spatial.doc

VC下MPEG2的视频编解码源码

The following changes have been made to debug spatial scalability:

gethdr.c
--------

Temporal_reference is used to compute the frame number of each frame,
named true_framenum.  The periodic reset at each GOP header as well as
the wrap of temporal_reference at 1024 cause a base value
temp_ref_base to be incremented accordingly.

spatscal.c
----------

getspatref()

A potential problem: Variable char fname[32] was dimensioned
statically and too small.

true_framenum is used instead of lower_layer_temporal_reference to
determine the lower layer frame to be read for spatial prediction.

The verification of lower_layer_temporal_reference is not possible
since the temporal reference values that have been encoded into the
base layer bitstream are not available to the enhancement layer
decoder.

Since there is no decoder timing information available, the rules on
which frames can legally be used as spatial prediction frames cannot
be checked.

Lower layer frames are read field-wise or frame-wise, depending on the
lower_layer_progressive_frame flag. Consistency between layers is
checked since the file format for frame and field pictures differs.

Note that the base layer decoder must not use the -f option to enforce
frame-wise storage.

Note further that only yuv image format (option -o0) is supported as
input format.

spatpred()

The code for the various combinations of llprog_frame, llfieldsel and
prog_frame has been completed and verified with the tceh_conf23
bitstream that uses all permissive combinations.


getpic.c
--------

A small bug when storing an I- or P-frame: The prog_frame flag that
the decoder knows when storing the oldrefframe belongs to the current
refframe. Therefore the old value of the flag needs to be memorized.


store.c
-------

A potential problem: the filename variables char outname[32],
tmpname[32] are statically dimensioned and quite small.


The concept of time in this video decoder software
--------------------------------------------------

When decoding a non-scalable bitstream, the frame number (i.e.
temporal position) of the current I- or P-frame can be derived
implicitly from the number of preceding B-frames after they have been
decoded. Therefore the temporal_reference entry in the picture header
is somewhat redundant and does not necessarily have to be evaluated in
the decoding process.

Decoding of the enhancement layer of a spatial scalable hierarchy,
however, requires to know the temporal position of each frame at the
instant when it is decoded, since data from a lower layer reference
frame has to be incorporated.

In the architecture of this video-only decoder decoding of a spatial
scalable hierarchy of bitstreams is done by calling mpeg2decode once
for the base layer bitstream and a second time for the enhancement
layer bitstream, indicating where the decoded base layer frames can be
found (option -s<filename>).

Here the concept of time is only present in the form of frame numbers.
Therefore spatial scalable bitstream hierarchies can only be handled
under the assumption that base and enhancement layer bitstreams are
decoded to image sequences where corresponding images of both layers
have identical frame numbers.

More specifically this means that base and enhancement layer
bitstreams must contain video with the same frame rate. Furthermore
only the temporally coincident frame of the base layer can be accessed
for spatial prediction by the enhancement layer decoder, since it is
not possible to resolve unambiguously the lower_layer_temporal_reference 
which is meant to further specify the lower layer reference frame.

======================== SPATIAL.DOC ========================0

Decoding a spatial scalable hierarchy of bitstreams
---------------------------------------------------

With this video-only decoder decoding of a spatial scalable hierarchy
of bitstreams is done by calling mpeg2decode once for the base layer
bitstream and a second time for the enhancement layer bitstream,
indicating where the decoded base layer frames can be found
(using option -s and supplying <spatial base filename>).

mpeg2decode -r -o0 base.mpg base%d%c
mpeg2decode -r -o0 -f -s base%d%c enh.mpg enh%d

Note that the base layer decoder must not use the -f option to enforce
frame-wise storage.

Note further that only yuv image format (option -o0) is supported as
input format.


Timing / layer synchronisation in this video decoder software
-------------------------------------------------------------

When decoding a non-scalable bitstream, the frame number (i.e.
temporal position) of the current I- or P-frame can be derived
implicitly from the number of preceding B-frames after they have been
decoded. Therefore the temporal_reference entry in the picture header
is somewhat redundant and does not necessarily have to be evaluated in
the decoding process.

Decoding of the enhancement layer of a spatial scalable hierarchy,
however, requires to know the temporal position of each frame at the
instant when it is decoded, since data from a lower layer reference
frame has to be incorporated.

The concept of time is only present in the form of frame numbers.
Therefore spatial scalable bitstream hierarchies can only be handled
under the assumption that base and enhancement layer bitstreams are
decoded to image sequences where corresponding images of both layers
have identical frame numbers.

More specifically this means that base and enhancement layer
bitstreams must contain video with the same frame rate. Furthermore
only the temporally coincident frame of the base layer can be accessed
for spatial prediction by the enhancement layer decoder, since it is
not possible to resolve unambiguously the lower_layer_temporal_reference
which is meant to further specify the lower layer reference frame.

Lower layer frames are read field-wise or frame-wise, depending on the
lower_layer_progressive_frame flag. Consistency between layers in this
respect is checked since the file format for frame and field pictures
differs.