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

[Mark Thompson]

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/957a269f02b00760b7e807643c821ee26abc529b
> ---
>  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