual-mono", OFFSET(dual_mono), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, A|F },
+    { "panlaw", "set a specific pan law for dual-mono files", OFFSET(pan_law), AV_OPT_TYPE_DOUBLE, {.dbl = -3.01029995663978}, -10.0, 0.0, A|F },
     { NULL },
 };
 
@@ -661,6 +665,10 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *insamples)
                 }
                 if (nb_integrated)
                     ebur128->integrated_loudness = LOUDNESS(integrated_sum / nb_integrated);
+                    /* dual-mono correction */
+                    if (nb_channels == 1 && ebur128->dual_mono) {
+                        ebur128->integrated_loudness -= ebur128->pan_law;
+                    }
             }
 
             /* LRA */
@@ -707,6 +715,12 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *insamples)
                 }
             }
 
+            /* dual-mono correction */
+            if (nb_channels == 1 && ebur128->dual_mono) {
+                loudness_400 -= ebur128->pan_law;
+                loudness_3000 -= ebur128->pan_law;
+            }
+
 #define LOG_FMT "M:%6.1f S:%6.1f     I:%6.1f LUFS     LRA:%6.1f LU"
 
             /* push one video frame */
@@ -855,6 +869,14 @@ static av_cold void uninit(AVFilterContext *ctx)
     int i;
     EBUR128Context *ebur128 = ctx->priv;
 
+    /* dual-mono correction */
+    if (ebur128->nb_channels == 1 && ebur128->dual_mono) {
+        ebur128->i400.rel_threshold -= ebur128->pan_law;
+        ebur128->i3000.rel_threshold -= ebur128->pan_law;
+        ebur128->lra_low -= ebur128->pan_law;
+        ebur128->lra_high -= ebur128->pan_law;
+    }
+
     av_log(ctx, AV_LOG_INFO, "Summary:\n\n"
            "  Integrated loudness:\n"
            "    I:         %5.1f LUFS\n"