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1085 | pj | 1 | The following changes have been made to debug spatial scalability: |
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3 | gethdr.c |
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4 | -------- |
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5 | |||
6 | Temporal_reference is used to compute the frame number of each frame, |
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7 | named true_framenum. The periodic reset at each GOP header as well as |
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8 | the wrap of temporal_reference at 1024 cause a base value |
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9 | temp_ref_base to be incremented accordingly. |
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10 | |||
11 | spatscal.c |
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12 | ---------- |
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13 | |||
14 | getspatref() |
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15 | |||
16 | A potential problem: Variable char fname[32] was dimensioned |
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17 | statically and too small. |
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18 | |||
19 | true_framenum is used instead of lower_layer_temporal_reference to |
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20 | determine the lower layer frame to be read for spatial prediction. |
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21 | |||
22 | The verification of lower_layer_temporal_reference is not possible |
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23 | since the temporal reference values that have been encoded into the |
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24 | base layer bitstream are not available to the enhancement layer |
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25 | decoder. |
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26 | |||
27 | Since there is no decoder timing information available, the rules on |
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28 | which frames can legally be used as spatial prediction frames cannot |
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29 | be checked. |
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30 | |||
31 | Lower layer frames are read field-wise or frame-wise, depending on the |
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32 | lower_layer_progressive_frame flag. Consistency between layers is |
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33 | checked since the file format for frame and field pictures differs. |
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34 | |||
35 | Note that the base layer decoder must not use the -f option to enforce |
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36 | frame-wise storage. |
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37 | |||
38 | Note further that only yuv image format (option -o0) is supported as |
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39 | input format. |
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40 | |||
41 | spatpred() |
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42 | |||
43 | The code for the various combinations of llprog_frame, llfieldsel and |
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44 | prog_frame has been completed and verified with the tceh_conf23 |
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45 | bitstream that uses all permissive combinations. |
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46 | |||
47 | |||
48 | getpic.c |
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49 | -------- |
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50 | |||
51 | A small bug when storing an I- or P-frame: The prog_frame flag that |
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52 | the decoder knows when storing the oldrefframe belongs to the current |
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53 | refframe. Therefore the old value of the flag needs to be memorized. |
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54 | |||
55 | |||
56 | store.c |
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57 | ------- |
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58 | |||
59 | A potential problem: the filename variables char outname[32], |
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60 | tmpname[32] are statically dimensioned and quite small. |
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61 | |||
62 | |||
63 | The concept of time in this video decoder software |
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64 | -------------------------------------------------- |
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65 | |||
66 | When decoding a non-scalable bitstream, the frame number (i.e. |
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67 | temporal position) of the current I- or P-frame can be derived |
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68 | implicitly from the number of preceding B-frames after they have been |
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69 | decoded. Therefore the temporal_reference entry in the picture header |
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70 | is somewhat redundant and does not necessarily have to be evaluated in |
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71 | the decoding process. |
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72 | |||
73 | Decoding of the enhancement layer of a spatial scalable hierarchy, |
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74 | however, requires to know the temporal position of each frame at the |
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75 | instant when it is decoded, since data from a lower layer reference |
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76 | frame has to be incorporated. |
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77 | |||
78 | In the architecture of this video-only decoder decoding of a spatial |
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79 | scalable hierarchy of bitstreams is done by calling mpeg2decode once |
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80 | for the base layer bitstream and a second time for the enhancement |
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81 | layer bitstream, indicating where the decoded base layer frames can be |
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82 | found (option -s<filename>). |
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83 | |||
84 | Here the concept of time is only present in the form of frame numbers. |
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85 | Therefore spatial scalable bitstream hierarchies can only be handled |
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86 | under the assumption that base and enhancement layer bitstreams are |
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87 | decoded to image sequences where corresponding images of both layers |
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88 | have identical frame numbers. |
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89 | |||
90 | More specifically this means that base and enhancement layer |
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91 | bitstreams must contain video with the same frame rate. Furthermore |
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92 | only the temporally coincident frame of the base layer can be accessed |
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93 | for spatial prediction by the enhancement layer decoder, since it is |
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94 | not possible to resolve unambiguously the lower_layer_temporal_reference |
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95 | which is meant to further specify the lower layer reference frame. |
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96 | |||
97 | ======================== SPATIAL.DOC ========================0 |
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98 | |||
99 | Decoding a spatial scalable hierarchy of bitstreams |
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100 | --------------------------------------------------- |
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101 | |||
102 | With this video-only decoder decoding of a spatial scalable hierarchy |
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103 | of bitstreams is done by calling mpeg2decode once for the base layer |
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104 | bitstream and a second time for the enhancement layer bitstream, |
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105 | indicating where the decoded base layer frames can be found |
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106 | (using option -s and supplying <spatial base filename>). |
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107 | |||
108 | mpeg2decode -r -o0 base.mpg base%d%c |
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109 | mpeg2decode -r -o0 -f -s base%d%c enh.mpg enh%d |
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110 | |||
111 | Note that the base layer decoder must not use the -f option to enforce |
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112 | frame-wise storage. |
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113 | |||
114 | Note further that only yuv image format (option -o0) is supported as |
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115 | input format. |
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116 | |||
117 | |||
118 | Timing / layer synchronisation in this video decoder software |
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119 | ------------------------------------------------------------- |
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120 | |||
121 | When decoding a non-scalable bitstream, the frame number (i.e. |
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122 | temporal position) of the current I- or P-frame can be derived |
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123 | implicitly from the number of preceding B-frames after they have been |
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124 | decoded. Therefore the temporal_reference entry in the picture header |
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125 | is somewhat redundant and does not necessarily have to be evaluated in |
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126 | the decoding process. |
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127 | |||
128 | Decoding of the enhancement layer of a spatial scalable hierarchy, |
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129 | however, requires to know the temporal position of each frame at the |
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130 | instant when it is decoded, since data from a lower layer reference |
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131 | frame has to be incorporated. |
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132 | |||
133 | The concept of time is only present in the form of frame numbers. |
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134 | Therefore spatial scalable bitstream hierarchies can only be handled |
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135 | under the assumption that base and enhancement layer bitstreams are |
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136 | decoded to image sequences where corresponding images of both layers |
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137 | have identical frame numbers. |
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138 | |||
139 | More specifically this means that base and enhancement layer |
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140 | bitstreams must contain video with the same frame rate. Furthermore |
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141 | only the temporally coincident frame of the base layer can be accessed |
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142 | for spatial prediction by the enhancement layer decoder, since it is |
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143 | not possible to resolve unambiguously the lower_layer_temporal_reference |
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144 | which is meant to further specify the lower layer reference frame. |
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145 | |||
146 | Lower layer frames are read field-wise or frame-wise, depending on the |
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147 | lower_layer_progressive_frame flag. Consistency between layers in this |
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148 | respect is checked since the file format for frame and field pictures |
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149 | differs. |
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150 | |||
151 | |||
152 | |||
153 | |||
154 |