[FFmpeg-devel] [PATCH v2 03/14] vvcdec: add sps, pps, sh parser

[Nuo Mi]

Hi James,
thank you for the review.

On Fri, Jul 7, 2023 at 10:28 PM James Almer <jamrial at gmail.com> wrote:

> On 7/7/2023 11:05 AM, Nuo Mi wrote:
> > ---
> >   libavcodec/vvc/Makefile   |    4 +-
> >   libavcodec/vvc/vvc_data.c | 3295 +++++++++++++++++++++++++++++++++++
> >   libavcodec/vvc/vvc_data.h |   69 +
> >   libavcodec/vvc/vvc_ps.c   | 3436 +++++++++++++++++++++++++++++++++++++
> >   libavcodec/vvc/vvc_ps.h   |  828 +++++++++
> >   libavcodec/vvc/vvcdec.h   |    3 +
> >   6 files changed, 7634 insertions(+), 1 deletion(-)
> >   create mode 100644 libavcodec/vvc/vvc_data.c
> >   create mode 100644 libavcodec/vvc/vvc_data.h
> >   create mode 100644 libavcodec/vvc/vvc_ps.c
> >   create mode 100644 libavcodec/vvc/vvc_ps.h
>
> I thought the plan for v2 was to use CBS? Hence you waiting until it was
> pushed.

No. The software decoder depends on frame split, I am waiting for it. :)

> CBS can store derived values and state, and you can always pass it a
> split AU in a loop if needed.
>
We have many problems when if we use CBS as a parameter parser
1. The derived value is only needed by the software decoder. like
VVCPH.lmcs_fwd_lut and VVCPPS.column_width. Storing it in the CBS structure
may have performance implications in on other user cases since they never
use it. for example, Metadata rewriter
2. CBS strictly follows the spec name, making it hard to write a loop like
this
https://github.com/ffvvc/FFmpeg/blob/ae76f43e2e47426544d4dedae1cdc3e16f546458/libavcodec/vvc/vvc_ctu.c#L2136.
We can use a derived variable, but this makes the variable duplicate the
original one.
3. There are so many intermediate names in cbs like abc_minus1. If we use
it directly, we may have need to calculate xxx_minus1 + 1 every time. If we
save abc as a derived value. It will be duplicated again.
4. Using CBS will not save too much code or logic in some cases. Take the
Scaling list as an example, you need a for loop in CBS to read the syntax.
Then you need a duplicated loop in the software decoder to reconstruct the
scaling list.

Again, if Mark agrees and nobody rejects it, I can send out v3 to use it.

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