Lack of Documentation of New Format Merging Ability and Its Syntax

[EveryDirection]

The documentation outputted by --help currently states only

-f, --format FORMAT
video format code, specify the order of preference using slashes: "-f 22/17/18". "-f mp4" and "-f flv" are also supported. You can also use the special names "best", "bestaudio", "worst", and "worstaudio". By default, youtube-dl will pick the best quality.

No mention is made of how the option now supports syntax like

-f, --format FORMATVIDEO+FORMATAUDIO

(by the way, this new option is pretty awesome; kudos to the developers responsible!)

to download both DASH video and audio and automatically merge them. The documentation should be updated to reflect the new syntax options and also to clarify possible ambiguities in its usage:

• does this only work for DASH formats, or for all formats?
• does this only work if one of the formats is video and the other is audio?
• can you switch the two arguments (e.g. audio format first, video format second)?
• what happens if more than two format arguments are supplied? for example, if three formats are supplied, does it download and attempt to merge all three formats?

[phihag]

This is intentional, we do not feel that the option has gotten the appropriate love in order to be documented. Before making it official, we'll likely want to respect keepvideo (#2108), more testing on more platforms, and add a facility to make it default for IEs that are interested (namely YouTube).

To answer your questions:

Like any other option (unless documented otherwise), the syntax does work for all formats in all extractors. As far as we know, it is only useful for YouTube DASH videos. We do not support more than two options or anything other than video and audio (or audio and video, you can switch these as you wish). However, there is no code preventing you from letting ffmpeg merge the file.

[jaimeMF]

Note that if you do {audio_format}+{video_format}, you will get a file with the m4a extension, instead of mp4