Introduction
This documentation serves as a quick overview on the first development phase of our wireless video compression and transmission testbed, i.e. H.264 realtime encoding, transmission, and decoding using two open source projects FFmpeg and x264.
FFmpeg is a comprehensive multimedia encoding and decoding library that consists of numerous audio, video, and container formats. For H.264 video encoding, FFmpeg uses external open source library x264. In our testbed, FFmpeg works as an encoding and decoding front-end, while x264 is the encoding engine. At the encoder side, frames are captured by camera and then encoded into H.264 bitstream in real time. Encoded packets are transmitted over UDP packets to the receiver. At the receiver side, packets are received, decoded, and displayed in real time too. We have also pruned FFmpeg such that source codes of unrelated codecs and formats have been removed from the package. It will make our future development easier.
In following sections, we will give brief introduction on FFmpeg and x264, as well as necessary external libraries in Section 2. We will also introduce how to use our software to perform real time H.264 encoding, transmission, and decoding in Section 3. This documentation is not intended to be comprehensive. Instead, hopefully it will provide enough information to interested readers to quickly set up and try out the software, or even to do their own development.
Source Packages and Libraries
x264
x264 is a free library for encoding H264/AVC video streams. It is used by many other projects. Below is its official website.
Website: http://www.videolan.org/developers/x264.html
Download: git clone git://git.videolan.org/x264.git
Following commands are used to build debug and release version of x264. After successful configurations, use “make” and “make install” to compile and install x264.
debug version: ./configure --disable-asm --enable-debug --enable-pic
--enable-shared --disable-mp4-output
release version: ./configure --enable-pic --enable-shared
--disable-mp4-output
FFmpeg
FFmpeg
is a complete open source solution to record, convert and stream audio
and video. It includes libavcodec, the leading audio/video codec
library used by several other projects. It also includes libavformat,
an audio/video container mux and demux library. FFmpeg is developed
under Linux, but it can compiled under most operating systems,
including Windows. For detailed
information and downloading, please visit following website.
Website: http://ffmpeg.mplayerhq.hu/
Download: git clone git://git.mplayerhq.hu/ffmpeg/
cd ffmpeg
git clone git://git.mplayerhq.hu/libswscale/
Following commands are used to build debug and release version of FFmpeg. Note that we must
configure
FFmpeg to use x264 library. After successful configurations, use “make”
and “make install” to compile and install FFmpeg.
debug version: ./configure --enable-gpl --enable-libx264 --enable-swscale
--enable-debug --disable-optimizations --disable-stripping
release version: ./configure --enable-gpl --enable-libx264 --enable-swscale
Git
Git is an open source version control system designed to handle very large projects with speed
and efficiency, but just as well suited for small personal repositories. We need Git to download
FFmpeg and x264 source codes.
Website: http://git.or.cz/
Download: http://www.kernel.org/pub/software/scm/git/
Subversion (SVN)
Subversion is an open source version control system. It is used to maintain current and historical
versions of files such as source code, web pages, and documentation. Subversion is well-known in
the open source community and is used on many open source projects and we need Subversion to down Yasm and SDL.
Website: http://subversion.tigris.org/
Download: http://subversion.tigris.org/getting.html#source-release
Yasm
Yasm
is a complete rewrite of the NASM assembler under the “new” BSD License
(some portions are under other licenses, see COPYING for details). Both
FFmpeg and x264 uses Yasm.
Website: http://www.tortall.net/projects/yasm/
Download: svn co http://www.tortall.net/svn/yasm/trunk/yasm yasm
Simple DirectMedia Layer (SDL)
Simple
DirectMedia Layer is a cross-platform multimedia library designed to
provide low level access to audio, keyboard, mouse, joystick, 3D
hardware via OpenGL, and 2D video framebuffer. It is used by MPEG
playback software, emulators, and many popular games and FFmpeg uses
SDL for displaying decoded video contents.
Website: http://www.libsdl.org/
Download: svn checkout http://svn.libsdl.org/branches/SDL-1.2
To Use FFmpeg and FFplay
Encoding
Encoding is done by binary ffmpeg in our trimmed FFmpeg package.
It calls x264 library to encode frames captured by camera into H.264
bitstreams. We have modified ffmpeg such that encoded bitstreams are
sent to receiver over UDP packets, or saved locally, or both. A common
command line input is shown below. In this example, the input of the
encoder is camera captured frames with CIF resolution. Frame rate is
10fps. The output is H.264 bitstream with bitrate at 50kbps. Intra
frame interval is set to 100. The encodeded bitstream is both sent to
localhost with port number 12349, and saved as a local file out.264 in
/bs directory.
./ffmpeg -g 100 -f video4linux -b 50k -s cif -r 10 -i /dev/video0
-vcodec libx264 -y -dest_ip 127.0.0.1 -dest_port 12349 -f h264
../bs/out.264
The command below only sends encoded bitstream over UDP packets:
./ffmpeg -g 100 -f video4linux -b 50k -s cif -r 10 -i /dev/video0
-vcodec libx264 -y -dest_ip 127.0.0.1 -dest_port 12349 -f h264
The command below only saves encoded bitstream to a local file:
./ffmpeg -g 100 -f video4linux -b 50k -s cif -r 10 -i /dev/video0
-vcodec libx264 -y -f h264 ../bs/out.264
In Table 1 we list some frequently used options of ffmpeg.
Decoding
Decoding is done by binary ffplay in our trimmed FFmpeg package.
It decodes H.264 bitstream and displays in real time. We have modified
ffplay such that it can both decodes local H.264 file and UDP packets
containing H.264 bitstream, but not simultaneously. The command below
decodes H.264 bitstream in UDP packets assuming port number 12349 is
used. Combined with the first ffmpeg command line example, real time
H.264 encoding and decoding will be performed on the same PC.
./ffplay -dest_port 12349 -f h264
The command below decodes local H.264 bitstream assuming the out.264 file is located in /bs directory.
./ffplay ../bs/out.264
Below we list some frequently used options and hot keys of ffplay.
Options:
- -h: show help
- -f: force format
- -s: set frame size (WxH or abbreviation)
- -dest_port: specify destination port number
- -fs: force full screen
- -x: force displayed width
- -y: force displayed height
Hot keys:
- q/ESC: quit
- f: toggle full screen
- p/SPC: pause
