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

[James Almer <jamrial@gmail.com>]

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.
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