H.265(libx265) codec for 8K videos, using same compress settings, why yuv444 10-bit is saving bitrates than yuv420 8-bit?
From Why does 10-bit save bandwidth (even when content is 8-bit)?


  When encoding with the 10-bit tool, the compression process is
  performed with at least 10-bit accuracy compared to only 8-bit
  otherwise. So there is less truncation errors, especially in the
  motion compensation  stage, increasing the efficiency of compression 
  tools. As a consequence, there is less need to quantize to achieve a
  given bit-rate.
  
  The net result is a better quality for the same bit-rate or conversely
  less bit-rate for the same quality: between 5% and 20% on typical
  sources.


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