I'm trying to build 5.1.0 right now. I'm getting this:

WARNING: allegro_video wanted but no supported backend found

I had built some earlier 5.1.0 revision but never got into building the video addon. Now I want to try it and I've already downloaded the FFmpeg dev binaries including the lib and header files. I was expecting for cmake to allow me to define something like FFMPEG_INCLUDE_DIR as for other build options but looks like I have to put the libs directly on my MinGW path. Is there a plan to change this?.

[EDIT:] I placed the FFmpeg files in the MinGW folder but I'm still getting the same warning. So, how do you build the video addon?.

Looks like the video addon is currently only enabled for Linux through cmake. I'm guessing the file cmake/AllegroFindFFMPEG.cmake is where the corresponding cmake changes must be done.

Unfortunately, I know only the very basics of cmake so I can't figure it out yet.

After doing some ugly changes to the cmake files, I managed to build the video addon. However, when I run ex_video I get audio but no video at all, al_get_video_frame() returns NULL. I wouldn't go as far as to tell that it's broken on Windows, though. Chances are I did something wrong. It could also be due to my too old machine/graphics card.

Finally, what's with this line in ex_video?:

fprintf(stderr, "SDL: could not set video mode - exiting\n");

This is what I did (yes, it's ugly).

Edit the file cmake/AllegroFindFFMPEG.cmake and add the following:

if (WIN32)
    set(FFMPEG_FOUND 1)
    set(FFMPEG_LIBRARIES avcodec avdevice avfilter avformat swscale swresample avutil postproc h:/libs/allegro-5.1.0/build/lib/liballegro_audio.dll.a)
endif()

For the allegro_audio lib I had to write the full path. Change the path accordingly.

Hope you can get it to work.