[FFmpeg-devel] [PATCH 2/2] avformat/riffdec: follow the MS docs more strictly for setting wav channel layouts

[Marton Balint]

- Only parse the defined masks in dwChannelMask, unless strict_std_compliance
  is less than normal. This matches with the behaviour of the wav muxer.
- Ignore additional bits in dwChannelMasks as the MS documentation suggests [1]
- Assume UNKNOWN channels for missing bits as the MS documentation suggests [1]

[1] https://learn.microsoft.com/en-us/previous-versions/windows/hardware/design/dn653308(v=vs.85)#details-about-dwchannelmask

Signed-off-by: Marton Balint <cus at passwd.hu>
---
 libavformat/riffdec.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/libavformat/riffdec.c b/libavformat/riffdec.c
index 6cc912133c..0c7b2c6bdb 100644
--- a/libavformat/riffdec.c
+++ b/libavformat/riffdec.c
@@ -58,7 +58,7 @@ enum AVCodecID ff_codec_guid_get_id(const AVCodecGuid *guids, ff_asf_guid guid)
  * an openended structure.
  */
 
-static void parse_waveformatex(AVFormatContext *ctx, AVIOContext *pb, AVCodecParameters *par)
+static int parse_waveformatex(AVFormatContext *ctx, AVIOContext *pb, AVCodecParameters *par, int channels)
 {
     ff_asf_guid subformat;
     int bps;
@@ -69,7 +69,26 @@ static void parse_waveformatex(AVFormatContext *ctx, AVIOContext *pb, AVCodecPar
         par->bits_per_coded_sample = bps;
 
     mask = avio_rl32(pb); /* dwChannelMask */
-    av_channel_layout_from_mask(&par->ch_layout, mask);
+    if (ctx->strict_std_compliance >= FF_COMPLIANCE_NORMAL)
+        mask &= 0x3ffff;
+
+    /* According to MS docs, most significant bits should be ignored if mask
+     * has more bits than the number of channels */
+    while (av_popcount64(mask) > channels)
+        mask &= ~(1LL << av_log2(mask));
+
+    /* If the mask has less bits than channels, then we assign the remaining
+     * channels as UNKNOWN. */
+    if (mask && av_popcount64(mask) < channels) {
+        int ret = av_channel_layout_custom_init(&par->ch_layout, channels);
+        if (ret < 0)
+            return ret;
+        for (int i = 0, idx = 0; mask; i++, mask >>= 1)
+            if (mask & 1)
+                par->ch_layout.u.map[idx++].id = i;
+    } else {
+        av_channel_layout_from_mask(&par->ch_layout, mask);
+    }
 
     ff_get_guid(pb, &subformat);
     if (!memcmp(subformat + 4,
@@ -88,6 +107,7 @@ static void parse_waveformatex(AVFormatContext *ctx, AVIOContext *pb, AVCodecPar
                    "unknown subformat:"FF_PRI_GUID"\n",
                    FF_ARG_GUID(subformat));
     }
+    return 0;
 }
 
 /* "big_endian" values are needed for RIFX file format */
@@ -145,7 +165,9 @@ int ff_get_wav_header(AVFormatContext *ctx, AVIOContext *pb,
         size  -= 18;
         cbSize = FFMIN(size, cbSize);
         if (cbSize >= 22 && id == 0xfffe) { /* WAVEFORMATEXTENSIBLE */
-            parse_waveformatex(ctx, pb, par);
+            ret = parse_waveformatex(ctx, pb, par, channels);
+            if (ret < 0)
+                return ret;
             cbSize -= 22;
             size   -= 22;
         }
-- 
2.35.3