fbui | Framebuffer-based graphical environment |
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libfbui/MPEG/SPATIAL.DOC
1 The following changes have been made to debug spatial scalability:
2
3 gethdr.c
4 --------
5
6 Temporal_reference is used to compute the frame number of each frame,
7 named true_framenum. The periodic reset at each GOP header as well as
8 the wrap of temporal_reference at 1024 cause a base value
9 temp_ref_base to be incremented accordingly.
10
11 spatscal.c
12 ----------
13
14 getspatref()
15
16 A potential problem: Variable char fname[32] was dimensioned
17 statically and too small.
18
19 true_framenum is used instead of lower_layer_temporal_reference to
20 determine the lower layer frame to be read for spatial prediction.
21
22 The verification of lower_layer_temporal_reference is not possible
23 since the temporal reference values that have been encoded into the
24 base layer bitstream are not available to the enhancement layer
25 decoder.
26
27 Since there is no decoder timing information available, the rules on
28 which frames can legally be used as spatial prediction frames cannot
29 be checked.
30
31 Lower layer frames are read field-wise or frame-wise, depending on the
32 lower_layer_progressive_frame flag. Consistency between layers is
33 checked since the file format for frame and field pictures differs.
34
35 Note that the base layer decoder must not use the -f option to enforce
36 frame-wise storage.
37
38 Note further that only yuv image format (option -o0) is supported as
39 input format.
40
41 spatpred()
42
43 The code for the various combinations of llprog_frame, llfieldsel and
44 prog_frame has been completed and verified with the tceh_conf23
45 bitstream that uses all permissive combinations.
46
47
48 getpic.c
49 --------
50
51 A small bug when storing an I- or P-frame: The prog_frame flag that
52 the decoder knows when storing the oldrefframe belongs to the current
53 refframe. Therefore the old value of the flag needs to be memorized.
54
55
56 store.c
57 -------
58
59 A potential problem: the filename variables char outname[32],
60 tmpname[32] are statically dimensioned and quite small.
61
62
63 The concept of time in this video decoder software
64 --------------------------------------------------
65
66 When decoding a non-scalable bitstream, the frame number (i.e.
67 temporal position) of the current I- or P-frame can be derived
68 implicitly from the number of preceding B-frames after they have been
69 decoded. Therefore the temporal_reference entry in the picture header
70 is somewhat redundant and does not necessarily have to be evaluated in
71 the decoding process.
72
73 Decoding of the enhancement layer of a spatial scalable hierarchy,
74 however, requires to know the temporal position of each frame at the
75 instant when it is decoded, since data from a lower layer reference
76 frame has to be incorporated.
77
78 In the architecture of this video-only decoder decoding of a spatial
79 scalable hierarchy of bitstreams is done by calling mpeg2decode once
80 for the base layer bitstream and a second time for the enhancement
81 layer bitstream, indicating where the decoded base layer frames can be
82 found (option -s<filename>).
83
84 Here the concept of time is only present in the form of frame numbers.
85 Therefore spatial scalable bitstream hierarchies can only be handled
86 under the assumption that base and enhancement layer bitstreams are
87 decoded to image sequences where corresponding images of both layers
88 have identical frame numbers.
89
90 More specifically this means that base and enhancement layer
91 bitstreams must contain video with the same frame rate. Furthermore
92 only the temporally coincident frame of the base layer can be accessed
93 for spatial prediction by the enhancement layer decoder, since it is
94 not possible to resolve unambiguously the lower_layer_temporal_reference
95 which is meant to further specify the lower layer reference frame.
96
97 ======================== SPATIAL.DOC ========================0
98
99 Decoding a spatial scalable hierarchy of bitstreams
100 ---------------------------------------------------
101
102 With this video-only decoder decoding of a spatial scalable hierarchy
103 of bitstreams is done by calling mpeg2decode once for the base layer
104 bitstream and a second time for the enhancement layer bitstream,
105 indicating where the decoded base layer frames can be found
106 (using option -s and supplying <spatial base filename>).
107
108 mpeg2decode -r -o0 base.mpg base%d%c
109 mpeg2decode -r -o0 -f -s base%d%c enh.mpg enh%d
110
111 Note that the base layer decoder must not use the -f option to enforce
112 frame-wise storage.
113
114 Note further that only yuv image format (option -o0) is supported as
115 input format.
116
117
118 Timing / layer synchronisation in this video decoder software
119 -------------------------------------------------------------
120
121 When decoding a non-scalable bitstream, the frame number (i.e.
122 temporal position) of the current I- or P-frame can be derived
123 implicitly from the number of preceding B-frames after they have been
124 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
128 Decoding of the enhancement layer of a spatial scalable hierarchy,
129 however, requires to know the temporal position of each frame at the
130 instant when it is decoded, since data from a lower layer reference
131 frame has to be incorporated.
132
133 The concept of time is only present in the form of frame numbers.
134 Therefore spatial scalable bitstream hierarchies can only be handled
135 under the assumption that base and enhancement layer bitstreams are
136 decoded to image sequences where corresponding images of both layers
137 have identical frame numbers.
138
139 More specifically this means that base and enhancement layer
140 bitstreams must contain video with the same frame rate. Furthermore
141 only the temporally coincident frame of the base layer can be accessed
142 for spatial prediction by the enhancement layer decoder, since it is
143 not possible to resolve unambiguously the lower_layer_temporal_reference
144 which is meant to further specify the lower layer reference frame.
145
146 Lower layer frames are read field-wise or frame-wise, depending on the
147 lower_layer_progressive_frame flag. Consistency between layers in this
148 respect is checked since the file format for frame and field pictures
149 differs.
150
151
152
153
154
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