Item description:

3Damnesic is a proof of concept media player for 3DS using ffmpeg, capable of playing MPEG4 and H264 video (no sound).

Features
Video.
MPEG4, H.264.
Hardware acceleration with Y2R for YUV -> RGB conversions.
Building
Requirements
DevkitArm.
Latest ctrulib.
ffmpeg compiled with the following instructions.
FFMPEG
Copy the ffmpeg-configure3ds script in your ffmpeg source folder.
Open a shell/command line in ffmpeg directory.
Windows users please use sh before starting the script.
./ffmpeg-configure3ds
make install
This will compile ffmpeg (with only a few features) with devkitArm and install it as a portlib.

3Damnesic
With the Makefile:

Simply use make.
With CMake:

mkdir cbuild && cd cbuild
On *NIX cmake -DCMAKE_TOOLCHAIN_FILE=DevkitArm3DS.cmake ..
On Windows cmake -DCMAKE_TOOLCHAIN_FILE=DevkitArm3DS.cmake -G"Unix Makefiles" ..
make
More information on the 3ds-cmake repository.

At the moment, you have to specify the file path in the main.c file at compilation time.

User guide
The file has to be named test.mp4 and be placed at the root of your SD card.

Use a video with dimensions multiple of 8 for best performance.

Videos up to 1024x1024 are supported (but will not run fullspeed and only if width<800, if you set up the framebuffers to be using RGBA).

This homebrew runs MPEG4 better than H.264 (H.264 is ~4 times slower). Some stats (video only, old 3DS):

400x240 mpeg4 -> 37fps.
400x240 h264 -> 16fps.