Add track audio-level metadata propagation to output frames.

src/framework/mlt_tractor.c

@@ -440,6 +440,10 @@ static int producer_get_audio(mlt_frame self,
                        "producer_consumer_fps",
                        mlt_properties_get(properties, "producer_consumer_fps"));
     mlt_frame_get_audio(frame, buffer, format, frequency, channels, samples);
+
+    // Propagate audio-level metadata onto the tractor output frame.
+    mlt_properties_copy(properties, frame_properties, "meta.");
+
     mlt_frame_set_audio(self,
                         *buffer,
                         *format,

src/modules/core/transition_mix.c

@@ -147,6 +147,7 @@ static int transition_get_audio(mlt_frame frame_a,
 
     // Get the properties of the b frame
     mlt_properties b_props = MLT_FRAME_PROPERTIES(frame_b);
+    mlt_properties a_props = MLT_FRAME_PROPERTIES(frame_a);
 
     transition_mix self = transition->child;
     float *buffer_b, *buffer_a;
@@ -160,6 +161,9 @@ static int transition_get_audio(mlt_frame frame_a,
     mlt_frame_get_audio(frame_b, (void **) &buffer_b, format, &frequency_b, &channels_b, &samples_b);
     mlt_frame_get_audio(frame_a, (void **) &buffer_a, format, &frequency_a, &channels_a, &samples_a);
 
+    // Merge b-frame level metadata onto a-frame after both callbacks run.
+    mlt_properties_copy(a_props, b_props, "meta.");
+
     // Prevent dividing by zero.
     if (!channels_a || !channels_b || !buffer_a || !buffer_b)
         return 1;

src/modules/normalize/filter_audiolevel.c

@@ -2,7 +2,7 @@
  * filter_audiolevel.c -- get the audio level of each channel
  * Copyright (C) 2002 Steve Harris
  * Copyright (C) 2010 Marco Gittler <g.marco@freenet.de>
- * Copyright (C) 2012-2023 Dan Dennedy <dan@dennedy.org>
+ * Copyright (C) 2012-2026 Dan Dennedy <dan@dennedy.org>
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -24,8 +24,42 @@
 #include <framework/mlt_log.h>
 
 #include <math.h>
+#include <stdio.h>
 #include <stdlib.h>
 
+static const char *kDefaultPrefix = "meta.media.audio_level.";
+
+static void append_int(char *out, size_t out_size, size_t *offset, int value)
+{
+    if (!out || !out_size || !offset || *offset >= out_size)
+        return;
+
+    const int n = snprintf(out + *offset, out_size - *offset, "%d", value);
+    if (n < 0)
+        return;
+    *offset += (size_t) n;
+    if (*offset >= out_size)
+        out[out_size - 1] = '\0';
+}
+
+static void make_audio_level_key(const char *prefix, int channel, char *key, size_t key_size)
+{
+    if (!key || key_size == 0)
+        return;
+
+    if (!prefix || !prefix[0])
+        prefix = kDefaultPrefix;
+
+    const int n = snprintf(key, key_size, "%s", prefix);
+    size_t offset = n > 0 ? (size_t) n : 0;
+    if (offset >= key_size)
+        offset = key_size - 1;
+    key[offset] = '\0';
+
+    append_int(key, key_size, &offset, channel);
+    key[offset < key_size ? offset : key_size - 1] = '\0';
+}
+
 #define AMPTODBFS(n) (log10(n) * 20.0)
 
 //----------------------------------------------------------------------------
@@ -67,6 +101,7 @@ static int filter_get_audio(mlt_frame frame,
 
     int iec_scale = mlt_properties_get_int(filter_props, "iec_scale");
     int dbPeak = mlt_properties_get_int(filter_props, "dbpeak");
+    const char *prefix = mlt_properties_get(filter_props, "prefix");
     *format = mlt_audio_s16;
     int error = mlt_frame_get_audio(frame, buffer, format, frequency, channels, samples);
     if (error || !buffer || !buffer[0])
@@ -76,7 +111,7 @@ static int filter_get_audio(mlt_frame frame,
     int num_samples = *samples > 200 ? 200 : *samples;
     int num_oversample = 0;
     int c, s;
-    char key[50];
+    char key[128];
     int16_t *pcm = (int16_t *) *buffer;
 
     for (c = 0; c < *channels; c++) {
@@ -119,7 +154,7 @@ static int filter_get_audio(mlt_frame frame,
             if (iec_scale)
                 level = IEC_Scale(AMPTODBFS(level));
         }
-        sprintf(key, "meta.media.audio_level.%d", c);
+        make_audio_level_key(prefix, c, key, sizeof(key));
         mlt_properties_set_double(MLT_FRAME_PROPERTIES(frame), key, level);
         sprintf(key, "_audio_level.%d", c);
         mlt_properties_set_double(filter_props, key, level);
@@ -152,6 +187,7 @@ mlt_filter filter_audiolevel_init(mlt_profile profile,
     if (filter) {
         filter->process = filter_process;
         mlt_properties_set_int(MLT_FILTER_PROPERTIES(filter), "iec_scale", 1);
+        mlt_properties_set(MLT_FILTER_PROPERTIES(filter), "prefix", kDefaultPrefix);
     }
     return filter;
 }

src/modules/normalize/filter_audiolevel.yml

@@ -2,7 +2,7 @@ schema_version: 7.2
 type: filter
 identifier: audiolevel
 title: Audio Levels
-version: 3
+version: 4
 copyright: Meltytech, LLC, Marco Gittler, and Steve Harris
 creator: Dan Dennedy
 contributor:
@@ -17,11 +17,20 @@ notes: >
   troughs of short duration like a VU meter.
   Applications can also get this data on the frame as meta.media.audio_level.<N>
   where <N> is the channel number starting with 0.
+  The prefix is configurable using the prefix property.
 tags:
   - Audio
 audio_formats:
   - s16
 parameters:
+  - identifier: prefix
+    title: Frame Property Prefix
+    description: >
+      Prefix for frame audio level keys. The filter appends the channel index
+      to this prefix.
+    type: string
+    default: meta.media.audio_level.
+
   - identifier: iec_scale
     title: Use IEC 60268-18 Scale
     type: boolean