[nmglug] Video format/quality question

[Anthony J. Bentley]

Hi LeRoy,

On Sun, Nov 1, 2020 at 8:06 PM LeRoy Diener
<leroy at choosetherightside.com> wrote:
> I'd like to learn about the quality of different video formats.

What a lot of people think of as "video" formats are actually
container formats: webm, mp4, mov, mkv, avi, etc. They're called this
because they can contain multiple data streams. For example, a movie
might have a single video stream, multiple audio streams (different
languages, commentary tracks, etc), and multiple subtitle streams
(different languages, "audio only" versus "signs and writing," etc).

Because of this, the container format doesn't generally affect the
quality of the video. In fact, as a rule you can convert video from
one container format to another (say, MP4 to MKV) without any loss in
quality. The quality is really a function of the streams it contains.
A MKV containing a FLAC audio stream will be higher quality than one
containing a MP3 audio stream, for example.

How this relates to video quality, then, is that your interest really
lies in the video *codec*, not the video *container*.

On YouTube you typically see two codecs, VP9 (open source) and AVC1
(proprietary, also called h.264). Other codecs I commonly encounter
are VP8 (VP9's predecessor) and HEVC (also called h.265, AVC1's
successor).

> If two videos have the same resolution, such as 1920x1080, do these two videos have the same quality?

Only if they use the same codec and bitrate.

The purpose behind ongoing development of new codecs is to provide
better quality at the same bitrate, or to provide similar quality at
smaller bitrates. A VP9 video of a particular bitrate will be better
quality than a VP8 video of the same bitrate, and better than a VP9
video of a lower bitrate (just like a 320 kbps MP3 is higher quality
than a 128 kbps MP3).

I'm not a video expert, so take the following with a grain of salt:
leaving aside various other reasons to pick a particular codec (open
source license, hardware decoding support, etc), I would expect VP9 to
provide better quality at a smaller filesize than AVC1. VP8 mostly
competes with AVC1, but VP9 competes with HEVC.

When creating home movies or storing videos long-term, I prefer to
convert .mov or .mp4 to a MKV container, using MKVToolNix or FFmpeg's
stream copy mode to migrate the video and audio in whatever codec they
came to me from the camera or video capture device. (Just like saving
a JPG multiple times, converting from one codec to another invariably
leads to a drop in quality.)

When creating new videos to upload to a website, I'll use VP9 and WebM
(which is essentially a special-case MKV that can only contain VP8,
VP9, and Vorbis or Opus audio).

For temporary videos I view from YouTube, I let youtube-dl do its
thing and pick the best quality.

-- 
Anthony J. Bentley