Re: [FFmpeg-devel] [PATCH] avcodec/libx264: fix extradata when config annexb=0

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Zhao Zhili:
> 
> 
>> -----Original Message-----
>> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of Andreas Rheinhardt
>> Sent: 2024年3月6日 22:19
>> To: ffmpeg-devel@ffmpeg.org
>> Subject: Re: [FFmpeg-devel] [PATCH] avcodec/libx264: fix extradata when config annexb=0
>>
>> Zhao Zhili:
>>> From: Zhao Zhili <zhilizhao@tencent.com>
>>>
>>> ---
>>>  configure            |   2 +-
>>>  libavcodec/libx264.c | 148 ++++++++++++++++++++++++++++++++++++-------
>>>  2 files changed, 125 insertions(+), 25 deletions(-)
>>>
>>> diff --git a/configure b/configure
>>> index bbf1a70731..ef58c3fe97 100755
>>> --- a/configure
>>> +++ b/configure
>>> @@ -3491,7 +3491,7 @@ libwebp_encoder_deps="libwebp"
>>>  libwebp_anim_encoder_deps="libwebp"
>>>  libx262_encoder_deps="libx262"
>>>  libx264_encoder_deps="libx264"
>>> -libx264_encoder_select="atsc_a53"
>>> +libx264_encoder_select="atsc_a53 h264parse"
>>>  libx264rgb_encoder_deps="libx264"
>>>  libx264rgb_encoder_select="libx264_encoder"
>>>  libx265_encoder_deps="libx265"
>>> diff --git a/libavcodec/libx264.c b/libavcodec/libx264.c
>>> index 10d646bd76..10dfda125d 100644
>>> --- a/libavcodec/libx264.c
>>> +++ b/libavcodec/libx264.c
>>> @@ -34,9 +34,12 @@
>>>  #include "avcodec.h"
>>>  #include "codec_internal.h"
>>>  #include "encode.h"
>>> +#include "get_bits.h"
>>> +#include "h264_ps.h"
>>>  #include "internal.h"
>>>  #include "packet_internal.h"
>>>  #include "atsc_a53.h"
>>> +#include "put_bits.h"
>>>  #include "sei.h"
>>>
>>>  #include <x264.h>
>>> @@ -865,6 +868,124 @@ static int convert_pix_fmt(enum AVPixelFormat pix_fmt)
>>>      return 0;
>>>  }
>>>
>>> +static int set_avcc_extradata(AVCodecContext *avctx, x264_nal_t *nal, int nnal)
>>> +{
>>> +    x264_nal_t *sps_nal = NULL;
>>> +    x264_nal_t *pps_nal = NULL;
>>> +    GetBitContext gbc;
>>> +    PutBitContext pbc;
>>> +    H264ParamSets ps = { 0 };
>>> +    uint8_t *p;
>>> +    int ret, profile;
>>> +
>>> +    /* We know it's in the order of SPS/PPS/SEI, but it's not documented in x264 API.
>>> +     * The x264 param i_sps_id implies there is a single pair of SPS/PPS.
>>> +     */
>>> +    for (int i = 0; i < nnal; i++) {
>>> +        if (nal[i].i_type == NAL_SPS)
>>> +            sps_nal = &nal[i];
>>> +        else if (nal[i].i_type == NAL_PPS)
>>> +            pps_nal = &nal[i];
>>> +    }
>>> +    av_assert0(sps_nal);
>>> +    av_assert0(pps_nal);
>>
>> We should never assert on the output of external libraries.
> 
> Ok, will just remove the assert. I don't think a check is needed.
> 
>>
>>> +
>>> +    // +11 for AVCDecoderConfigurationRecord, will shrink to the real size finally
>>> +    avctx->extradata_size = sps_nal->i_payload + pps_nal->i_payload + 11;
>>> +    avctx->extradata = av_mallocz(avctx->extradata_size + AV_INPUT_BUFFER_MIN_SIZE);
>>
>> AV_INPUT_BUFFER_PADDING_SIZE
> 
> Oops.
> 
>>
>>> +    if (!avctx->extradata)
>>> +        return AVERROR(ENOMEM);
>>> +
>>> +    // Skip size part
>>> +    p = sps_nal->p_payload + 4;
>>> +    init_get_bits8(&gbc, p, sps_nal->i_payload - 4);
>>> +    skip_bits(&gbc, 8);
>>> +    ret = ff_h264_decode_seq_parameter_set(&gbc, avctx, &ps, 0);
>>
>> This is completely overblown. You only read three fields: profile_idc,
>> the constraint fields and level_idc. All three are at the start of the
>> SPS and occupy a whole byte, so there is no need for a GetBitContext at all.
>> (Yes, I am aware of the hypothetical scenario in which we could have a
>> 0x03 escape byte before level_idc (in case of a hypothetical future
>> profile zero), but even then using ff_h264_decode_seq_parameter_set() is
>> overblown.)
> 
> You missed chroma_format_idc, bit_depth_luma/bit_depth_chroma.
> I can get that from pixel format, but there are a lot of them.
> 

Ok, but even then I fail to see a convenience advantage here. After all,
using it e.g. forces the following loop onto you. And it forces cleanup
code here. Also, there is a gotcha here. See below.

>>
>>> +    if (ret < 0)
>>> +        return ret;
>>> +
>>> +    for (int i = 0; i < FF_ARRAY_ELEMS(ps.sps_list); i++) {
>>> +        if (ps.sps_list[i]) {
>>> +            ps.sps = ps.sps_list[i];
>>> +            break;
>>> +        }
>>> +    }
>>> +
>>> +    // Now create AVCDecoderConfigurationRecord
>>> +    init_put_bits(&pbc, avctx->extradata, avctx->extradata_size);
>>> +    put_bits(&pbc, 8, 1); // version
>>> +    put_bits(&pbc, 8, ps.sps->profile_idc); // AVCProfileIndication
>>> +    put_bits(&pbc, 8, ps.sps->constraint_set_flags); // profile_compatibility

Sure this is correct at all? It seems to me that h264_ps.c reverses the
order of the constraint_set_flags (constraint_set0_flag (which is the
0x80 bit in a file) will be mapped to 0x01 etc.) and it discards
reserved_zero_2bits. You should instead simply copy the relevant byte
from the SPS.

>>> +    put_bits(&pbc, 8, ps.sps->level_idc); // AVCLevelIndication
>>> +    put_bits(&pbc, 11, 0x7FF);
>>> +
>>> +    p = sps_nal->p_payload + 4;
>>> +    put_bits(&pbc, 5, 1); // numOfSequenceParameterSets
>>> +    put_bits(&pbc, 16, sps_nal->i_payload - 4); // sequenceParameterSetLength
>>
>> I also think that using a PutBitContext is overblown here (most fields
>> don't cross a byte-boundary, the bit-alignment of every field is fixed),
>> in particular if you copy the NALUs eight bits at a time:
> 
> It's readable, close related to the spec, and it counts how many bytes actually used
> automatically. I choose it for these convenience over performance.
> 

I regard a simple memcpy() as more convenient than the following loop:

>>
>>> +    for (int i = 0; i < sps_nal->i_payload - 4; i++)
>>> +        put_bits(&pbc, 8, p[i]);
>>> +
>>> +    p = pps_nal->p_payload + 4;
>>> +    put_bits(&pbc, 8, 1); // numOfPictureParameterSets
>>> +    put_bits(&pbc, 16, pps_nal->i_payload - 4); // pictureParameterSetLength
>>> +    for (int i = 0; i < pps_nal->i_payload - 4; i++)
>>> +        put_bits(&pbc, 8, p[i]);
>>> +
>>> +    profile = ps.sps->profile_idc;
>>> +    if (profile != 66 && profile != 77 && profile != 88) {
>>> +        put_bits(&pbc, 6, 0x3F);
>>> +        put_bits(&pbc, 2, ps.sps->chroma_format_idc);
>>> +        put_bits(&pbc, 5, 0x1F);
>>> +        put_bits(&pbc, 3, ps.sps->bit_depth_luma - 8);
>>> +        put_bits(&pbc, 5, 0x1F);
>>> +        put_bits(&pbc, 3, ps.sps->bit_depth_chroma - 8);
>>> +        put_bits(&pbc, 8, 0);
>>> +    }
>>> +    flush_put_bits(&pbc);
>>> +    avctx->extradata_size = put_bytes_output(&pbc);
>>> +
>>> +    ff_h264_ps_uninit(&ps);
>>> +
>>> +    return 0;
>>> +}
>>> +
>>> +static int set_extradata(AVCodecContext *avctx)
>>> +{
>>> +    X264Context *x4 = avctx->priv_data;
>>> +    x264_nal_t *nal;
>>> +    uint8_t *p;
>>> +    int nnal, s;
>>> +
>>> +    s = x264_encoder_headers(x4->enc, &nal, &nnal);
>>> +    if (s < 0)
>>> +        return AVERROR_EXTERNAL;
>>> +
>>> +    if (!x4->params.b_annexb)
>>> +        return set_avcc_extradata(avctx, nal, nnal);
>>> +
>>> +    avctx->extradata = p = av_mallocz(s + AV_INPUT_BUFFER_PADDING_SIZE);
>>> +    if (!p)
>>> +        return AVERROR(ENOMEM);
>>> +
>>> +    for (int i = 0; i < nnal; i++) {
>>> +        /* Don't put the SEI in extradata. */
>>> +        if (nal[i].i_type == NAL_SEI) {
>>> +            av_log(avctx, AV_LOG_INFO, "%s\n", nal[i].p_payload + 25);
>>> +            x4->sei_size = nal[i].i_payload;
>>> +            x4->sei = av_malloc(x4->sei_size);
>>> +            if (!x4->sei)
>>> +                return AVERROR(ENOMEM);
>>> +            memcpy(x4->sei, nal[i].p_payload, nal[i].i_payload);
>>> +            continue;
>>> +        }
>>> +        memcpy(p, nal[i].p_payload, nal[i].i_payload);
>>> +        p += nal[i].i_payload;
>>> +    }
>>> +    avctx->extradata_size = p - avctx->extradata;
>>> +
>>> +    return 0;
>>> +}
>>> +
>>>  #define PARSE_X264_OPT(name, var)\
>>>      if (x4->var && x264_param_parse(&x4->params, name, x4->var) < 0) {\
>>>          av_log(avctx, AV_LOG_ERROR, "Error parsing option '%s' with value '%s'.\n", name, x4->var);\
>>> @@ -1233,30 +1354,9 @@ FF_ENABLE_DEPRECATION_WARNINGS
>>>          return AVERROR_EXTERNAL;
>>>
>>>      if (avctx->flags & AV_CODEC_FLAG_GLOBAL_HEADER) {
>>> -        x264_nal_t *nal;
>>> -        uint8_t *p;
>>> -        int nnal, s, i;
>>> -
>>> -        s = x264_encoder_headers(x4->enc, &nal, &nnal);
>>> -        avctx->extradata = p = av_mallocz(s + AV_INPUT_BUFFER_PADDING_SIZE);
>>> -        if (!p)
>>> -            return AVERROR(ENOMEM);
>>> -
>>> -        for (i = 0; i < nnal; i++) {
>>> -            /* Don't put the SEI in extradata. */
>>> -            if (nal[i].i_type == NAL_SEI) {
>>> -                av_log(avctx, AV_LOG_INFO, "%s\n", nal[i].p_payload+25);
>>> -                x4->sei_size = nal[i].i_payload;
>>> -                x4->sei      = av_malloc(x4->sei_size);
>>> -                if (!x4->sei)
>>> -                    return AVERROR(ENOMEM);
>>> -                memcpy(x4->sei, nal[i].p_payload, nal[i].i_payload);
>>> -                continue;
>>> -            }
>>> -            memcpy(p, nal[i].p_payload, nal[i].i_payload);
>>> -            p += nal[i].i_payload;
>>> -        }
>>> -        avctx->extradata_size = p - avctx->extradata;
>>> +        ret = set_extradata(avctx);
>>> +        if (ret < 0)
>>> +            return ret;
>>>      }
>>>
>>>      cpb_props = ff_encode_add_cpb_side_data(avctx);
>>

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