doc/reST/lossless.rst

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