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[/] [mpeg2fpga/] [trunk/] [tools/] [mpeg2dec/] [SPATIAL.DOC] - Blame information for rev 2

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1 2 kdv
The following changes have been made to debug spatial scalability:
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gethdr.c
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--------
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Temporal_reference is used to compute the frame number of each frame,
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named true_framenum.  The periodic reset at each GOP header as well as
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the wrap of temporal_reference at 1024 cause a base value
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temp_ref_base to be incremented accordingly.
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spatscal.c
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----------
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getspatref()
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A potential problem: Variable char fname[32] was dimensioned
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statically and too small.
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true_framenum is used instead of lower_layer_temporal_reference to
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determine the lower layer frame to be read for spatial prediction.
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The verification of lower_layer_temporal_reference is not possible
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since the temporal reference values that have been encoded into the
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base layer bitstream are not available to the enhancement layer
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decoder.
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Since there is no decoder timing information available, the rules on
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which frames can legally be used as spatial prediction frames cannot
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be checked.
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Lower layer frames are read field-wise or frame-wise, depending on the
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lower_layer_progressive_frame flag. Consistency between layers is
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checked since the file format for frame and field pictures differs.
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Note that the base layer decoder must not use the -f option to enforce
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frame-wise storage.
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Note further that only yuv image format (option -o0) is supported as
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input format.
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spatpred()
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The code for the various combinations of llprog_frame, llfieldsel and
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prog_frame has been completed and verified with the tceh_conf23
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bitstream that uses all permissive combinations.
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getpic.c
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--------
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A small bug when storing an I- or P-frame: The prog_frame flag that
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the decoder knows when storing the oldrefframe belongs to the current
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refframe. Therefore the old value of the flag needs to be memorized.
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store.c
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-------
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A potential problem: the filename variables char outname[32],
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tmpname[32] are statically dimensioned and quite small.
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The concept of time in this video decoder software
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--------------------------------------------------
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When decoding a non-scalable bitstream, the frame number (i.e.
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temporal position) of the current I- or P-frame can be derived
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implicitly from the number of preceding B-frames after they have been
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decoded. Therefore the temporal_reference entry in the picture header
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is somewhat redundant and does not necessarily have to be evaluated in
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the decoding process.
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Decoding of the enhancement layer of a spatial scalable hierarchy,
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however, requires to know the temporal position of each frame at the
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instant when it is decoded, since data from a lower layer reference
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frame has to be incorporated.
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In the architecture of this video-only decoder decoding of a spatial
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scalable hierarchy of bitstreams is done by calling mpeg2decode once
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for the base layer bitstream and a second time for the enhancement
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layer bitstream, indicating where the decoded base layer frames can be
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found (option -s).
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Here the concept of time is only present in the form of frame numbers.
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Therefore spatial scalable bitstream hierarchies can only be handled
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under the assumption that base and enhancement layer bitstreams are
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decoded to image sequences where corresponding images of both layers
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have identical frame numbers.
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More specifically this means that base and enhancement layer
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bitstreams must contain video with the same frame rate. Furthermore
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only the temporally coincident frame of the base layer can be accessed
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for spatial prediction by the enhancement layer decoder, since it is
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not possible to resolve unambiguously the lower_layer_temporal_reference
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which is meant to further specify the lower layer reference frame.
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======================== SPATIAL.DOC ========================0
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Decoding a spatial scalable hierarchy of bitstreams
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---------------------------------------------------
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With this video-only decoder decoding of a spatial scalable hierarchy
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of bitstreams is done by calling mpeg2decode once for the base layer
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bitstream and a second time for the enhancement layer bitstream,
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indicating where the decoded base layer frames can be found
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(using option -s and supplying ).
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mpeg2decode -r -o0 base.mpg base%d%c
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mpeg2decode -r -o0 -f -s base%d%c enh.mpg enh%d
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Note that the base layer decoder must not use the -f option to enforce
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frame-wise storage.
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Note further that only yuv image format (option -o0) is supported as
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input format.
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Timing / layer synchronisation in this video decoder software
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-------------------------------------------------------------
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When decoding a non-scalable bitstream, the frame number (i.e.
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temporal position) of the current I- or P-frame can be derived
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implicitly from the number of preceding B-frames after they have been
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decoded. Therefore the temporal_reference entry in the picture header
125
is somewhat redundant and does not necessarily have to be evaluated in
126
the decoding process.
127
 
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Decoding of the enhancement layer of a spatial scalable hierarchy,
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however, requires to know the temporal position of each frame at the
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instant when it is decoded, since data from a lower layer reference
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frame has to be incorporated.
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The concept of time is only present in the form of frame numbers.
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Therefore spatial scalable bitstream hierarchies can only be handled
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under the assumption that base and enhancement layer bitstreams are
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decoded to image sequences where corresponding images of both layers
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have identical frame numbers.
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More specifically this means that base and enhancement layer
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bitstreams must contain video with the same frame rate. Furthermore
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only the temporally coincident frame of the base layer can be accessed
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for spatial prediction by the enhancement layer decoder, since it is
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not possible to resolve unambiguously the lower_layer_temporal_reference
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which is meant to further specify the lower layer reference frame.
145
 
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Lower layer frames are read field-wise or frame-wise, depending on the
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lower_layer_progressive_frame flag. Consistency between layers in this
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respect is checked since the file format for frame and field pictures
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differs.
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