Answers.Body: <p>In strict mathematical terms, no.</p>
  
  <p>In terms of maintaining an acceptable image quality, yes. If you use CRF <code>18</code> and a preset like <code>ultrafast</code> now, you should be able to get a smaller acceptable file with preset <code>veryslow</code> later on.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-12-11T08:16:17.283Z
Answers.Text: Ahh too bad. Upon testing a longer clip, the size reduction is around 20%. I can live with that I guess. Thanks for the quick answer!
Answers.UserDisplayName: Yifeng Mu
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-12-11T06:24:57.807Z
Answers.LastActivityDate: 2015-12-11T06:24:57.807Z
Answers.UserDisplayName: Gyan
Body: <p>My situation:</p>
  
  <ol>
  <li>Transcoding 1080p bluray videos to HEVC. Audio untouched;</li>
  <li>My rig is old (pre-2012) and to speed up the process I'm using <code>--preset ultrafast</code>;</li>
  <li>From x265 documentation, I understand that video quality is independent of the preset used. And the slower the preset, the better the compression ratio it seems;</li>
  <li>I'm using FFmpeg.</li>
  </ol>
  
  <p>My Reasoning:</p>
  
  <ol>
  <li>I want to start converting to HEVC now because the codec is good enough for me and the space saving is phenomenal;</li>
  <li>I've seen people reporting a 35GB video compressed to 1GB or so with <code>crf=18</code> and <code>--preset veryslow</code>. It sounds ludicrous but an experiment of my own shows about 40% additional space saving between presets <code>ultrafast</code> and <code>veryslow</code>.</li>
  </ol>
  
  <p>The Question:</p>
  
  <p>When I get a new rig with a more capable CPU (Skylake or later, I hear), is it possible for me, to "re-compress" my HEVC videos using eg. <code>--preset veryslow</code>, without quality loss due to encoding a second time?</p>
  
Tags: <ffmpeg><encoding><compression><h.265><lossless>
Title: x265: Is it possible to re-compress HEVC video without quality loss?
UserDisplayName: Yifeng Mu
ThreadText: # x265: Is it possible to re-compress HEVC video without quality loss?
  
  (Asked by: username_0 on 2015-12-11)
  
  My situation:
  
  1. Transcoding 1080p bluray videos to HEVC. Audio untouched;
  2. My rig is old (pre-2012) and to speed up the process I'm using `--preset ultrafast`;
  3. From x265 documentation, I understand that video quality is independent of the preset used. And the slower the preset, the better the compression ratio it seems;
  4. I'm using FFmpeg.
  
  My Reasoning:
  
  1. I want to start converting to HEVC now because the codec is good enough for me and the space saving is phenomenal;
  2. I've seen people reporting a 35GB video compressed to 1GB or so with `crf=18` and `--preset veryslow`. It sounds ludicrous but an experiment of my own shows about 40% additional space saving between presets `ultrafast` and `veryslow`.
  
  The Question:
  
  When I get a new rig with a more capable CPU (Skylake or later, I hear), is it possible for me, to "re-compress" my HEVC videos using eg. `--preset veryslow`, without quality loss due to encoding a second time?
  
  **Tags:** ffmpeg, encoding, compression, h.265, lossless
  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_1 on 2015-12-11. Score: 3 | Accepted answer
  
  In strict mathematical terms, no.
  
  In terms of maintaining an acceptable image quality, yes. If you use CRF `18` and a preset like `ultrafast` now, you should be able to get a smaller acceptable file with preset `veryslow` later on.
  
  > **Comments:**
  > **username_0**: Ahh too bad. Upon testing a longer clip, the size reduction is around 20%. I can live with that I guess. Thanks for the quick answer!  
  
  ---
  
  