[Gstreamer-bugs] [Bug 141686] New: - Proposal for printf-style tag formatting element

[bugzilla-daemon at widget.gnome.org]

http://bugzilla.gnome.org/show_bug.cgi?id=141686
GStreamer | gstreamer (core) | Ver: HEAD CVS

           Summary: Proposal for printf-style tag formatting element
           Product: GStreamer
           Version: HEAD CVS
          Platform: Other
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: Normal
         Component: gstreamer (core)
        AssignedTo: gstreamer-maint at bugzilla.gnome.org
        ReportedBy: jyasskin at mail.utexas.edu
         QAContact: gstreamer-maint at bugzilla.gnome.org


Right now, the only way I know of to get the tags out of a file from the command
line is to actually decode the file with `gst-launch --tags`. I suggest that a
printf-style tag formatting element be written that could be used in a command
line like: `gst-launch filesrc="file" ! spider ! formattags
format="%{album}:%{title}" ! filesink=/dev/stdout`. This could then be easily
wrapped up in a shell script.

The element should have a sink of type application/x-gst-tags and a source of
type text/plain.

I think format specifiers like %{tagname} work well for inserting tag values
because the names are variable length and more will be added. For other
metadata, the more traditional %c style could be used, or other delimiters, ex.
%(non-tag_data)

At the very least, all tags gstreamer knows about should be supported by %{}, %%
should insert a '%', and all C '\' escapes should work. If possible, there
should be format specifiers that insert the filename and mime type (%f and %m?).
It might be nice to also support format-specific tag names, like id3's TPE4, etc.

I have a simple program that does this formatting for mp3 and vorbis files in
arch. Archive at http://bluejay.dyndns.org/archives/2003/ version
jyasskin at mail.utexas.edu--2003/mprintf--main--0.1 (sorry, it's in haskell)

We need to decide what to do when a tag doesn't exist. So far, I've just been
inserting the empty string, but it might be better to let the user decide. Also,
tags with a list of values need to have those values separated with something.
I've been using ',', but again the user should have a choice. Finally, if we
support format-specific tags, a way to disambiguate their names from gstreamer
offical tag names is needed. Perhaps "artist" would select the gst tag, while
"id3v1:artist" would take the id3v1 tag specifically. (This solution would
constrain gst tag names to never contain ':')

I hope to be able to start writing this element after the 15th, but I figure a
bug is appropriate in case I don't get around to it or someone has a better idea.

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