opus: Read ReplayGain values from R128 tags

Also ignore peak tags since Opus does not support them.

See also:
- https://datatracker.ietf.org/doc/html/rfc7845#section-5.2.1
- https://github.com/orgs/audacious-media-player/discussions/101
- audacious-media-player/audacious#1138
- https://github.com/Moonbase59/loudgain?tab=readme-ov-file#how-i-handle-opus-opus-audio-files
- beetbox/beets#2557

Co-authored-by: Almoped <[email protected]>

src/opus/opus.cc

@@ -180,8 +180,8 @@ static bool update_replay_gain(OggOpusFile * opus_file,
     if (!tags)
         return false;
 
-    const char * album_gain = opus_tags_query(tags, "REPLAYGAIN_ALBUM_GAIN", 0);
-    const char * track_gain = opus_tags_query(tags, "REPLAYGAIN_TRACK_GAIN", 0);
+    const char * album_gain = opus_tags_query(tags, "R128_ALBUM_GAIN", 0);
+    const char * track_gain = opus_tags_query(tags, "R128_TRACK_GAIN", 0);
 
     /* stop if we have no gain values */
     if (!album_gain && !track_gain)
@@ -193,34 +193,17 @@ static bool update_replay_gain(OggOpusFile * opus_file,
     if (!track_gain)
         track_gain = album_gain;
 
-    rg_info->album_gain = str_to_double(album_gain);
-    rg_info->track_gain = str_to_double(track_gain);
+    /* Q7.8 number to float .. 2^8 = 256 */
+    /* EBU-R128 (-23 LUFS) to RG 2.0 (-18 LUFS) .. +5 dB */
+    rg_info->album_gain = str_to_double(album_gain) / 256 + 5.0f;
+    rg_info->track_gain = str_to_double(track_gain) / 256 + 5.0f;
 
-    const char * album_peak = opus_tags_query(tags, "REPLAYGAIN_ALBUM_PEAK", 0);
-    const char * track_peak = opus_tags_query(tags, "REPLAYGAIN_TRACK_PEAK", 0);
-
-    if (!album_peak && !track_peak)
-    {
-        /* okay, we have gain but no peak values */
-        rg_info->album_peak = 0;
-        rg_info->track_peak = 0;
-    }
-    else
-    {
-        /* fill in missing values if we can */
-        if (!album_peak)
-            album_peak = track_peak;
-        if (!track_peak)
-            track_peak = album_peak;
-
-        rg_info->album_peak = str_to_double(album_peak);
-        rg_info->track_peak = str_to_double(track_peak);
-    }
+    /* Opus intentionally does not support peak tags */
+    rg_info->album_peak = 0;
+    rg_info->track_peak = 0;
 
     AUDDBG("Album gain: %s (%f)\n", album_gain, rg_info->album_gain);
     AUDDBG("Track gain: %s (%f)\n", track_gain, rg_info->track_gain);
-    AUDDBG("Album peak: %s (%f)\n", album_peak, rg_info->album_peak);
-    AUDDBG("Track peak: %s (%f)\n", track_peak, rg_info->track_peak);
 
     return true;
 }