[FFmpeg-devel] [PATCH 1/3][RFC] avutil/frame: add AV_FRAME_FLAG_DISPOSABLE

[Michael Niedermayer]

On Tue, Aug 27, 2019 at 09:12:53AM +0200, Marton Balint wrote:
> 
> 
> On Mon, 26 Aug 2019, James Almer wrote:
> 
> >Used to signal frames that can be safely discarded without losing
> >any picture data, side data, or metadata other than timing info.
> >
> >Signed-off-by: James Almer <jamrial at gmail.com>
> >---
> >This implements the "disposable frame" solution to allow library
> >users to drop duplicate frames before further processing if desired,
> >instead of forcing decoders to output vfr content when cfr is coded
> >in the bitstream.
> >
> >doc/APIchanges      | 3 +++
> >libavutil/frame.h   | 5 +++++
> >libavutil/version.h | 2 +-
> >3 files changed, 9 insertions(+), 1 deletion(-)
> >
> >diff --git a/doc/APIchanges b/doc/APIchanges
> >index 682b67aa25..b28d702bae 100644
> >--- a/doc/APIchanges
> >+++ b/doc/APIchanges
> >@@ -15,6 +15,9 @@ libavutil:     2017-10-21
> >
> >API changes, most recent first:
> >
> >+2019-08-xx - xxxxxxxxxx - lavu 58.34.100 - avframe.h
> >+  Add AV_FRAME_FLAG_DISPOSABLE
> >+
> >2019-08-xx - xxxxxxxxxx - lavf 58.31.101 - avio.h
> >  4K limit removed from avio_printf.
> >
> >diff --git a/libavutil/frame.h b/libavutil/frame.h
> >index 5d3231e7bb..e1bf8795d2 100644
> >--- a/libavutil/frame.h
> >+++ b/libavutil/frame.h
> >@@ -522,6 +522,11 @@ typedef struct AVFrame {
> > * A flag to mark the frames which need to be decoded, but shouldn't be output.
> > */
> >#define AV_FRAME_FLAG_DISCARD   (1 << 2)
> >+/**
> >+ * A flag to indicate frames that can be discarded by the encoder. I.e. frames
> >+ * that are an exact duplicate of the previous one.
> >+ */
> >+#define AV_FRAME_FLAG_DISPOSABLE    (1 << 3)
> 
> Such a flag has the problem that it is not really an attribute of a frame in
> itself, but of a frame of the frame chain. That can cause various issues: If
> the first frame is dropped, the next frame no longer becomes disposable.
> 

> For the issue we are trying to solve I feel that it is better if we create a
> video filter which checks the buffers (and side data and crop parameters) if
> they are the same and drops the frame. It also allows more granular control
> (or setting parameters, like making sure you get a frame every 1 second or
> so, which is common requirement even for VFR), plus it can work for existing
> cases without this flag implemented.

One disadvantage of this with the current ref counting system is that in codecs
which do in place changes in a buffer. 
Keeping a reference to a frame means the decoder must copy the frame to get a
new writable one to do changes in. Not keeping a ref avoids this.
A method which detects duplicates without keeping a +1 reference on the 
previous frame would be better in this respect ...

Thanks


[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The misfortune of the wise is better than the prosperity of the fool.
-- Epicurus
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