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1 Lossless
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2 --------
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3
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4 Lossless Encoding
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5 =================
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6
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7 x265 can encode HEVC bitstreams that are entirely lossless (the
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8 reconstructed images are bit-exact to the source images) by using the
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9 :option:`--lossless` option. Lossless operation is theoretically
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10 simple. Rate control, by definition, is disabled and the encoder
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11 disables all quality metrics since they would only waste CPU cycles.
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12 Instead, x265 reports only a compression factor at the end of the
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13 encode.
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14
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15 In HEVC, lossless coding means bypassing both the DCT transforms and
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16 bypassing quantization (often referred to as transquant bypass). Normal
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17 predictions are still allowed, so the encoder will find optimal inter or
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18 intra predictions and then losslessly code the residual (with transquant
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19 bypass).
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20
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21 All :option:`--preset` options are capable of generating lossless video
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22 streams, but in general the slower the preset the better the compression
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23 ratio (and the slower the encode). Here are some examples::
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24
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25 ./x265 ../test-720p.y4m o.bin --preset ultrafast --lossless
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26 ... <snip> ...
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27 encoded 721 frames in 238.38s (3.02 fps), 57457.94 kb/s
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28
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29 ./x265 ../test-720p.y4m o.bin --preset faster --lossless
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30 ... <snip> ...
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31 x265 [info]: lossless compression ratio 3.11::1
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32 encoded 721 frames in 258.46s (2.79 fps), 56787.65 kb/s
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33
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34 ./x265 ../test-720p.y4m o.bin --preset slow --lossless
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35 ... <snip> ...
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36 x265 [info]: lossless compression ratio 3.36::1
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37 encoded 721 frames in 576.73s (1.25 fps), 52668.25 kb/s
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38
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39 ./x265 ../test-720p.y4m o.bin --preset veryslow --lossless
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40 x265 [info]: lossless compression ratio 3.76::1
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41 encoded 721 frames in 6298.22s (0.11 fps), 47008.65 kb/s
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42
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43 .. Note::
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44 In HEVC, only QP=4 is truly lossless quantization, and thus when
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45 encoding losslesly x265 uses QP=4 internally in its RDO decisions.
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46
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47 Near-lossless Encoding
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48 ======================
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49
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50 Near-lossless conditions are a quite a bit more interesting. Normal ABR
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51 rate control will allow one to scale the bitrate up to the point where
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52 quantization is entirely bypassed (QP <= 4), but even at this point
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53 there is a lot of SSIM left on the table because of the DCT transforms,
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54 which are not lossless::
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55
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56 ./x265 ../test-720p.y4m o.bin --preset medium --bitrate 40000 --ssim
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57 encoded 721 frames in 326.62s (2.21 fps), 39750.56 kb/s, SSIM Mean Y: 0.9990703 (30.317 dB)
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58
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59 ./x265 ../test-720p.y4m o.bin --preset medium --bitrate 50000 --ssim
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60 encoded 721 frames in 349.27s (2.06 fps), 44326.84 kb/s, SSIM Mean Y: 0.9994134 (32.316 dB)
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61
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62 ./x265 ../test-720p.y4m o.bin --preset medium --bitrate 60000 --ssim
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63 encoded 721 frames in 360.04s (2.00 fps), 45394.50 kb/s, SSIM Mean Y: 0.9994823 (32.859 dB)
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64
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65 For the encoder to get over this quality plateau, one must enable
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66 lossless coding at the CU level with :option:`--cu-lossless`. It tells
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67 the encoder to evaluate trans-quant bypass as a coding option for each
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68 CU, and to pick the option with the best rate-distortion
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69 characteristics.
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70
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71 The :option:`--cu-lossless` option is very expensive, computationally,
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72 and it only has a positive effect when the QP is extremely low, allowing
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73 RDO to spend a large amount of bits to make small improvements to
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74 quality. So this option should only be enabled when you are encoding
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75 near-lossless bitstreams::
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76
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77 ./x265 ../test-720p.y4m o.bin --preset medium --bitrate 40000 --ssim --cu-lossless
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78 encoded 721 frames in 500.51s (1.44 fps), 40017.10 kb/s, SSIM Mean Y: 0.9997790 (36.557 dB)
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79
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80 ./x265 ../test-720p.y4m o.bin --preset medium --bitrate 50000 --ssim --cu-lossless
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81 encoded 721 frames in 524.60s (1.37 fps), 46083.37 kb/s, SSIM Mean Y: 0.9999432 (42.456 dB)
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82
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83 ./x265 ../test-720p.y4m o.bin --preset medium --bitrate 60000 --ssim --cu-lossless
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84 encoded 721 frames in 523.63s (1.38 fps), 46552.92 kb/s, SSIM Mean Y: 0.9999489 (42.917 dB)
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85
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86 .. Note::
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87 It is not unusual for bitrate to drop as you increase lossless coding.
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88 Having "perfectly coded" reference blocks reduces residual in later
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89 frames. It is quite possible for a near-lossless encode to spend
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90 more bits than a lossless encode.
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91
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92 Enabling psycho-visual rate distortion will improve lossless coding.
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93 :option:`--psy-rd` influences the RDO decisions in favor of energy
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94 (detail) preservation over bit cost and results in more blocks being
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95 losslessly coded. Our psy-rd feature is not yet assembly optimized, so
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96 this makes the encodes run even slower::
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97
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98 ./x265 ../test-720p.y4m o.bin --preset medium --bitrate 40000 --ssim --cu-lossless --psy-rd 1.0
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99 encoded 721 frames in 581.83s (1.24 fps), 40112.15 kb/s, SSIM Mean Y: 0.9998632 (38.638 dB)
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100
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101 ./x265 ../test-720p.y4m o.bin --preset medium --bitrate 50000 --ssim --cu-lossless --psy-rd 1.0
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102 encoded 721 frames in 587.54s (1.23 fps), 46284.55 kb/s, SSIM Mean Y: 0.9999663 (44.721 dB)
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103
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104 ./x265 ../test-720p.y4m o.bin --preset medium --bitrate 60000 --ssim --cu-lossless --psy-rd 1.0
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105 encoded 721 frames in 592.93s (1.22 fps), 46839.51 kb/s, SSIM Mean Y: 0.9999707 (45.334 dB)
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106
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107 :option:`--cu-lossless` will also be more effective at slower
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108 presets which perform RDO at more levels and thus may find smaller
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109 blocks that would benefit from lossless coding::
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110
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111 ./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 40000 --ssim --cu-lossless
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112 encoded 721 frames in 12969.25s (0.06 fps), 37331.96 kb/s, SSIM Mean Y: 0.9998108 (37.231 dB)
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113
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114 ./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 50000 --ssim --cu-lossless
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115 encoded 721 frames in 46217.84s (0.05 fps), 42976.28 kb/s, SSIM Mean Y: 0.9999482 (42.856 dB)
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116
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117 ./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 60000 --ssim --cu-lossless
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118 encoded 721 frames in 13738.17s (0.05 fps), 43864.21 kb/s, SSIM Mean Y: 0.9999633 (44.348 dB)
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119
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120 And with psy-rd and a slow preset together, very high SSIMs are
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121 possible::
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122
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123 ./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 40000 --ssim --cu-lossless --psy-rd 1.0
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124 encoded 721 frames in 11675.81s (0.06 fps), 37819.45 kb/s, SSIM Mean Y: 0.9999181 (40.867 dB)
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125
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126 ./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 50000 --ssim --cu-lossless --psy-rd 1.0
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127 encoded 721 frames in 12414.56s (0.06 fps), 42815.75 kb/s, SSIM Mean Y: 0.9999758 (46.168 dB)
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128
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129 ./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 60000 --ssim --cu-lossless --psy-rd 1.0
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130 encoded 721 frames in 11684.89s (0.06 fps), 43324.48 kb/s, SSIM Mean Y: 0.9999793 (46.835 dB)
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131
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132
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133 It's important to note in the end that it is easier (less work) for the
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134 encoder to encode the video losslessly than it is to encode it
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135 near-losslessly. If the encoder knows up front the encode must be
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136 lossless, it does not need to evaluate any lossy coding methods. The
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137 encoder only needs to find the most efficient prediction for each block
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138 and then entropy code the residual.
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139
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140 It is not feasible for :option:`--cu-lossless` to turn itself on when
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141 the encoder determines it is encoding a near-lossless bitstream (ie:
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142 when rate control nearly disables all quantization) because the feature
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143 requires a flag to be enabled in the stream headers. At the time the
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144 stream headers are being coded we do not know whether
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145 :option:`--cu-lossless` would be a help or a hinder. If very few or no
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146 blocks end up being coded as lossless, then having the feature enabled
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147 is a net loss in compression efficiency because it adds a flag that must
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148 be coded for every CU. So ignoring even the performance aspects of the
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149 feature, it can be a compression loss if enabled without being used. So
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150 it is up to the user to only enable this feature when they are coding at
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151 near-lossless quality.
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152
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153 Transform Skip
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154 ==============
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155
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156 A somewhat related feature, :option:`--tskip` tells the encoder to
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157 evaluate transform-skip (bypass DCT but with quantization still enabled)
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158 when coding small 4x4 transform blocks. This feature is intended to
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159 improve the coding efficiency of screen content (aka: text on a screen)
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160 and is not really intended for lossless coding. This feature should
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161 only be enabled if the content has a lot of very sharp edges in it, and
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162 is mostly unrelated to lossless coding.
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