Re: [libav-devel] [PATCH] movenc: Allow use of hvc1 tag for h.265

[Martin Storsjö]

On Tue, 13 Jun 2017, John Stebbins wrote:

On 06/13/2017 12:13 PM, Martin Storsjö wrote:
On Mon, 12 Jun 2017, John Stebbins wrote:
If AVCodecParameters.codec_tag is 'hvc1' use it instead of 'hev1' for
h.265 streams. QuickTime (and other Apple software) requires 'hvc1'.
---
libavformat/movenc.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index e389029..8d89a7a 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -726,7 +726,10 @@ static int mov_write_hvcc_tag(AVIOContext *pb, MOVTrack 
*track)

    avio_wb32(pb, 0);
    ffio_wfourcc(pb, "hvcC");
-    ff_isom_write_hvcc(pb, track->vos_data, track->vos_len, 0);
+    if (track->tag == MKTAG('h','v','c','1'))
+        ff_isom_write_hvcc(pb, track->vos_data, track->vos_len, 1);
+    else
+        ff_isom_write_hvcc(pb, track->vos_data, track->vos_len, 0);
    return update_size(pb, pos);
}

@@ -786,6 +789,8 @@ static int mp4_get_codec_tag(AVFormatContext *s, MOVTrack 
*track)
        return 0;

    if      (track->par->codec_id == AV_CODEC_ID_H264)      tag = 
MKTAG('a','v','c','1');
+    else if (track->par->codec_id == AV_CODEC_ID_HEVC &&
+             tag == MKTAG('h','v','c','1'))                 ;
    else if (track->par->codec_id == AV_CODEC_ID_HEVC)      tag = 
MKTAG('h','e','v','1');
    else if (track->par->codec_id == AV_CODEC_ID_AC3)       tag = 
MKTAG('a','c','-','3');
    else if (track->par->codec_id == AV_CODEC_ID_DIRAC)     tag = 
MKTAG('d','r','a','c');
@@ -4496,7 +4501,9 @@ AVOutputFormat ff_mp4_muxer = {
    .write_packet      = mov_write_packet,
    .write_trailer     = mov_write_trailer,
    .flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH | 
AVFMT_TS_NEGATIVE,
-    .codec_tag         = (const AVCodecTag* const []){ ff_mp4_obj_type, 0 },
+    .codec_tag         = (const AVCodecTag* const []){
+        ff_codec_movvideo_tags, ff_codec_movaudio_tags,
+        ff_codec_movsubtitle_tags, 0 },
    .priv_class        = &mp4_muxer_class,
I guess this change is because libavformat refuses to allow you to set a
custom codec_tag unless it is found in the codec_tag list, right? Does
this have any other practical implications? I feel a little uneasy about
this part of it...



Yes, this change was to fix result of validate_codec_tag in mux.c.

I was curious about this myself.  I could only find 2 places that this 
codec_tag array is used.  One in
validate_codec_tag which is the instance that needed fixing, and another in 
avconv where it is checking if the source
codec_tag is valid for the output format when doing stream copy.  In both of 
these cases ff_mp4_obj_type is wrong for
codec_tag so I'm not sure why this was used in the first place. According to 
git history, this was used from the
beginning, but makes no sense. I also looked at who calls validate_codec_tag 
just to be sure it's being used
consistently. It's only used by avformat_write_header.

I also ran fate to check for regressions.

Ok, that makes sense.

In that case though, I would very much prefer to do this in two separate 
steps; one that removes the .codec_tag part from the AVOutputFormat, with 
a thorough explanation of this (pretty much what you just wrote) and 
saying that this will allow the caller to indicate that actual codec tag 
he wants to be used, and then this one on top that allows you to use hvc1.

// Martin
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