Just an FYI (a few years late. Just a tip for anyone else that may come and see this) the reason why the poster AND why it's the best setting for quality is 1 instead of 0 for the quality value is because 0 is LITERALLY lossless. And what that means when converting from a webm to say a mp4, is that re-econded mp4 file will end up being decently larger than the webm file. Now you may say it's supposed to do that...it's not. See there is additional data that is not needed for mp4's with 265 encoding. So in lamens/short terms' what happens is artifacts can appear and there can be issues with a/v and synchronization and lots of other little bugs. It's not like those bluetooth audio codecs where lossless is ideal for things like latency and audio quality. It works differently for video.
So setting you quality to 0 can in many instances create a video mp4 that is actually larger in size than the webm video and has the same quality or worse than the webm video.
Setting the quality to 1 instead indicates that total lossless conversion is NOT to be used but to set the quality to the highest possible outcome. And in some programs...you can even set it further to 0.5 (handbrake for windows for example) allows settings it by increments of .5. Anything above 0 works.
It's highly confusing. As most program simply state the scale is 51-0 ranking from worst-best...so most intelligent human beings would take that as 0 is the best quality possible. While in all reality it's not. 1 is. Or 0.5 (granted i have yet to see a noticable improvement when using 0.5 instead of 1 for handbrake) for handbrake.
Also note that if you are choosing h.265 for your encoding...your resulting file will almost 100% definitely be decently smaller is total file size than the webm video file. That's because h.265 is a newer standard codec that supposed to allow for smaller file sizes with minimal or no quality or performance loss. And in some cases that quality and performance is even better than h.264.
For example, last week I converted a 250MB video webm file. The webm video contained zero tracking or scroll info and was missing lots a meta data so I couldn't fast forward or rewind through it at all and there were no thumbnails. It was a lecture that I only needed like 2 minutes from and the video was like 25 min long. A major pain... so I needed to convert it to mp4.
The original webm video was 720p. However that was recorded from a laptop screen browser window. And this new mp4 I was creating would be displayed on a massive projector screen. So I decided to upscale it to 4k so try to minimize any massive pixels and pixelation. (Upsclaing a video won't really do anything for video quality tho, note. It's just simply adding more pixels or re-sizing the pixels already existing. It can't add in any new details or such unless you are using some sort of ai upscaling solution)
Anyways, I essentially more than quadrupled the resolution. And I added in lots of filters with decently heavy usage settings and also used medium denoising... that in theory SHOULD have meant a much larger mp4 file size than 250MB like the webm video file.
Nope. The end mp4 h.265 video ended up being 65MB total in size. And there was zero missing video segments and zero bugs or "glitches" and audio/visual was perfectly aligned and audio tracks were crisp and not missing any segments.
So that kinda shows you just how much h.265 encoding can shrink a file. To give examples I have often seen video files as large as 16GB using h.264 encoding shrink all the way down to 6GB using h.265 encoding. Same video quality and resolution. Only thing that changed was encoding method used.
H.265 is really big right now for things like VR media as even a Oculus Quest 2 from Yeats and Yeats ago.. a basic vr movie was like 12-26GB per file if you went uo towards 6k/8k resolutions. So using h.265 to cut that file size is half is really important as those headsets have limited storage space and streaming those massive files instead from a computer or Nas or whatever will almost certainly induce lots of buffering for those massive file sizes.
Cheers