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x265/doc/reST/lossless.rst
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x265/doc/reST/lossless.rst
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Lossless
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--------
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Lossless Encoding
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=================
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x265 can encode HEVC bitstreams that are entirely lossless (the
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reconstructed images are bit-exact to the source images) by using the
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:option:`--lossless` option. Lossless operation is theoretically
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simple. Rate control, by definition, is disabled and the encoder
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disables all quality metrics since they would only waste CPU cycles.
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Instead, x265 reports only a compression factor at the end of the
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encode.
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In HEVC, lossless coding means bypassing both the DCT transforms and
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bypassing quantization (often referred to as transquant bypass). Normal
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predictions are still allowed, so the encoder will find optimal inter or
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intra predictions and then losslessly code the residual (with transquant
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bypass).
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All :option:`--preset` options are capable of generating lossless video
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streams, but in general the slower the preset the better the compression
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ratio (and the slower the encode). Here are some examples::
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./x265 ../test-720p.y4m o.bin --preset ultrafast --lossless
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... <snip> ...
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encoded 721 frames in 238.38s (3.02 fps), 57457.94 kb/s
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./x265 ../test-720p.y4m o.bin --preset faster --lossless
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... <snip> ...
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x265 [info]: lossless compression ratio 3.11::1
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encoded 721 frames in 258.46s (2.79 fps), 56787.65 kb/s
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./x265 ../test-720p.y4m o.bin --preset slow --lossless
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... <snip> ...
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x265 [info]: lossless compression ratio 3.36::1
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encoded 721 frames in 576.73s (1.25 fps), 52668.25 kb/s
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./x265 ../test-720p.y4m o.bin --preset veryslow --lossless
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x265 [info]: lossless compression ratio 3.76::1
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encoded 721 frames in 6298.22s (0.11 fps), 47008.65 kb/s
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.. Note::
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In HEVC, only QP=4 is truly lossless quantization, and thus when
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encoding losslesly x265 uses QP=4 internally in its RDO decisions.
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Near-lossless Encoding
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======================
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Near-lossless conditions are a quite a bit more interesting. Normal ABR
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rate control will allow one to scale the bitrate up to the point where
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quantization is entirely bypassed (QP <= 4), but even at this point
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there is a lot of SSIM left on the table because of the DCT transforms,
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which are not lossless::
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./x265 ../test-720p.y4m o.bin --preset medium --bitrate 40000 --ssim
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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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./x265 ../test-720p.y4m o.bin --preset medium --bitrate 50000 --ssim
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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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./x265 ../test-720p.y4m o.bin --preset medium --bitrate 60000 --ssim
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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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For the encoder to get over this quality plateau, one must enable
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lossless coding at the CU level with :option:`--cu-lossless`. It tells
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the encoder to evaluate trans-quant bypass as a coding option for each
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CU, and to pick the option with the best rate-distortion
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characteristics.
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The :option:`--cu-lossless` option is very expensive, computationally,
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and it only has a positive effect when the QP is extremely low, allowing
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RDO to spend a large amount of bits to make small improvements to
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quality. So this option should only be enabled when you are encoding
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near-lossless bitstreams::
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./x265 ../test-720p.y4m o.bin --preset medium --bitrate 40000 --ssim --cu-lossless
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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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./x265 ../test-720p.y4m o.bin --preset medium --bitrate 50000 --ssim --cu-lossless
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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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./x265 ../test-720p.y4m o.bin --preset medium --bitrate 60000 --ssim --cu-lossless
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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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.. Note::
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It is not unusual for bitrate to drop as you increase lossless coding.
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Having "perfectly coded" reference blocks reduces residual in later
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frames. It is quite possible for a near-lossless encode to spend
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more bits than a lossless encode.
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Enabling psycho-visual rate distortion will improve lossless coding.
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:option:`--psy-rd` influences the RDO decisions in favor of energy
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(detail) preservation over bit cost and results in more blocks being
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losslessly coded. Our psy-rd feature is not yet assembly optimized, so
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this makes the encodes run even slower::
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./x265 ../test-720p.y4m o.bin --preset medium --bitrate 40000 --ssim --cu-lossless --psy-rd 1.0
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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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./x265 ../test-720p.y4m o.bin --preset medium --bitrate 50000 --ssim --cu-lossless --psy-rd 1.0
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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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./x265 ../test-720p.y4m o.bin --preset medium --bitrate 60000 --ssim --cu-lossless --psy-rd 1.0
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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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:option:`--cu-lossless` will also be more effective at slower
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presets which perform RDO at more levels and thus may find smaller
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blocks that would benefit from lossless coding::
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./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 40000 --ssim --cu-lossless
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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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./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 50000 --ssim --cu-lossless
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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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./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 60000 --ssim --cu-lossless
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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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And with psy-rd and a slow preset together, very high SSIMs are
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possible::
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./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 40000 --ssim --cu-lossless --psy-rd 1.0
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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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./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 50000 --ssim --cu-lossless --psy-rd 1.0
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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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./x265 ../test-720p.y4m o.bin --preset veryslow --bitrate 60000 --ssim --cu-lossless --psy-rd 1.0
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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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It's important to note in the end that it is easier (less work) for the
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encoder to encode the video losslessly than it is to encode it
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near-losslessly. If the encoder knows up front the encode must be
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lossless, it does not need to evaluate any lossy coding methods. The
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encoder only needs to find the most efficient prediction for each block
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and then entropy code the residual.
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It is not feasible for :option:`--cu-lossless` to turn itself on when
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the encoder determines it is encoding a near-lossless bitstream (ie:
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when rate control nearly disables all quantization) because the feature
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requires a flag to be enabled in the stream headers. At the time the
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stream headers are being coded we do not know whether
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:option:`--cu-lossless` would be a help or a hinder. If very few or no
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blocks end up being coded as lossless, then having the feature enabled
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is a net loss in compression efficiency because it adds a flag that must
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be coded for every CU. So ignoring even the performance aspects of the
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feature, it can be a compression loss if enabled without being used. So
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it is up to the user to only enable this feature when they are coding at
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near-lossless quality.
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Transform Skip
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==============
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A somewhat related feature, :option:`--tskip` tells the encoder to
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evaluate transform-skip (bypass DCT but with quantization still enabled)
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when coding small 4x4 transform blocks. This feature is intended to
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improve the coding efficiency of screen content (aka: text on a screen)
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and is not really intended for lossless coding. This feature should
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only be enabled if the content has a lot of very sharp edges in it, and
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is mostly unrelated to lossless coding.
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