x265 has a lossless mode which is used for ultra-high bitrates with zero loss of quality. I tried using this mode with avconv as follows:

avconv -i input.mp4 -c:v libx265 -x265-params lossless -c:a copy output.mp4

However, using this mode has an opposite effect : video came out to be of extremely low quality. It was totally lossy!!

Am I using the parameter incorrectly? Or am I somehow misunderstanding the definition of the word "lossless"?

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  • Should it not be --lossless ?
    – Linef4ult
    Sep 23, 2015 at 7:20
  • @Linef4ult : No. That doesn't work. --lossless is used when using the x265 binary directly. However, when using avconv, parameters are passed differently. For example, CRF value is passed as --crf 28 when using x265 directly, however it is passed as -x265-params crf=28 when using avconv.
    – shivams
    Sep 23, 2015 at 7:51

1 Answer 1


FFmpeg/Libav require a "key=value" format so it requires a "=1". So for your example:

avconv -i input.mp4 -c:v libx265 -x265-params lossless=1 -c:a copy output.mp4

To confirm if it works (via command line) you should see the following log:

x265 [info]: Rate Control                        : Lossless


  • This does work!! Thanks @TheBiggerGuy. I tried converting a x264 file to x265 losslessly, and it worked. The file size of x265 came out to be larger, which was kind of a bummer, though not unexpected. I will accept your answer soon after a bit of further testing.
    – shivams
    Jun 4, 2016 at 4:11

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