[FFmpeg-devel] [PATCH RFC v2 2/3] libavcodec: Add thumbnail output to vaapi_h264 decoder

[Zhou, Zachary]

> -----Original Message-----
> From: ffmpeg-devel [mailto:ffmpeg-devel-bounces at ffmpeg.org] On Behalf Of
> Mark Thompson
> Sent: Wednesday, April 10, 2019 6:27 AM
> To: ffmpeg-devel at ffmpeg.org
> Subject: Re: [FFmpeg-devel] [PATCH RFC v2 2/3] libavcodec: Add thumbnail
> output to vaapi_h264 decoder
> 
> On 08/04/2019 09:53, Zachary Zhou wrote:
> > This is sample code for reference
> >
> > HW support for decode+scaling in a single HW command (VDBOX+SFC).
> > The primary target usage is video analytics, but can be used playback,
> > transcoding, etc.
> >
> > For VAAPI -
> > https://github.com/intel/libva
> > basically, it allows multiple outputs (in different resolutions) using the
> decode context in a single call (you can search for “additional_outputs” in
> va.h).
> >
> > VAAPI sample code -
> > https://github.com/intel/libva-utils/commit/957a269f02b00760b7e807643c
> > 821ee26abc529b
> > ---
> >  libavcodec/avcodec.h       |   8 +++
> >  libavcodec/decode.c        |  16 +++++
> >  libavcodec/options_table.h |   4 ++
> >  libavcodec/vaapi_decode.c  | 122
> > ++++++++++++++++++++++++++++++++++---
> >  libavcodec/vaapi_decode.h  |  30 +++++++++
> >  libavcodec/vaapi_h264.c    |  13 ++++
> >  6 files changed, 185 insertions(+), 8 deletions(-)
> 
> This doesn't fit into the libavcodec API at all.  If it is desirable to have some
> sort of generic multiple-output mechanism in libavcodec, please open a
> separate discussion considering that API specifically.
> 
> For the particular hardware instance you are talking about here, can you
> explain what this mechanism actually helps with?  My understanding was that
> the VDBOX / SFC chaining existed to save a small amount of power in
> restricted playback cases going directly to the display (that is, for maximising
> playback time on battery devices).  Since you are writing everything back to
> memory immediately here anyway, I don't see how it helps compared to just
> doing the scale as a filter operation.
> 
> - Mark

Thanks Mark, I understand it doesn't fit the libavcodec API currently, and I will open a discussion regarding multiple-output mechanism in libavcodec.

you may already know following document -
https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-skl-vol15-sfc.pdf

As my investigation -
- This feature can save some power and memory bandwidth. 
- This feature can save the ffmpeg filters code writing for end user. it also support ARGB output for some AI use case, I think some AI cases may need ARGB input.

-Zach

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