Re: [FFmpeg-devel] [PATCH 3/6] Add CRYO APC muxer

[Michael Niedermayer <michael@niedermayer.cc>]

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Hi

On Tue, Dec 26, 2023 at 04:52:47PM +0100, Tomas Härdin wrote:
> 

[...]
> +
> +static int apc_write_header(AVFormatContext *s)
> +{
> +    AVIOContext *pb = s->pb;
> +    AVCodecParameters *par;
> +    AVStream *st;
> +
> +    if (s->nb_streams != 1) {
> +        av_log(s, AV_LOG_ERROR, "Must have exactly one stream\n");
> +        return AVERROR(EINVAL);
> +    }
> +
> +    st = s->streams[0];
> +    par = st->codecpar;
> +
> +    if (par->ch_layout.nb_channels <= 0 || par->ch_layout.nb_channels > 2) {
> +        av_log(s, AV_LOG_ERROR, "Must be mono or stereo\n");
> +        return AVERROR(EINVAL);
> +    }
> +
> +    if (par->extradata_size != 0 && par->extradata_size != 8) {
> +        av_log(s, AV_LOG_ERROR,
> +            "Must have exactly 0 or 8 bytes of extradata, got %i\n",
> +            par->extradata_size);
> +        return AVERROR(EINVAL);
> +    }
> +
> +    ffio_wfourcc(pb, "CRYO");
> +    ffio_wfourcc(pb, "_APC");
> +    ffio_wfourcc(pb, "1.20");

> +    avio_wl32(pb, 0); // number or samples

please add to the comment "updated in apc_write_trailer()"


> +    avio_wl32(pb, par->sample_rate);
> +
> +    // write extradata if we have it (remuxing)
> +    // else write dummy values and wait for AV_PKT_DATA_NEW_EXTRADATA (encoding)
> +    if (par->extradata_size) {
> +        avio_write(pb, par->extradata, par->extradata_size);
> +    } else {
> +        avio_wl64(pb, 0);
> +    }
> +
> +    avio_wl32(pb, par->ch_layout.nb_channels - 1);
> +    avpriv_set_pts_info(st, 64, 1, par->sample_rate);
> +    return 0;
> +}
> +

> +static int apc_write_packet(AVFormatContext *s, AVPacket *pkt)
> +{
> +    size_t extradata_size = 0;

> +    const uint8_t *extradata = av_packet_get_side_data(
> +            pkt, AV_PKT_DATA_NEW_EXTRADATA, &extradata_size);
> +
> +    if (extradata_size == 8) {
> +        // we got predictors from encoder
> +        // try to seek back end write them
> +        int64_t pos = avio_tell(s->pb), err;
> +        if ((err = avio_seek(s->pb, 20, SEEK_SET)) >= 0) {
> +            avio_write(s->pb, extradata, extradata_size);
> +            avio_seek(s->pb, pos, SEEK_SET);
> +        } else {
> +            av_log(s, AV_LOG_ERROR, "Got predictors from encoder but couldn't seek back to write them\n");
> +            // fail since we should always be able to do this within the avio cache
> +            // unless the encoder gave us predictors way too late for some reason
> +            return err;
> +        }
> +    }

I think the encoder should buffer data or use 2 passes
if it needs future data. seeking back in the muxer is a bit odd.

if it becomes available always in teh first packet then maybe the
whole header could be written in the first packet

I would suggest, you add APC support to nut.
Thats a good test to ensure the packet and extradata is set at the right time
for a generic muxer



> +
> +    avio_write(s->pb, pkt->data, pkt->size);
> +    return 0;
> +}
> +
> +static int apc_write_trailer(AVFormatContext *s)
> +{
> +    int64_t file_size = avio_tell(s->pb);
> +
> +    // write length, if we're able to seek back
> +    if (avio_seek(s->pb, 12, SEEK_SET) >= 0) {

> +        if (file_size - APC_HEADER_SIZE >
> +                UINT32_MAX * s->streams[0]->codecpar->ch_layout.nb_channels / 2) {
> +            av_log(s, AV_LOG_ERROR, "File too large\n");
> +            return AVERROR(EINVAL);
> +        }
> +
> +        avio_wl32(s->pb, (file_size - APC_HEADER_SIZE) *
> +                            2 / s->streams[0]->codecpar->ch_layout.nb_channels);

I think something like this is slightly cleaner:


int64_t number_of_samples = (file_size - APC_HEADER_SIZE) *
                            2 / s->streams[0]->codecpar->ch_layout.nb_channels;

if (number_of_samples > UINT32_MAX)
    ...
[...]

thx

--
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Avoid a single point of failure, be that a person or equipment.

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