[FFmpeg-devel] [PATCH] avcodec: add AVCODEC_REQUIRED_INPUT_BUFFER_PADDING_SIZE, split FF_INPUT_BUFFER_PADDING_SIZE

[Michael Niedermayer]

On Fri, Jun 13, 2014 at 02:49:01AM +0200, wm4 wrote:
> On Thu, 12 Jun 2014 23:06:13 +0200
> Michael Niedermayer <michaelni at gmx.at> wrote:
> 
> > TODO bump minor version, update APIChanges
> > 
> > Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
> > ---
> >  libavcodec/avcodec.h |   11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> > index 68b1f26..ca66f3c 100644
> > --- a/libavcodec/avcodec.h
> > +++ b/libavcodec/avcodec.h
> > @@ -608,13 +608,22 @@ typedef struct AVCodecDescriptor {
> >  
> >  /**
> >   * @ingroup lavc_decoding
> > + * The amount of padding that is allocated at the end of the input bitstream by
> > + * libavformat and other libs, this affects all buffers which may reasonably be
> > + * directly used as avcodec input buffers.
> > + * The value of this symbol may be increased without major version bump.
> > + */
> > +#define FF_INPUT_BUFFER_PADDING_SIZE 32
> 
> Why is there something about libavformat and "other libs" in libavcodec?

it has to be somewhere, and it is about buffers that are input into
libavcodec.
also existing API requires it to be included when libavcodec/avcodec.h
is


> 
> The name of this define is more informative than these 4 lines of help
> text.

what do you suggest instead ?


> 
> > +
> > +/**
> > + * @ingroup lavc_decoding
> >   * Required number of additionally allocated bytes at the end of the input bitstream for decoding.
> >   * This is mainly needed because some optimized bitstream readers read
> >   * 32 or 64 bit at once and could read over the end.<br>
> >   * Note: If the first 23 bits of the additional bytes are not 0, then damaged
> >   * MPEG bitstreams could cause overread and segfault.
> >   */
> > -#define FF_INPUT_BUFFER_PADDING_SIZE 32
> > +#define AVCODEC_REQUIRED_INPUT_BUFFER_PADDING_SIZE 16
> 
> I don't understand the difference between these two.

Each lib has its own requirement for padding

to be able to pass through input buffers from libavcodec over
output AVFrames into libavfilter, it is neccessary that the input
to libavcodec has been padded sufficiently for both libavfilter and
libavcodec. This case can occurs with raw video for example

AVCODEC_REQUIRED_INPUT_BUFFER_PADDING_SIZE represents the amount of
padding that is required for input to libavcodec

FF_INPUT_BUFFER_PADDING_SIZE represents the amount that we pad.
Using this we can pass the output from libavcodec to other libs like
libavfilter in the case where (due to rawvideo) the input has been
passed through

I hope above is understandable

[...]

PS: iam not sure you even need 16byte padding for libavcodec, possibly
less will be fine

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Let us carefully observe those good qualities wherein our enemies excel us
and endeavor to excel them, by avoiding what is faulty, and imitating what
is excellent in them. -- Plutarch
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