Issues linking libav (FFMPEG)

[virtuald]

In branch libav, built with libav from this branch in roborio-packages.

Basically, it looks like if libav is setup for shared libraries, you end up with something like this:

• Some exe depends on libopencv_videoio.so
• libopencv_videoio.so depends on libopencv_highgui.so and libav*.so
• libopencv_highgui.so depends on libav*.so

The error is something like...

../../lib/libopencv_videoio.so.3.1.0: undefined reference to `av_freep@LIBAVUTIL_54'
../../lib/libopencv_videoio.so.3.1.0: undefined reference to `avformat_write_header@LIBAVFORMAT_56'
../../lib/libopencv_videoio.so.3.1.0: undefined reference to `av_read_frame@LIBAVFORMAT_56'
../../lib/libopencv_videoio.so.3.1.0: undefined reference to `avpicture_get_size@LIBAVCODEC_56'
../../lib/libopencv_videoio.so.3.1.0: undefined reference to `av_free@LIBAVUTIL_54'
../../lib/libopencv_videoio.so.3.1.0: undefined reference to `av_dict_free@LIBAVUTIL_54'
...

And if you look at the rpath entries for libopencv_ libs, it points at the correct path to find the libav modules. However, the link fails, so I suspect for some reason LD isn't recursively adding the rpath as it encounters a new library. When running the compile command with an additional -Wl,-rpath-link,/path/to/libav flag, then it succeeds -- but I can't figure out how to convince cmake to add that for me.

I've spent a bit more time on this than I would like, so I'm giving up for now -- but I'm going to see if statically linking libav will work instead.

[virtuald]

I setup libav with --disable-shared --enable-fpic and now OpenCV compiles fine with it. Slightly annoying, as I think I'd rather use the shared library version of livav. The binaries are really large... looks like it's because the debug information wasn't stripped. Will need a better way to separate the debug info from a package and create a separate debug package for it.

I'll have to think about this some more.

[virtuald]

I think what I should do is build both in release mode with debug symbols, and in roborio-packages have a step to strip out the symbols automagically. We already have such a step, but it's only executed on things that are installed, and not packages like this.