I assume that media handling in gwenview, just like in the rest of kde, is done via phonon. The phonon-xine backend has been obsoleted for some time now, and it's not even supported in Gentoo anymore. So, I'd look at either vlc or gstreamer, which are the two backends that are supported in portage that can handle video. They can be adjusted via USE flags, when compiling media-libs/phonon.
There're also some configuration options that might be relevant in the kde control panel.
Note that I don't know much about gwenview, so maybe they disabled the feature for some other reason and I am not aware of it.
Pilsner wrote:This could well be the source of this problem. I have gstreamer use flag enabled in phonon, though but vlc is deselected.
Well, I chose vlc because it's simple to setup, and can play anything that I care about. GStreamer has lots of separate packages for many things (don't ask me how they are handled in Gentoo, I have no idea). You might need to emerge something like gst-plugins-ugly or some other random media-libs/gst-* package(s).
I can't know for sure if switching to vlc will fix your problem. But if you have both USE flags set, make sure that you select the right backend in kde's systemsettings to be prefered over the other(s).
I tried to switch to vlc, but there are some "hidden strings" that pull back to gstreamer. After I compile phonon with "-gstreame rvlc next emerge -auDv asks to enable gstreamer flag back. Moreover gwenview crashes when phonon is compiled with vlc flag. Seems, more ebuilds have to be re-compiled at once. Need to figure out all those dependencies.
You can use "qlist -U gstreamer" to see what packages have been installed with gstreamer enabled. That can give you an idea, but might not be enough to find the offenders.