Makefile to Compile OpenCV Code in C++ on Ubuntu/Linux

Question

I am learning OpenCV using Learning OpenCV book.

One problem I am facing while compiling the code is that I have to write a long command to compile and get the executable.

This is the command I am using

g++ `pkg-config –cflags opencv` file_name.cpp -o output_file `pkg-config –libs opencv`

I am no Make expert but I think I can eliminate writing that long command using make. Before that I should explain my work flow. I have created a directory called opencv in my home directory (~/opencv/). I am reading the book section by section and coding the examples or exercises into new cpp source code files in that directory. So I don't know the names of the files before hand.

Now what I want make to do is,

Suppose I have coded a new file named facedetect.cpp in my opencv directory, and if I call make like this

make facedetect

then I want make to execute the following command for me

g++ `pkg-config --cflags opencv` facedetect.cpp -o facedetect `pkg-config --libs opencv`

so that whenever I make a new file named abc.cpp, I will execute make abc so that I can run

$ ./abc

at my command line to test my abc.cpp

Please give that make file so that I can save the frustration of typing that long command each time.

PS: I have Googled for help on this and found this on using CMake but I could not understand what that does. Kindly also explain how can I use CMake for the same task.

Solution 1

You can create a file called Makefile in you working directory like this:

CFLAGS = `pkg-config --cflags opencv`
LIBS = `pkg-config --libs opencv`

% : %.cpp
        g++ $(CFLAGS) $(LIBS) -o $@ $<

then you can use this file for all your single-file programms. Just call make with the basename of the file you want to compile. For facedetect.cpp that would be

make facedetect

Here some more details:

The general format of a makefile is like this:

target : dependecy1 dependenc2 ...
    command that generates the target

So for your example you could write:

facedetect : facedetect.cpp
    g++  $(CFLAGS) $(LIBS) -o facedetect facedetect.cpp

For each new example you can now create a new target. But you can also make it more general:

% : %.cpp
    g++  $(CFLAGS) $(LIBS) -o $@ $<

Here % matches any nonempty substring. The automatic variables $@ and $< substitute the names of the target file and the source file. For more information you can consult the make documentation.

Solution 2

GNU Make is pretty smart and the Makefile you need for this doesn't need to be as verbose as in the other answers given so far. Here is a simple Makefile which you can use to compile the OpenCV examples:

CPPFLAGS = $(shell pkg-config --cflags opencv)
LDLIBS = $(shell pkg-config --libs opencv)

That's it. The Makefile can be this short thanks to Make's implicit rules.

Then run make as usual:

make facedetect

This assumes there is a facedetect.c or facedetect.cpp in the same directory.

I recommend the following (free!) book if you want to learn Make: http://oreilly.com/catalog/make3/book/index.csp

Comments

[jumpdiffusion]

It looks like only g++ $(CFLAGS) -o $@ $< $(LIBS) will work. But I think that is a opencv thing. Thanks for the Makefile. Can u explain what does that mean or give any good resource to learn about Make

[morynicz]

Good resources about make: make manual. It's not as scary and hard to understand as it looks.

[Cosimo]

Thanks Siva Prasad Varma, it was indeed the order of the arguments. $(LIBS) must go at the end of the compilation command, or OpenCV examples themselves will not compile!

[Admin]

today, you can simply install cmake(if not already installed => sudo apt-get install cmake); open a terminal; navigate to opencv folder; cmake .; make; note the "."(dot) after cmake, now, wait for everything to be built

[Serrano]

@ComputerSaysNo: I think you misunderstood the question. He's not trying to compile OpenCV, but source files that he wrote himself from exercises in a book (which use the OpenCV libraries).

[Admin]

ahhh, yes, misunderstood badly, I'm sorry

[nn0p]

You can just use LDLIBS = `pkg-config --libs opencv` without explicitly writing the build command.

[mrgloom]

stackoverflow.com/questions/33530363/order-in-makefile also about the order.