On 04/02/2019 14:41, Michael Dirks wrote:
> On 04.02.2019 11:05, Mark Thompson wrote:
>> Can you explain what this "skip frame" actually does in the encoder? The
>> concept does not exist in H.264 or H.265, as far as I know.
>
> I believe this has to do with the pic_struct flag which has a value of 7 for
> frame doubling and 8 for frame tripling, see page 345 (PDF page 367) Table
> D-1 Interpretation of pic_struct in Rec. ITU-T H.264 (04/2017). However I do
> not work for AMD (and their encoder is closed source and a piece of
> hardware), so I can't confirm that this is actually the behavior, nor do I
> have any tools that can read and show all H.264 headers. An alternative would
> be that AMDs encoder is instead choosing to emit a frame that has maximum
> compression and references the previous frame for all data, thus causing a
> copy of it.
You can see this with the trace_headers bitstream filter. (Try "-c:v h264_amf
-options... -bsf:v trace_headers -f null -", or it can be used with streamcopy
to examine an existing file.)
>>> + case AV_PICTURE_TYPE_I:
>>> + AMF_ASSIGN_PROPERTY_INT64(res, surface,
>>> AMF_VIDEO_ENCODER_FORCE_PICTURE_TYPE, AMF_VIDEO_ENCODER_PICTURE_TYPE_I);
>> Consider whether you need to set IDR here rather than I, and maybe add a
>> comment explaining the choice? The normal use of this is to indicate that
>> an IRAP should be generated, not just a single intra picture. (Compare
>> libx264, which has an additional flag controlling whether the forced picture
>> is IDR or I, but I believe it is still always an IRAP there.)
>>> + // Keyframe overrides previous assignment.
>>> + if (frame->key_frame) {
>> This flag shouldn't be read by encoders. (I believe it is set by the
>> decoder and never cleared, so with this test you will have surprising
>> behaviour where the output stream gets key frames everywhere that the input
>> stream had them, in addition to those dictated by its own GOP structure.)
> I went by the documentation in the individual header files, which does not
> actually claim key_frame to be a decoder only field (libavutil/frame.h):
>
>> /**
>> * 1 -> keyframe, 0-> not
>> */
>> int key_frame;
>
> Therefore this patch uses the field exactly as it is documented in the
> frame.h file, and also treats AV_PICTURE_TYPE_I as a request for an Intra
> Picture instead of an IDR Picture.
Well yes, but that's not actually going to work - try it with a set of JPEGs as
input and you'll observe the problem. (Note that no other encoder uses this
field on input, though some do use it internally.)
- Mark
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