Hi Hans, Nicolas,
Thanks for your comments.
On 2020-06-12 14:41, Hans Verkuil wrote:
Hi Dikshita, Nicolas,
On 11/06/2020 16:22, Nicolas Dufresne wrote:
Le jeudi 11 juin 2020 à 15:55 +0530, Dikshita Agarwal a écrit :
LTR (Long Term Reference) frames are the frames that are encoded
sometime in the past
and stored in the DPB buffer list to be used as reference to
encode
future frames.
One usage of LTR encoding is to reduce error propagation for
video
transmission
in packet lossy networks. For example, encoder may want to
specify
some key frames as
LTR pictures and use them as reference frames for encoding. With
extra protection
selectively on these LTR frames or synchronization with the
receiver
of reception of
the LTR frames during transmission, decoder can receive reference
frames more reliably
than other non-reference frames. As a result, transmission error
can
be effectively
restricted within certain frames rather than propagated to future
frames.
We are introducing below V4l2 Controls for this feature
1. V4L2_CID_MPEG_VIDEO_LTRCOUNT
a. This is used to query or configure the number of LTR
frames.
This is a static control and is controlled by the client.
b. The LTR index varies from 0 to the max LTR-1.
c. If LTR Count is more than max supported LTR count (max
LTR) by
driver, it will be rejected.
d. Auto Marking : If LTR count is non zero,
1) first LTR count frames would be mark as LTR
automatically
after
every IDR frame (inclusive).
2) For multilayer encoding: first LTR count base layer
reference frames starting after
every IDR frame (inclusive) in encoding order would be
marked as LTR frames by the encoder.
3) Auto marking of LTR due to IDR should consider
following
conditions:
1. The frame is not already set to be marked as LTR.
2. The frame is part of the base layer in the
hierarchical layer case.
3. The number of frames currently marked as LTR is
less
than the maximum LTR frame index plus 1.
e. Encoder needs to handle explicit Mark/Use command when
encoder
is still doing "auto" marking
I don't follow this, quite possibly due to lack of experience with
encoders.
I kind of would expect to see two modes: either automatic where
encoders can
mark up to LTR_COUNT frames as long term reference, and userspace
just
sets
LTR_COUNT and doesn't have to do anything else.
Or it is manual mode where userspace explicitly marks long term
reference
frames.
From the proposal above it looks like you can mix auto and manual
modes.
BTW, how do you 'unmark' long term reference frames?
This feature is for stateful encoders, right?
Perhaps we are missing a LONG_TERM_REFERENCE_MODE ? I bet some
encoder
can select by themself long term references and even some encoders
may
not let the user decide.
(not huge han of LTR acronyme, but that could be fine too,
assuming
you
add more _).
Userspace sets LTR count which signifies the number of LTR frames
encoder needs to generate or keep.
The encoder has to build-up its internal buffer reference list (aka
DBP
list or recon buffer list).
In order to achieve that encoder will fill It's LTR (long term
references) list and STR (short term references) list
by auto marking n frames as LTR frames(n is equal to LTR count)
based on
auto-marking dictated by the encoder spec.
The client then can replace those automatically marked frames with
new
frames using V4L2_CID_MPEG_VIDEO_MARKLTRFRAME and can ask
encoder to refer the newly marked frame for encoding the next frame
using V4L2_CID_MPEG_VIDEO_USELTRFRAME.
2. V4L2_CID_MPEG_VIDEO_MARKLTRFRAME :
a. This signals to mark the current frame as LTR frame. It is
a
dynamic control and also provide the LTR index to be used.
b. the LTR index provided by this control should never exceed
the
max LTR-1. Else it will be rejected.
The "current" frame seems a bit loose. Perhaps you wanted to use
buffer
flags ? A bit like what we have to signal TOP/BOTTOM fields in
alternate interlacing.
I was thinking the same thing. Using a control for this doesn't
seem
right.
the client sets this to replace automatically marked frames by the
encoder with a particular frame.
this provides an index that ranges from 0 to LTR count-1 and then
the
particular frame will be marked with that index.
this can be achieved through request by associating this control
with a
specific buffer to make it synchronized.
3. V4L2_CID_MPEG_VIDEO_USELTRFRAME :
a. This specifies the LTR frame(s) to be used for encoding
the
current frame. This is a dynamic control.
b. LTR Use Bitmap : this consists of bits [0, 15]. A total of
N
LSB bits of this field are valid,
where N is the maximum number of LTRs supported. All the
other
bits are invalid and should be rejected.
The LSB corresponds to the LTR index 0. Bit N-1 from the
LSB
corresponds to the LTR index max LTR-1.
How would userspace know this? Esp. with auto marking since
userspace
would have
to predict how auto marking works (if I understand this correctly).
Client sets LTR count which tells about the number of LTR frames
automatically marked by the encoder.
so client can use LTR index (0 to LTR count -1) to ask encoder to
refer
any particular
frame (marked automatically by driver or marked by client with
V4L2_CID_MPEG_VIDEO_MARKLTRFRAME) as a reference to encode the next
frame.
For which HW encoder is this meant?
This is primarily meant for H264 and HEVC.
