Re: [FFmpeg-devel] [PATCH v23 04/10] avformat/evc_demuxer: Added demuxer to handle reading EVC video files

["Dawid Kozinski/Multimedia \(PLT\) /SRPOL/Staff Engineer/Samsung Electronics" <d.kozinski@samsung.com>]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of James
> Almer
> Sent: środa, 7 czerwca 2023 15:40
> To: ffmpeg-devel@ffmpeg.org
> Subject: Re: [FFmpeg-devel] [PATCH v23 04/10] avformat/evc_demuxer: Added
> demuxer to handle reading EVC video files
> 
> On 6/7/2023 10:36 AM, Dawid Kozinski/Multimedia (PLT) /SRPOL/Staff
> Engineer/Samsung Electronics wrote:
> >
> >
> >
> >> -----Original Message-----
> >> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> >> James Almer
> >> Sent: poniedziałek, 29 maja 2023 15:08
> >> To: ffmpeg-devel@ffmpeg.org
> >> Subject: Re: [FFmpeg-devel] [PATCH v23 04/10] avformat/evc_demuxer:
> >> Added demuxer to handle reading EVC video files
> >>
> >> On 5/26/2023 7:31 AM, Dawid Kozinski wrote:
> >>> +static int get_temporal_id(const uint8_t *bits, int bits_size) {
> >>> +    int temporal_id = 0;
> >>> +    short t = 0;
> >>> +
> >>> +    if (bits_size >= EVC_NALU_HEADER_SIZE) {
> >>> +        unsigned char *p = (unsigned char *)bits;
> >>> +        // forbidden_zero_bit
> >>> +        if ((p[0] & 0x80) != 0)
> >>> +            return -1;
> >>> +
> >>> +        for (int i = 0; i < EVC_NALU_HEADER_SIZE; i++)
> >>> +            t = (t << 8) | p[i];
> >>
> >> I think this can be replaced with AV_RB16.
> >>
> >>> +
> >>> +        temporal_id = (t >> 6) & 0x0007;
> >>> +    }
> >>> +
> >>> +    return temporal_id;
> >>> +}
> >>
> >> [...]
> >>
> >>> +static uint32_t read_nal_unit_length(const uint8_t *bits, int
> >>> +bits_size) {
> >>> +    uint32_t nalu_len = 0;
> >>> +
> >>> +    if (bits_size >= EVC_NALU_LENGTH_PREFIX_SIZE) {
> >>> +
> >>> +        int t = 0;
> >>> +        unsigned char *p = (unsigned char *)bits;
> >>> +
> >>> +        for (int i = 0; i < EVC_NALU_LENGTH_PREFIX_SIZE; i++)
> >>> +            t = (t << 8) | p[i];
> >>
> >> AV_RB32.
> >>
> >>> +
> >>> +        nalu_len = t;
> >>> +        if (nalu_len == 0)   // Invalid bitstream size
> >>> +            return 0;
> >>> +    }
> >>> +
> >>> +    return nalu_len;
> >>> +}
> >>
> >> [...]
> >>
> >>> +static int evc_frame_merge_filter(AVBSFContext *bsf, AVPacket *out) {
> >>> +    EVCMergeContext *ctx = bsf->priv_data;
> >>> +    AVPacket *in = ctx->in;
> >>> +
> >>> +    int free_space = 0;
> >>> +    size_t  nalu_size = 0;
> >>> +    uint8_t *nalu = NULL;
> >>> +    int au_end_found = 0;
> >>> +    int err;
> >>> +
> >>> +    err = ff_bsf_get_packet_ref(bsf, in);
> >>> +    if (err < 0)
> >>> +        return err;
> >>> +
> >>> +    nalu_size = read_nal_unit_length(in->data,
> >> EVC_NALU_LENGTH_PREFIX_SIZE);
> >>> +    if(nalu_size <= 0) {
> >>> +        av_packet_unref(in);
> >>> +        return AVERROR_INVALIDDATA;
> >>> +    }
> >>> +
> >>> +    nalu = in->data + EVC_NALU_LENGTH_PREFIX_SIZE;
> >>> +    nalu_size = in->size - EVC_NALU_LENGTH_PREFIX_SIZE;
> >>> +
> >>> +    // NAL unit parsing needed to determine if end of AU was found
> >>> +    err = parse_nal_unit(nalu, nalu_size, bsf);
> >>> +    if (err < 0) {
> >>> +        av_log(bsf, AV_LOG_ERROR, "NAL Unit parsing error\n");
> >>> +        av_packet_unref(in);
> >>> +
> >>> +        return err;
> >>> +    }
> >>> +
> >>> +    au_end_found = end_of_access_unit_found(bsf);
> >>> +
> >>> +    free_space = ctx->au_buffer.capacity - ctx->au_buffer.data_size;
> >>> +    while( free_space < in->size ) {
> >>> +        ctx->au_buffer.capacity *= 2;
> >>> +        free_space = ctx->au_buffer.capacity -
> >>> + ctx->au_buffer.data_size;
> >>> +
> >>> +        if(free_space >= in->size) {
> >>> +            ctx->au_buffer.data = av_realloc(ctx->au_buffer.data,
> >>> + ctx-
> >>> au_buffer.capacity);
> >>> +        }
> >>> +    }
> >>> +
> >>> +    memcpy(ctx->au_buffer.data + ctx->au_buffer.data_size,
> >>> + in->data,
> >>> + in->size);
> >>
> >> This is including the Annex-B's nal_unit_lenght value into the
> >> assembled
> > packet. I
> >> assume libxevd expects this? Also, is it defined as part of the
> >> ISOBMFF encapsulation? Patch 07/10 does not seem to strip it.
> >
> > Could you expand on this topic?
> > What do you mean by "Patch 07/10 does not seem to strip it"?
> > Patch 7/10 expands the implementation of the MOV Muxer to support the
> > EVC stream.
> > The function ff_isom_write_evcc parses the received data, extracts NAL
> > units of types SPS, PPS, APS, SEI, and inserts their contents into the
> > corresponding elements of the arrays array, which is a component of
> > the EVCDecoderConfigurationRecord structure. Additionally, the
> > ff_isom_write_evcc function populates the relevant fields of the
> > EVCDecoderConfigurationRecord structure based on the parsed SPS NAL
> > unit data, and then calls the evcc_write function, which writes the
> > EVCDecoderConfigurationRecord to the stream.
> >
> > It seems that I don't understand what's wrong with our implementation
> > ( I mean MOV muxer for EVC) and it seems I need your help in this
> > matter. I need an explanation of what's wrong and what should be done to fix
> the issue.
> 
> I mean the packets that go into ISOBMFF Sample, not the evcc atom. Is the
> encapsulation supposed to include the nal_unit_length from Annex-B?

The function mov_write_single_packet(s, pkt) receives packets containing the Access Units (AU) of an EVC stream. Each AU consists of a sequence of NAL units, each preceded by a 4-byte prefix that contains information about the length of the NAL unit. The entire AU is then copied to AVIOContext using avio_write(pb, pkt->data, size).

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