FastMJPG is a command line tool for capturing, sending, receiving, rendering, piping, and recording MJPG video with extremely low latency. It is optimized for running on constrained hardware and battery powered devices.

It is written entirely in C, and leverages UDP for network transport, v4l2 for video capture, libturbojpeg for JPEG decoding, libglfw for OpenGL rendering, and ffmpeg for Matroska video packing.

It can be integrated directly into your C application as a library, piped to your application via a file descriptor, or used exclusively as a command line tool.

• Alpha Notice
• Install
• Syntax
  • Capture (Input)
  • Receive (Input)
  • Render (Output)
  • Record (Output)
  • Send (Output)
  • Pipe (Output)
• Measuring Latency
• Author

FastMJPG is currently in a public alpha state. It is feature complete, and all known bugs have been fixed, though more issues are expected to be discovered as it is used by more people. It is not recommended for use in critical production environments at this time.

# Install dependencies:
sudo apt-get update
sudo apt-get install git build-essential libturbojpeg libturbojpeg0-dev libglfw3-dev v4l-utils libavutil-dev libavcodec-dev libavformat-dev libswscale-dev

# Clone:
git clone [email protected]:adrianseeley/FastMJPG.git
cd ./FastMJPG

# Build:
chmod +x ./build
./build
cd ./bin

# Display help:
./FastMJPG help

# List compatible devices, resolutions, and framerates:
./FastMJPG devices

# Run FastMJPG:
./FastMJPG [input] [output 0] [output 1] ... [output n]

Exactly one input is required, either:

• capture from a video capture device.
• receive from a from another FastMJPG process over a network.

At least one output is also required, one or more of:

• render to the screen using an OpenGL window.
• record to a Matroska file as an MJPG stream.
• send to another FastMJPG process over a network.
• pipe to a file descriptor that your application provides.

FastMJPG capture DEVICE_NAME RESOLUTION_WIDTH RESOLUTION_HEIGHT TIMEBASE_NUMERATOR TIMEBASE_DENOMINATOR ...

1. You must provide a resolution and framerate that is supported by the video device in MJPG streaming capture mode using MMAP buffers. FastMJPG will crash if the requested configuration is unsupported.
2. You can use FastMJPG devices to list all compatible devices, resolutions, and framerates.

FastMJPG receive LOCAL_IP_ADDRESS LOCAL_PORT MAX_PACKET_LENGTH MAX_JPEG_LENGTH RESOLUTION_WIDTH RESOLUTION_HEIGHT TIMEBASE_NUMERATOR TIMEBASE_DENOMINATOR ...

1. FastMJPG only supports receiving from other FastMJPG processes, it will not work with other software as it uses a custom application layer UDP protocol.
2. All stream configuration settings must exactly match that of the sender, otherwise it will result in undefined behaviour.
3. MAX_PACKET_LENGTH should be the largest value that fits into your network's MTU minus the overhead of the UDP header and IP header. MTU fragmented packets will result in undefined behaviour.
4. MAX_JPEG_LENGTH must be larger than the maximum JPEG frame size produced by the capture, otherwise it will result in undefined behaviour.

FastMJPG ... render WINDOW_WIDTH WINDOW_HEIGHT ...

1. Video will be rescaled to fit the window size on the GPU incurring little additional overhead. This rescaling will not affect any other outputs.
2. OpenGL windows are free-timed with the video stream, meaning the window may be declared unresponsive by the operating system if the video stream is paused or interrupted for any reason, this is normal behaiour and can safely be ignored.
3. OpenGL windows will not respond to the 'X' button to close for safety reasons, instead you must use CTRL+C to close the process with a sigint signal.
4. There can be at most one OpenGL window per process, attempting to create more than one will crash.

FastMJPG ... record FILE_NAME ...

1. The sigint signal (CTRL+C) will cause FastMJPG to attempt to write trailers to the mkv file before shutting down, though the nature of MJPG streams in a Matroska container does not require this finalization for playback, allowing you to safely kill the process at any time.
2. If there are pauses in the video stream, the last frame before the pause will last for the duration of the pause, this is expected behaviour.

FastMJPG ... send LOCAL_IP_ADDRESS LOCAL_PORT REMOTE_IP_ADDRESS REMOTE_PORT MAX_PACKET_LENGTH MAX_JPEG_LENGTH SEND_ROUNDS ...

1. FastMJPG only supports sending to other FastMJPG processes, it will not work with other software as it uses a custom application layer UDP protocol.
2. All stream configuration settings must exactly match that of the receiver, otherwise it will result in undefined behaviour.
3. MAX_PACKET_LENGTH should be the largest value that fits into your network's MTU minus the overhead of the UDP header and IP header. MTU fragmented packets will result in undefined behaviour.
4. MAX_JPEG_LENGTH must be larger than the maximum JPEG frame size produced by the capture, otherwise it will result in undefined behaviour.

FastMJPG ... pipe PIPE_FILE_DESCRIPTOR RGB_OR_JPEG MAX_PACKET_LENGTH ...

1. You must manage the pipe yourself, ensure that is it open and ready to write to before starting FastMJPG, and ensure that it is closed after FastMJPG has exited.
2. The pipe protocol is as follows.

Per Frame (RGB or JPEG):
1. big endian uint64_t uSeconds (capture timestamp)
2. big endian uint32_t length (frame length in bytes)
3. uint8_t[length] data (frame data)

3. RGB data is provided as three unsigned bytes per pixel, in the order of red, green, blue, with no padding and no alpha channel.

// A 2x2 pixel RGB frame of all red would be 12 bytes long, 3 bytes per pixel, 4 pixels total, 2 pixels wide, 2 pixels tall and produce the following byte stream:
0xFF 0x00 0x00 0xFF 0x00 0x00 0xFF 0x00 0x00 0xFF 0x00 0x00

4. JPEG data does not contain MJPG frame separators, and is provided instead as a single properly formed JPEG.

FastMJPG has a built in latency measurement tool that can be used to measure the latency of the entire pipeline. It is disabled by default, and must be enabled at compile time as follows:

# Build with measure enabled:
./build measure

# Run FastMJPG:
./FastMJPG [input] [output 0] [output 1] ... [output n]

Once enabled you can run FastMJPG as normal, and upon sending a sigint signal (CTRL+C) it will print all of the latency measurements to the console.

All measurements are the amount of microseconds (not milliseconds) since the capture timestamp of the frame as reported by v4l2. Literally how long has the frame been going stale for upon reaching that point in the pipeline.

Caveats:

1. Measuring incurs a significant processing overhead especially on constrained hardware, and will increase the latency of the pipeline. It is recommended to only use this feature for debugging purposes.
2. Different computers have different internal clocks, meaning you must use a network time protocol synchronization tool such as ntpd to synchronize the clocks of the computers you are measuring between. Even still this is all best effort, and the measurements will never be 100% accurate.
3. The measurements provided are effectively only useful for comparing the relative latency of different configurations of FastMJPG, and are not useful for comparing the latency of FastMJPG to other software without careful scrutiny and an understanding of how the measurements are taken in both FastMJPG and the compared.
4. The measurements provided are a very rough approximation of the actual latency of the pipeline, and should serve only as an indicator to guide further investigation.
5. The first step in any FastMJPG instance (capture or receive) has no capture timestamp to measure against when it starts, meaning the start and delta of those steps will not be tracked or printed.
6. Each instance tracks it's own measurements, and does not communicate with other instances beyond the normally provided capture timestamp.

All code and documentation was written by Adrian Seeley, who can be reached by email: [email protected]