[FFmpeg-devel] cbs_av1: Make fake OBU size length field a write option

This is an option to modify the behaviour of the writer, not a syntax
field.
---
Tested by hacking av1_metadata.  For example, adding:

av_opt_set_int(ctx->common.output->priv_data, "fixed_obu_size_length", 7, 0);

gets you OBU headers that look like:

[trace_headers @ 0x55706fcb2880] OBU header
[trace_headers @ 0x55706fcb2880] 0           obu_forbidden_bit                                           0 = 0
[trace_headers @ 0x55706fcb2880] 1           obu_type                                                 0100 = 4
[trace_headers @ 0x55706fcb2880] 5           obu_extension_flag                                          0 = 0
[trace_headers @ 0x55706fcb2880] 6           obu_has_size_field                                          1 = 1
[trace_headers @ 0x55706fcb2880] 7           obu_reserved_1bit                                           0 = 0
[trace_headers @ 0x55706fcb2880] 8           obu_size  10101110100010101000000010000000100000001000000000000000 = 1326

It's not obvious that there is any value in exposing this option more generally, though?  It could made a visible option of av1_metadata or others if there is any use-case for it.

Thanks,

- Mark


  libavcodec/cbs_av1.c          | 31 +++++++++++++++++++++----------
  libavcodec/cbs_av1.h          |  5 ++++-
  libavcodec/vaapi_encode_av1.c |  4 +++-
  3 files changed, 28 insertions(+), 12 deletions(-)

On Mon, 2023-09-25 at 14:53 +0100, Mark Thompson wrote:
> This is an option to modify the behaviour of the writer, not a syntax
> field.
> ---
> Tested by hacking av1_metadata.  For example, adding:
> 
> av_opt_set_int(ctx->common.output->priv_data,
> "fixed_obu_size_length", 7, 0);
> 
> gets you OBU headers that look like:
> 
> [trace_headers @ 0x55706fcb2880] OBU header
> [trace_headers @ 0x55706fcb2880]
> 0           obu_forbidden_bit                                        
>    0 = 0
> [trace_headers @ 0x55706fcb2880]
> 1           obu_type                                                 
> 0100 = 4
> [trace_headers @ 0x55706fcb2880]
> 5           obu_extension_flag                                       
>    0 = 0
> [trace_headers @ 0x55706fcb2880]
> 6           obu_has_size_field                                       
>    1 = 1
> [trace_headers @ 0x55706fcb2880]
> 7           obu_reserved_1bit                                        
>    0 = 0
> [trace_headers @ 0x55706fcb2880]
> 8           obu_size  10101110100010101000000010000000100000001000000
> 000000000 = 1326
> 
> It's not obvious that there is any value in exposing this option more
> generally, though?  It could made a visible option of av1_metadata or
> others if there is any use-case for it.
> 
> Thanks,
> 
> - Mark
> 
> 
>   libavcodec/cbs_av1.c          | 31 +++++++++++++++++++++----------
>   libavcodec/cbs_av1.h          |  5 ++++-
>   libavcodec/vaapi_encode_av1.c |  4 +++-
>   3 files changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/libavcodec/cbs_av1.c b/libavcodec/cbs_av1.c
> index 4e687ace79..ed9a7b80d4 100644
> --- a/libavcodec/cbs_av1.c
> +++ b/libavcodec/cbs_av1.c
> @@ -138,19 +138,25 @@ static int
> cbs_av1_read_leb128(CodedBitstreamContext *ctx, GetBitContext *gbc,
>       return 0;
>   }
> 
> -/** Minimum byte length will be used to indicate the len128 of value
> if byte_len is 0. */
>   static int cbs_av1_write_leb128(CodedBitstreamContext *ctx,
> PutBitContext *pbc,
> -                                const char *name, uint64_t value,
> uint8_t byte_len)
> +                                const char *name, uint64_t value,
> int fixed_length)
>   {
>       int len, i;
>       uint8_t byte;
> 
>       CBS_TRACE_WRITE_START();
> 
> -    if (byte_len)
> -        av_assert0(byte_len >= (av_log2(value) + 7) / 7);
> +    len = (av_log2(value) + 7) / 7;
> 
> -    len = byte_len ? byte_len : (av_log2(value) + 7) / 7;
> +    if (fixed_length) {
> +        if (fixed_length < len) {
> +            av_log(ctx->log_ctx, AV_LOG_ERROR, "OBU is too large for
> "
> +                   "fixed length size field (%d > %d).\n",
> +                   len, fixed_length);
> +            return AVERROR(EINVAL);
> +        }
> +        len = fixed_length;
> +    }
> 
>       for (i = 0; i < len; i++) {
>           if (put_bits_left(pbc) < 8)
> @@ -1006,8 +1012,8 @@ static int
> cbs_av1_write_obu(CodedBitstreamContext *ctx,
> 
>       if (obu->header.obu_has_size_field) {
>           pbc_tmp = *pbc;
> -        if (obu->obu_size_byte_len) {
> -            for (int i = 0; i < obu->obu_size_byte_len; i++)
> +        if (priv->fixed_obu_size_length) {
> +            for (int i = 0; i < priv->fixed_obu_size_length; i++)
>                   put_bits(pbc, 8, 0);
>           } else {
>               // Add space for the size field to fill later.
> @@ -1133,7 +1139,8 @@ static int
> cbs_av1_write_obu(CodedBitstreamContext *ctx,
>       end_pos   /= 8;
> 
>       *pbc = pbc_tmp;
> -    err = cbs_av1_write_leb128(ctx, pbc, "obu_size", obu->obu_size,
> obu->obu_size_byte_len);
> +    err = cbs_av1_write_leb128(ctx, pbc, "obu_size", obu->obu_size,
> +                               priv->fixed_obu_size_length);
>       if (err < 0)
>           goto error;
> 
> @@ -1150,10 +1157,12 @@ static int
> cbs_av1_write_obu(CodedBitstreamContext *ctx,
>       }
> 
>       if (obu->obu_size > 0) {
> -        if (!obu->obu_size_byte_len) {
> -            obu->obu_size_byte_len = start_pos - data_pos;
> +        if (!priv->fixed_obu_size_length) {
>               memmove(pbc->buf + data_pos,
>                       pbc->buf + start_pos, header_size);
> +        } else {
> +            // The size was fixed so the following data was
> +            // already written in the correct place.
>           }
>           skip_put_bytes(pbc, header_size);
> 
> @@ -1273,6 +1282,8 @@ static const CodedBitstreamUnitTypeDescriptor
> cbs_av1_unit_types[] = {
>   static const AVOption cbs_av1_options[] = {
>       { "operating_point",  "Set operating point to select layers to
> parse from a scalable bitstream",
>                             OFFSET(operating_point), AV_OPT_TYPE_INT,
> { .i64 = -1 }, -1, AV1_MAX_OPERATING_POINTS - 1, 0 },
> +    { "fixed_obu_size_length", "Set fixed length of the obu_size
> field",
> +      OFFSET(fixed_obu_size_length), AV_OPT_TYPE_INT, { .i64 = 0 },
> 0, 8, 0 },
>       { NULL }
>   };
> 
> diff --git a/libavcodec/cbs_av1.h b/libavcodec/cbs_av1.h
> index a9e2d2284f..7924257164 100644
> --- a/libavcodec/cbs_av1.h
> +++ b/libavcodec/cbs_av1.h
> @@ -401,7 +401,6 @@ typedef struct AV1RawOBU {
>       AV1RawOBUHeader header;
> 
>       size_t obu_size;
> -    uint8_t obu_size_byte_len;
> 
>       union {
>           AV1RawSequenceHeader sequence_header;
> @@ -468,6 +467,10 @@ typedef struct CodedBitstreamAV1Context {
> 
>       // AVOptions
>       int operating_point;
> +    // When writing, fix the length in bytes of the obu_size field.
> +    // Writing will fail with an error if an OBU larger than can be
> +    // represented by the fixed size is encountered.
> +    int fixed_obu_size_length;
>   } CodedBitstreamAV1Context;
> 
> 
> diff --git a/libavcodec/vaapi_encode_av1.c
> b/libavcodec/vaapi_encode_av1.c
> index 3ff1c47b53..861bf4a13b 100644
> --- a/libavcodec/vaapi_encode_av1.c
> +++ b/libavcodec/vaapi_encode_av1.c
> @@ -133,6 +133,9 @@ static av_cold int
> vaapi_encode_av1_configure(AVCodecContext *avctx)
>       priv->cbc->trace_context = ctx;
>       priv->cbc->trace_write_callback =
> vaapi_encode_av1_trace_write_log;
> 
> +    av_opt_set_int(priv->cbc->priv_data, "fixed_obu_size_length",
> +                   priv->attr_ext2.bits.obu_size_bytes_minus1 + 1,
> 0);
> +

This should be put after querying priv.attr_ext2 in
vaapi_encode_av1_init().

Thanks
Fei  

>       if (ctx->rc_mode->quality) {
>           priv->q_idx_p = av_clip(ctx->rc_quality, 0, AV1_MAX_QUANT);
>           if (fabs(avctx->i_quant_factor) > 0.0)
> @@ -634,7 +637,6 @@ static int
> vaapi_encode_av1_init_picture_params(AVCodecContext *avctx,
>           }
>       }
> 
> -    fh_obu->obu_size_byte_len = priv-
> >attr_ext2.bits.obu_size_bytes_minus1 + 1;
>       ret = vaapi_encode_av1_add_obu(avctx, obu,
> AV1_OBU_FRAME_HEADER, &priv->fh);
>       if (ret < 0)
>           goto end;

Mark Thompson:
> This is an option to modify the behaviour of the writer, not a syntax
> field.
> ---
> Tested by hacking av1_metadata.  For example, adding:
> 
> av_opt_set_int(ctx->common.output->priv_data, "fixed_obu_size_length",
> 7, 0);
> 
> gets you OBU headers that look like:
> 
> [trace_headers @ 0x55706fcb2880] OBU header
> [trace_headers @ 0x55706fcb2880] 0          
> obu_forbidden_bit                                           0 = 0
> [trace_headers @ 0x55706fcb2880] 1          
> obu_type                                                 0100 = 4
> [trace_headers @ 0x55706fcb2880] 5          
> obu_extension_flag                                          0 = 0
> [trace_headers @ 0x55706fcb2880] 6          
> obu_has_size_field                                          1 = 1
> [trace_headers @ 0x55706fcb2880] 7          
> obu_reserved_1bit                                           0 = 0
> [trace_headers @ 0x55706fcb2880] 8           obu_size 
> 10101110100010101000000010000000100000001000000000000000 = 1326
> 
> It's not obvious that there is any value in exposing this option more
> generally, though?  It could made a visible option of av1_metadata or
> others if there is any use-case for it.
> 
> Thanks,
> 
> - Mark
> 
> 
>  libavcodec/cbs_av1.c          | 31 +++++++++++++++++++++----------
>  libavcodec/cbs_av1.h          |  5 ++++-
>  libavcodec/vaapi_encode_av1.c |  4 +++-
>  3 files changed, 28 insertions(+), 12 deletions(-)

What is the advantage of this? Why would people want to waste space?

- Andreas

On 26/09/2023 10:19, Andreas Rheinhardt wrote:
> Mark Thompson:
>> This is an option to modify the behaviour of the writer, not a syntax
>> field.
>> ---
>> Tested by hacking av1_metadata.  For example, adding:
>>
>> av_opt_set_int(ctx->common.output->priv_data, "fixed_obu_size_length",
>> 7, 0);
>>
>> gets you OBU headers that look like:
>>
>> [trace_headers @ 0x55706fcb2880] OBU header
>> [trace_headers @ 0x55706fcb2880] 0
>> obu_forbidden_bit                                           0 = 0
>> [trace_headers @ 0x55706fcb2880] 1
>> obu_type                                                 0100 = 4
>> [trace_headers @ 0x55706fcb2880] 5
>> obu_extension_flag                                          0 = 0
>> [trace_headers @ 0x55706fcb2880] 6
>> obu_has_size_field                                          1 = 1
>> [trace_headers @ 0x55706fcb2880] 7
>> obu_reserved_1bit                                           0 = 0
>> [trace_headers @ 0x55706fcb2880] 8           obu_size
>> 10101110100010101000000010000000100000001000000000000000 = 1326
>>
>> It's not obvious that there is any value in exposing this option more
>> generally, though?  It could made a visible option of av1_metadata or
>> others if there is any use-case for it.
>>
>> Thanks,
>>
>> - Mark
>>
>>
>>   libavcodec/cbs_av1.c          | 31 +++++++++++++++++++++----------
>>   libavcodec/cbs_av1.h          |  5 ++++-
>>   libavcodec/vaapi_encode_av1.c |  4 +++-
>>   3 files changed, 28 insertions(+), 12 deletions(-)
> 
> What is the advantage of this? Why would people want to waste space?

Because they want to be able to write a different size into the field later without realigning all the following bytes.

VAAPI seems to require this, hence the previous code which added a fake syntax element field to the OBU structure containing the size to be written with, which this patch is replacing with an external option.

Thanks,

- Mark