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: 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.

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