OpenMAX IL demos for Raspberry Pi

I wanted to develop a program that utilizes the RaspiCam camera board for the Raspberry Pi. The Raspberry Pi Foundation provides RaspiVid command-line application that can record video using the RaspiCam. However, this wasn't flexible enough for my needs so I decided to learn how one can utilize the RaspiCam programmatically from within one's own application code.

Initially I had no idea what would be needed. I did my research and found out that you could use OpenMAX IL API to drive the RaspiCam and VideoCore hardware video encoder on the RaspberryPi to get H.264 encoded high-definition video out of the system in real time.

So I spent countless of hours reading the OpenMAX IL API specification and some existing code using OpenMAX IL API (see [References] section below). However, it was all very difficult to understand due to the complexity of the spec and layering of the sample code. I wanted to go to the basics and write trivial, uncapsulated demo code in order to learn how things fundamentally worked.

And here is the result. Because of the lack of simple sample code I decided to document, package and share my work. Maybe it helps other people in similar situation.

Downloading

OpenMAX IL demos for Raspberry Pi home page is at http://solitudo.net/software/raspberrypi/rpi-openmax-demos/ and it can be downloaded by cloning the public Git repository at git://scm.solitudo.net/rpi-openmax-demos.git. Gitweb interface is available at http://scm.solitudo.net/gitweb/public/rpi-openmax-demos.git. The software is also available at GitHub page https://github.com/tjormola/rpi-openmax-demos/.

Installation

This code has been developed and tested on Raspbian operating system running on Raspberry Pi Model B Rev. 1.0. You need the standard GCC compiler toolchain, GNU Make and Raspberry Pi system libraries development files installed, i.e. the following packages must be installed.

gcc make libraspberrypi-dev

The binaries can be then compiled by running make command in the cloned Git repository base directory. No special installation is required, you can just run the self-contained binaries directly from the source directory.

Code structure

This is not elegant or efficient code. It's aiming to be as simple as possible and each demo program is fully self-contained. That means hundreds of lines of duplicated helper and boiler-plate code in each demo program source code file. The relevant OpenMAX IL code is all sequentally placed inside a single main routine in each demo program in order to make it simple to follow what is happening. Error handling is dead simple - if something goes wrong, report the error and exit immediatelly. Other anti-patterns, such as busy waiting instead of proper signaling based control of the flow of execution, are also emloyed in the name of simplicity. Try not to be distracted by these flaws. This code is not for production usage but to show how things work in a simple way.

The program flow in each demo program goes as described here.

Comment header with usage instructions
Hard-coded configuration parameters
General helper routines
Signal and event handler routines
Main routine implementing the program logic
1. Initialization
2. Component creation
3. Component configuration
4. Enable component execution
  1. Tunneling of the subsequent components in the pipeline
  2. Enabling of relevant component ports
  3. Changing of the component states
5. Buffer allocation
6. Main program loop
7. Clean up and resource de-allocation
  1. Flushing of the buffers
  2. Disabling of relevant component ports
  3. De-allocation of the buffers
  4. Changing of the component states
  5. De-allocation of the component handles
8. Program exit

Usage

This section describes each program included in this demo bundle. Common to each program is that any data is written to stdout and read from stdin. Quite verbose status messages are printed to stderr. There is no command-line switches. All configuration is hard-coded and can be found at the top of the each .c source code file. Execution of each program can be stopped by sending INT, TERM or QUIT signal to the process e.g. by pressing Ctrl-C when the program is running.

rpi-camera-encode

rpi-camera-encode records video using the RaspiCam module and encodes the stream using the VideoCore hardware encoder using H.264 codec. The raw H.264 stream is emitted to stdout. In order to properly display the encoded video, it must be wrapped inside a container format, e.g. Matroska.

The following exaple uses mkvmerge tool from the MKVToolNix software package to create a Matroska video file from the recorded H.264 file and then play it using omxplayer (although omxplayer happens to deal also with the raw H.264 stream, but generally other players, such avplay, don't).

$ ./rpi-camera-encode >test.h264
# Press Ctrl-C to interrupt the recording...
$ mkvmerge -o test.mkv test.h264
$ omxplayer test.mkv

rpi-camera-encode uses camera, video_encode and null_sink components. camera video output port is tunneled to video_encode input port and camera preview output port is tunneled to null_sink input port. H.264 encoded video is read from the buffer of video_encode output port and dumped to stdout.

rpi-camera-playback

rpi-camera-playback records video using the RaspiCam module and displays it on the Raspberry Pi frame buffer display device, i.e. it should be run on the Raspbian console.

$ ./rpi-camera-playback

rpi-camera-playback uses camera, video_render and null_sink components. camera video output port is tunneled to video_render input port and camera preview output port is tunneled to null_sink input port. video_render component uses a display region to show the video on local display.

rpi-camera-dump-yuv

rpi-camera-dump-yuv records video using the RaspiCam module and dumps the raw YUV planar 4:2:0 (I420) data to stdout.

$ ./rpi-camera-dump-yuv >test.yuv

rpi-camera-dump-yuv uses camera and null_sink components. Uncompressed YUV planar 4:2:0 (I420) frame data is read from the buffer of camera video output port and dumped to stdout and camera preview output port is tunneled to null_sink.

However, the camera is sending a frame divided into multiple buffers. Each buffer contains a slice of the Y, U, and V planes. This means that the plane data is fragmented if printed out just as is. Search for the definition of OMX_COLOR_FormatYUV420PackedPlanar in the OpenMAX IL specification for more details. Thus in order to produce valid I420 data to output file, you first have to save the received buffers until the whole frame has been delivered unpacking the plane slices in the process. Then the whole frame can be written to output file.

rpi-encode-yuv

rpi-encode-yuv reads YUV planar 4:2:0 (I420) frame data from stdin, encodes the stream using the VideoCore hardware encoder using H.264 codec and emits the H.264 stream to stdout.

$ ./rpi-encode-yuv <test.yuv >test.h264

rpi-encode-yuv uses the video_encode component. Uncompressed YUV 4:2:0 (I420) frame data is read from stdin and passed to the buffer of input port of video_encode. H.264 encoded video is read from the buffer of video_encode output port and dumped to stdout.

But similarly as described above in the [rpi-camera-dump-yuv] section, also video_encode component requires its buffers to be formatted in OMX_COLOR_FormatYUV420PackedPlanar. Thus we need to pack the I420 data to the desired format while reading from input file and writing to video_encode input buffer. Luckily no buffering is required here, you can just read the data for each of the Y, U, and V planes directly to the video_encode input buffer with proper alignment between the planes in the buffer.

Bugs

There's probably many bugs in component configuration and freeing of resources and also buffer management (things done in incorrect order etc.). Feel free to submit patches in order to clean things up.

References

The following resources on the Web were studied and found to be useful when this code was developed.

Copyright and licensing

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.