Linux Multimedia

Environment

1. python3 -m venv /path/to/new/virtual/environment
2. source /path/to/new/virtual/environment/bin/activate
3. sudo apt-get install libpython3.6-dev python3-dbg
4. pip install -r requirements.txt

Build

1. mkdir build && mkdir include
2. make

Tests

1. make test

Usage

Standard Capture

build/multimedia -c <number-of-frames-to-capture> -o <output-file-string>

Standard Capture with Cropping

build/multimedia -c <number-of-frames-to-capture> -w x1,y1,x2,y2 -o <output-file-string>

Notes

Make

A makefile typically has a start where variables are declared at the start, eg:

CC=gcc
LIB_DIR=lib
INCLUDE_DIR=include
SRC_DIR=src
BUILD_DIR=build

These variables may be referred to later by $(variable-name), in which case the reference to the variable is replaced by the expanded value of the variable. E.g.: $(CC) gets expanded to gcc

The remaining parts of the makefile are of the form:

<target-name>: <dependency-1> <dependency-2>
    <action-for-building-target>

A call to make of the form make <target-name> will check for the presence and timestamp of the target. It will also check the presence and timestamps of the dependencies. If they don't exist it throws an error. If present and their timestamps indicate that they are more up to date than the target or if the target does not exist, then the action for building the target is performed.

For example:

default: $(BUILD_DIR)/multimedia pymultimedia

pymultimedia: setup.py $(SRC_DIR)/pymultimedia.pyx $(SRC_DIR)/camera.c $(SRC_DIR)/imageprocessing.c
    python3 setup.py build_ext --inplace && rm -f $(SRC_DIR)/pymultimedia.c

$(BUILD_DIR)/multimedia: $(SRC_DIR)/multimedia.c $(SRC_DIR)/camera.c $(SRC_DIR)/imageprocessing.c
    $(CC) -g3 $^ -o $@

clean:
    rm -f *.o *.a *.so $(BUILD_DIR)/multimedia

The above script will attempt to build the targets multimedia and pymultimedia by running python3 setup.py build_ext --inplace && rm -f src/pymultimedia.c and $(CC) -g3 src/multimedia.c src/camera.c src/imageprocessing.c -o build/multimedia at the command line.

It will only attempt to build those targets if their respective dependencies have fresher timestamps. If for some reason a dependency is missing, then make will throw an error.

Cython

Cython is a compiled language that is a superset of Python, with the addition of C like syntax. This amounts to Python type free syntax, together with C like typed syntax which compiles to more efficient runtime code. Cython also allows for the possibility of calling out to C or C++ modules and functionality.

A Cython module gets compiled to C, which then gets compiled to a dynamic library file, i.e. an *.so file on Linux. This library can then be loaded from within Python by simply importing the module by name, and its functionality can then be accessed from within Python.

All the information needed for compiling the dynamic library so file is specified within the modules setup.py file.

Building C / C++ extensions for Python

An excellent resource on this is here The TLDR is as follows:

If the following script is in setup.py

from distutils.core import setup, Extension

module1 = Extension('demo',
                    define_macros = [('MAJOR_VERSION', '1'),
                                     ('MINOR_VERSION', '0')],
                    include_dirs = ['/usr/local/include'],
                    libraries = ['tcl83'],
                    library_dirs = ['/usr/local/lib'],
                    sources = ['demo.c'])

setup (name = 'PackageName',
       version = '1.0',
       description = 'This is a demo package',
       author = 'Martin v. Loewis',
       author_email = '[email protected]',
       url = 'https://docs.python.org/extending/building',
       ext_modules = [module1])

Then running python setup.py build_ext --inplace will result in distutils calling out to the compiler on Linux as follows:

gcc -DNDEBUG -g -O3 -Wall -Wstrict-prototypes -fPIC -DMAJOR_VERSION=1 -DMINOR_VERSION=0 -I/usr/local/include -I/usr/local/include/python2.2 -c demo.c -o build/temp.linux-i686-2.2/demo.o

gcc -shared build/temp.linux-i686-2.2/demo.o -L/usr/local/lib -ltcl83 -o build/lib.linux-i686-2.2/demo.so

Using GDB

Gdb is excellent for debugging. Using it with C code is relatively straightforward.

1. Compile the C code with the -g3 argument to make sure that debugging symbols are included.
2. Set any breakpoints you need. For example break code_filename.c:162 to set a breakpoint at line 162 of code_filename.c.
3. Then run the program with r <executable-name> <arguments>

You can also use gdb to debug Python code. This is especially useful for debugging issues with the Pymultimedia Cython extension. In order to do this you need to have the Python debugging symbols installed. To do this for Python3 simply do:

sudo apt-get install python3-dbg

Once those are installed you can debug as follows:

1. gdb python
2. Set any breakpoints you need, even breakpoints in C files.
3. r <program-name>.py <arguments>