[FFmpeg-devel] [PATCH 2/2] avcodec/h264_slice: Never call get_format() for just checking if the format matches

[Hendrik Leppkes]

On Sun, Oct 11, 2015 at 10:43 PM, wm4 <nfxjfg at googlemail.com> wrote:
> On Sun, 11 Oct 2015 21:55:12 +0200
> Michael Niedermayer <michael at niedermayer.cc> wrote:
>
>> On Sun, Oct 11, 2015 at 09:39:32PM +0200, wm4 wrote:
>> > On Sun, 11 Oct 2015 21:16:27 +0200
>> > Michael Niedermayer <michaelni at gmx.at> wrote:
>> >
>> > > From: Michael Niedermayer <michael at niedermayer.cc>
>> > >
>> > > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>> > > ---
>> > >  libavcodec/h264_slice.c |    4 ++++
>> > >  1 file changed, 4 insertions(+)
>> > >
>> > > diff --git a/libavcodec/h264_slice.c b/libavcodec/h264_slice.c
>> > > index cce97d9..daa3737 100644
>> > > --- a/libavcodec/h264_slice.c
>> > > +++ b/libavcodec/h264_slice.c
>> > > @@ -985,6 +985,10 @@ static enum AVPixelFormat get_pixel_format(H264Context *h, int force_callback)
>> > >      for (i=0; choices[i] != AV_PIX_FMT_NONE; i++)
>> > >          if (non_j_pixfmt(choices[i]) == non_j_pixfmt(h->avctx->pix_fmt) && !force_callback)
>> > >              return choices[i];
>> > > +
>> > > +    if (!force_callback)
>> > > +        return AV_PIX_FMT_NONE;
>> > > +
>> > >      return ff_thread_get_format(h->avctx, choices);
>> > >  }
>> > >
>> >
>> > So if I can see this right, the whole purpose of this is to check
>> > whether the h264 parameters map to a lavc pixfmt, and bail out or
>> > reinitialize if it doesn't. Why can't this be delayed later? What
>> > actually needs it sooner than the first "real" get_format? For dealing
>>
>> > with paramater changes, why can't it check the raw parameters instead?
>>
>> The raw parameters are checked as well but iam not sure these checks
>> are complete, they are more complex.
>>
>
> I've checked again. 3 parameters influence the pixfmt:
> bit_depth_luma, chroma_format_idc, and colorspace. Maybe add
> color_range because of the J formats.  The first two are already
> checked (lazily, if the SPS changes). A colorspace change also triggers
> reinit, although the check for it is buried somewhat deeper in the
> code. (Also I see a potential bug there: are colorspace and other
> fields not reset correctly if the SPS changes, and the new SPS doesn't
> have these fields set anymore?) A changing color_range does not force
> reinit; this must be fixed (although I'd prefer dropping the J hack
> completely).
>
> So do you agree that checks for colorspace and color_range should be
> added, and that they should trigger reinit, and that this combined with
> my patch would fix all the potential issues the patch could introduce?

 Color Range and Color Space should not trigger a re-init, its
pointless and may disrupt playback if a HWAccel re-inits.
The in-memory representaiton, and as such the surface format, do not
change when only these two change, so re-initing makes no sense to me.
I have specifically hacked my local fork to avoid this because it can
disrupt playback.

>
> Note that because of hwaccels, get_format should always be triggered
> if the SPS changes in any way, because some hwaccels might rely on the
> current SPS information.
>
> I'm also questioning why there are so many "clever" fine grained reinit
> checks. Wouldn't it be better to fully reinit every time there is a new
> SPS, and the new SPS is different than the previous SPS on the byte
> level? (Don't worry, I'm only speaking hypothetically.)
>
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