title: Optimally using hevc_videotoolbox and ffmpeg on OSX
body: <p>I'm using ffmpeg 4.3.1 to convert videos from h264 to h265 and initially I was excited to discover that I can use my Mac's GPU to speed up the conversion with the flag hevc_videotoolbox.</p>
  <p>My Mac hardware is the 10th generation Intel i5 with AMD Radeon Pro 5300</p>
  <p>I'm using this command:</p>
  <pre><code>ffmpeg -i input_h264.mp4 -c:v hevc_videotoolbox -b:v 6000K -c:a copy -crf 19 -preset veryslow output_h265.mp4
  </code></pre>
  <p>The conversion speeds increased from 0.75x to 4x, almost a 500% improvement !</p>
  <p>But then I noticed large filesizes and slightly fuzzy results.  Then I noticed that changing the crf or the preset makes no difference, ffmpeg seems to ignore those settings.  The only setting that seems to work is the video bit rate (-b:v).</p>
  <p>So I started to google around to see how I could get better results.</p>
  <p>But except for a few posts here and there, I'm mostly coming up blank.</p>
  <p>Where can I get documentation on how to get better results using hevc_videotoolbox?
  How can I find out what settings work and which ones are ignored?</p>