How to: OpenCV in Qt creator and ImageQ

In this post I'll cover how to glue the OpenCV libraries to Qt creator via an example. And I'll also release ImageQ, a Qt based image processing application.

OpenCV

OpenCV is an open source library that contains various functions for computer vision and image processing. OpenCV is written in C/C++ but there are ports for Java and Python.

Qt framework

Qt is a framework for cross development of Graphical User Interfaces (GUI) in C++. Qt libraries are open source.

Qt creator is an Integrated Development Environment (IDE) for Qt applications. Qt creator was created using the Qt framework as well.

Dependencies

You'll need the following libraries/software:

• Qt creator
• Qt libraries
• OpenCV libraries

Ubuntu

Open a linux terminal and type the following commands:

sudo apt-get install qtcreator # Qt creator + Qt libraries
sudo apt-get install libopencv-dev # OpenCV libraries

On Ubuntu 12.04 you'll end with Qt libraries version 4.8 and OpenCV libraries version 2.3.

Windows

First, you need Qt creator and Qt libraries. Follow the steps in this post and come back.

Now, grab and unzip OpenCV from here.

Next, install CMake (win32 exe) from here.

You'll need to add C:/mingw/bin to your PATH environment variable. Check this post for a how to.

Now, launch cmake-gui.

Select as source folder: The unzipped version of OpenCV and as build folder: any folder you want. Then click on the configure button, from the drop down menu select the MinGW makefiles and accept. You should get the following output.

OpenCV build configuration.

Go with the default configuration and click the generate button.

Now open a cmd and head to your "build folder" (the one you selected in CMake) and run the following command:

mingw32-make

Now go get a coffee, a sandwich or take a nap while the compiler does it job.

(coffee break)

OK, after thousands of compiler messages, you finally obtained the necessary binaries.

Execute the next command:

mingw32-make install

A new folder name "install" will be created in your "build folder". Copy the contents of this "install" folder in any folder you like (e.g. C:/opencv). You can now delete the "source code folder" (the one you download from OpenCV website) and the intermediate compiled objects (a.k.a. your "build folder").

Now you'll need to modify some environment variables. Check this post if you don't know how.

I'm assuming you are using the C:\opencv folder, change the path if necessary.

Append to the PATH (or Path, is the same, do NOT create a new variable) variable: C:\opencv\bin

Append to the CPLUS_INCLUDE_PATH variable: C:\opencv\include

Append to the LIBRARY_PATH variable: C:\opencv\lib

Setting up the Qt project

(The rest of the article is the same for Ubuntu and Windows users)

• Launch Qt creator.
• Start a new project (usually Qt Gui Application).
• Head to your .pro file.
• Inside the .pro file add these lines:

Linking OpenCV libraries (Linux)

Note: You probably won't need to link all the libraries, only link the ones you are going to use. The most common libraries are core, highgui and imgproc.

Adding the headers

OpenCV library headers are inside the opencv2 folder. To add these headers to your application add the following line:

Adding OpenCV library headers.

Displaying the images

OpenCV offers a function named cv::imshow for displaying images. However this method won't integrate to your Qt application.

In Qt, images are usually displayed in QLabels using the method setPixmap(). As the name implies the argument for this method must be a QPixmap object. QPixmap objects are optimized for displaying, meanwhile QImage objects are optimized for pixel manipulation. Using the static method QPixmap::fromImage() a QImage can be converted in a QPixmap.

Is easier to convert a cv::Mat, the basic OpenCV image object, to a QImage object than to a QPixmap objetc.

In summary, the necessary steps are listed below:

• Convert the cv::Mat object to a QImage object.
• Convert the QImage object to a QPixmap object, using QPixmap::fromImage().
• Add the QPixmap object to a QLabel, using setPixmap().

To convert a cv::Mat to a QImage, you can use my implementation available on the following links: header and source.

An example

Let's put everything together in an example. Head to your .ui file and add a label (leave the default name "label"), as shown in the following image. (I have also added grid layout, not necessary but helps visualization)

Toy application layout

Next, add the mat2qimage.cpp and mat2qimage.h to your project.

Now, on your mainwindow.cpp source file type the following code:

Toy application source code.

Note: In Windows, I had to use the full path to the image and the '\' characters needed to be written as "\\" insted. Example: "C:\\Users\\Jorge\\Desktop\\myimage.JPG".

This is the output, after running the example:

Toy example output. (Image from wikipedia)

ImageQ

Using these basics, I have developed ImageQ in Qt. ImageQ provides GUI access to some basic OpenCV functions. You can use ImageQ to learn the basics of image processing or to experiment with OpenCV function parameters.

ImageQ: Coin image after Sobel operator.

You can get ImageQ source code from this github repository.

P.S. For ImageQ you'll also need Qwt libraries, you can install these in Ubuntu using the following command. (Windows users, check this post instead)

sudo apt-get install libqwt-dev