--thumbnail-from-video option to extract the thumbnail from the video

[johelegp]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2020.05.08
• I've searched the bugtracker for similar feature requests including closed ones

Description

Feature request: A --thumbnail-from-video option to extract the thumbnail (to be used by the other thumbnail options) from the video (for better image quality).

I'm personally going to use this with music videos that have a static image throughout the whole video. Some have a mostly static image, so to avoid corner cases, perhaps some further-refining options may be required. Like for:

• getting the image from the middle of the video to avoid any initial effects,
• and maybe only if seeking is supported to save bandwidth.
• If the above refinements aren't used, and the default is to use the first frame of the video, check that the image isn't unreasonable (like all black) and fall back to the usual thumbnail.

[johelegp]

For now, I've gone with the following as a post-process step on my m4a files:

--restrict-filenames
--output "%(title)s.%(id)s.%(ext)s"
--format bestvideo[height<=720]
--exec "hq_cover {}"

#!/bin/bash
# "$1" is the video
set -e
file=${1%.*}
ffmpeg -sseof -00:00:20 -i "$1" -frames:v 1 "$file.png"
ffmpeg -i "$file.m4a" -i "$file.png" -c copy "$file.with_cover.mp4"
rm "$1" "$file.png"
mv "$file.with_cover.mp4" "$file.m4a"

[johelegp]

For https://www.youtube.com/watch?v=cEFStFTtBxI, here's what --embed-thumbnail gives me:

That's better than what --write-thumbnail, --write-all-thumbnails and --list-thumbnails have to offer.
This is the quality I get with the post-processing in the comment above:

Then there's https://www.youtube.com/watch?v=VzGxhMVd0pA, whose thumbnail is as tiny as it seems (--embed-thumbnail):

This is the quality I get with the post-processing in the comment above:

I might not want the author's thumbnail (--embed-thumbnail, https://www.youtube.com/watch?v=wF0bWnQXGqY):

This is the thumbnail I get with the post-processing in the comment above: