[FFmpeg-devel] [PATCH 2/4] avformat/mxf: get rid of samples per frame array usage

[Baptiste Coudurier]

> On Mar 3, 2020, at 11:37 AM, Tomas Härdin <tjoppen at acc.umu.se> wrote:
> 
> tis 2020-03-03 klockan 11:22 -0800 skrev Baptiste Coudurier:
>> Hey guys,
>> 
>> 
>>> On Mar 3, 2020, at 10:54 AM, Tomas Härdin <tjoppen at acc.umu.se> wrote:
>>> 
>>> mån 2020-03-02 klockan 21:35 +0100 skrev Marton Balint:
>>>> On Mon, 2 Mar 2020, Tomas Härdin wrote:
>>>> 
>>>>> fre 2020-02-28 klockan 01:37 +0100 skrev Marton Balint:
>>>>>> Signed-off-by: Marton Balint <cus at passwd.hu>
>>>>>> ---
>>>>>> libavformat/mxf.c    | 44 ++++----------------------------------------
>>>>>> libavformat/mxf.h    |  6 ------
>>>>>> libavformat/mxfdec.c | 23 +++--------------------
>>>>>> libavformat/mxfenc.c | 24 ++++++------------------
>>>>>> 4 files changed, 13 insertions(+), 84 deletions(-)
>>>>>> int ff_mxf_get_content_package_rate(AVRational time_base)
>>>>>> {
>>>>>> -    int idx = av_find_nearest_q_idx(time_base, mxf_time_base);
>>>>>> -    AVRational diff = av_sub_q(time_base, mxf_time_base[idx]);
>>>>>> -
>>>>>> -    diff.num = FFABS(diff.num);
>>>>>> -
>>>>>> -    if (av_cmp_q(diff, (AVRational){1, 1000}) >= 0)
>>>>>> -        return -1;
>>>>>> -
>>>>>> -    return mxf_content_package_rates[idx];
>>>>>> +    for (int i = 0; mxf_time_base[i].num; i++)
>>>>>> +        if (!av_cmp_q(time_base, mxf_time_base[i]))
>>>>> 
>>>>> I see this gets rid of the inexact check for an exact one. Approve!
>>>>> 
>>>>>> @@ -3307,20 +3307,17 @@ static int mxf_get_next_track_edit_unit(MXFContext *mxf, MXFTrack *track, int64_
>>>>>> static int64_t mxf_compute_sample_count(MXFContext *mxf, AVStream *st,
>>>>>>                                        int64_t edit_unit)
>>>>>> {
>>>>>> -    int i, total = 0, size = 0;
>>>>>>    MXFTrack *track = st->priv_data;
>>>>>>    AVRational time_base = av_inv_q(track->edit_rate);
>>>>>>    AVRational sample_rate = av_inv_q(st->time_base);
>>>>>> -    const MXFSamplesPerFrame *spf = NULL;
>>>>>> -    int64_t sample_count;
>>>>>> 
>>>>>>    // For non-audio sample_count equals current edit unit
>>>>>>    if (st->codecpar->codec_type != AVMEDIA_TYPE_AUDIO)
>>>>>>        return edit_unit;
>>>>>> 
>>>>>> -    if ((sample_rate.num / sample_rate.den) == 48000)
>>>>>> -        spf = ff_mxf_get_samples_per_frame(mxf->fc, time_base);
>>>>>> -    if (!spf) {
>>>>>> +    if ((sample_rate.num / sample_rate.den) == 48000) {
>>>>>> +        return av_rescale_q(edit_unit, sample_rate, track->edit_rate);
>>>>> 
>>>>> Should be OK, per previous rounding argument
>>>>> 
>>>>>>                }
>>>>>>                sc->index = INDEX_D10_AUDIO;
>>>>>>                sc->container_ul = ((MXFStreamContext*)s->streams[0]->priv_data)->container_ul;
>>>>>> -                sc->frame_size = 4 + 8 * spf[0].samples_per_frame[0] * 4;
>>>>>> +                sc->frame_size = 4 + 8 * av_rescale_rnd(st-
>>>>>>> codecpar->sample_rate, mxf->time_base.num, mxf->time_base.den,
>>>>>> AV_ROUND_UP) * 4;
>>>>> 
>>>>> I was going to say this is only used for CBR video, but closer
>>>>> inspection reveals it's used to prevent 1/1.001 rate audio packets from
>>>>> making their way into CBR files. This is a bit surprising to me, since
>>>>> D-10 supports NTSC (without audio?)
>>>> 
>>>> I thought D10 can only be CBR and and it can only use a constant edit unit 
>>>> size, 1/1.001 audio packet size difference is handled using KLV 
>>>> padding. So what we compute here is a _maximum_ frame size.
>>> 
>>> Now I remember: D-10 is indeed CBR, but at the MXF/CP level. To make it
>>> CBR you must pad the content packages with fill KLVs as necessary. This
>>> filling was actually removed by Baptiste a while back, and we had a set
>>> of patches attempting to fix support for NTSC but sadly the ratecontrol
>>> in lavc is not up to snuff to support that for files longer than about
>>> an hour. This also means the video essence needs to be coded at a
>>> bitrate slightly lower than the D-10 rate, say 49.9 Mbps for 50 Mbps D-
>>> 10 (+-audio).
>> 
>> IIRC, and correct me if I’m wrong, the VBV delay in D-10 is actually fixed, 
>> so the essence needs to be CBR at the essence (mpeg-2) level.
>> Anything else is just broken IMHO.
> 
> For practical D-10 decoders probably, but I think the spec is a bit
> more lax. You only need to set up the DeltaEntry array in the
> IndexTableSegment correctly
> 
>>> I believe at some point there was code for padding the MPEG-2 essence
>>> with zeroes to make CBR, but that obviously doesn't work with audio due
>>> to 1601.6 samples per EditUnit
>> 
>> Yeah, the pattern of audio samples is very strict in MXF, and every 5 frames, 
>> Audio and video durations match perfectly.
> 
> Yep, which luckily this patchset does. For audio there definitely needs
> to be KLV fill since the size of the audio packets will vary

Wait, is it what it does ? The pattern is very strict and ordered.
For 30000/1001 and 60000/1001, it seems to me that it would not be correct.

— 
Baptiste