In this tutorial we will learn how to configure Qt to use OpenCV. Although the tutorial is targeted for OSX users, you can modify my suggestions for use in Linux and Windows. I assume you have a working knowledge of Qt and you have at least built a “hello world” application using it.
Of the few different ways of configuring Qt for OpenCV, we will use the one that involves pkg-config.
I have created a simple Qt application that uses OpenCV to capture frames from a webcam and display them using QGraphicsView. A screenshot is shown below.
Install OpenCV
For this tutorial I am assuming you have installed OpenCV 2 or OpenCV 3. If you have not, you can install them using Homebrew. The basic installation commands are shown below.
Install OpenCV 2.4.x on OSX using Homebrew
brew tap homebrew/science
brew install opencv
Install OpenCV 3 on OSX using Homebrew
brew tap homebrew/science
brew install opencv3
You can find detailed instructions for installing OpenCV using Homebrew by clicking here. Using OpenCV 3 in a Qt application is a bit tricky for OSX because opencv and opencv3 packages contain the same libraries and so opencv3 is not deployed to /usr/local/lib like other packages. I will help you navigate through these complexities.
Install Qt Creator
You can download Qt Creator and follow the onscreen instructions for installing it. There are some restrictions on using Qt in a commercial application and you should make sure you know about licensing issues. You may find this discussion helpful.
Master Generative AI for CV
Build settings for Qt / OpenCV project
The very first thing to do is instruct Qt where to find pkg-config. The default location for pkg-config is /usr/local/bin.
which pkg-config
# returns /usr/local/bin/pkg-config
We need to add /usr/local/bin to PATH. Go to Project, expand Build Environment and add /usr/local/bin to PATH. Don’t forget to add a colon (:) before appending /usr/local/bin! See the screenshot below.
If you are using OpenCV 3 you also need to add a new variable called PKG_CONFIG_PATH and set it to the directory that contains opencv.pc for your OpenCV 3 installation. You can find it using the following command
find /usr/local -name "opencv.pc"
# /usr/local/Cellar/opencv3/3.1.0_3/lib/pkgconfig/opencv.pc
You may not need the above step for OpenCV 2.4.x.
How to modify Qt project file ( .pro ) for OpenCV
Now that the paths are correctly set up, we need to add a few lines to our project file (.pro) to tell qmake to use pkg-config for OpenCV.
# The following lines tells Qmake to use pkg-config for opencv
QT_CONFIG -= no-pkg-config
CONFIG += link_pkgconfig
PKGCONFIG += opencv
If you are using OpenCV 3, the compiler may complain
ld: library not found for -lippicv
How to fix compiler error “ld: library not found for -lippicv”
The ippicv library the compiler is complaining about is actually included in your OpenCV installation, but it is not in the right path. Let’s first locate the library
find /usr/local/Cellar -name "libippicv*"
# /usr/local/Cellar/opencv3/3.1.0_3/share/OpenCV/3rdparty/lib/libippicv.a
You can tell the compiler where to look for libippicv.a by adding the following line to the .pro file
LIBS += -L/usr/local/Cellar/opencv3/3.1.0_3/share/OpenCV/3rdparty/lib/
Alternatively, you can symlink libippicv.a to /usr/local/lib using the following command on the terminal
ln -s /usr/local/Cellar/opencv3/3.1.0_3/share/OpenCV/3rdparty/lib/libippicv.a /usr/local/lib
and then add the following to the .pro file
LIBS += -L/usr/local/lib/
# See cautionary note below
Caution : When both opencv and opencv3 packages are installed
When both opencv and opencv3 packages are installed, and you are using opencv3 in your Qt application, adding /usr/local/lib to your path ( as shown in the previous section ) can lead to linking errors. The best thing to do in such a case is unlink opencv.
brew unlink opencv
I have spent an embarrassing amount of time debugging the above problem in El Capitan. It does not show up on Yosemite.
Runtime error on OSX El Capitan
You may receive a runtime error on El Capitan. Note the actual error message may vary sometimes.
dyld: Symbol not found: __cg_jpeg_resync_to_restart
Referenced from: /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
Expected in: /usr/local/lib/libJPEG.dylib
in /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
The program has unexpectedly finished.
To fix it go to Project -> Run -> Run Environment -> Unset DYLD_LIBRARY_PATH. See image below.
That’s all you need to know to build a Qt application with OpenCV, but let me throw in another goodie.
How to convert OpenCV Mat to QImage
The following piece of code converts OpenCV Mat to QImage. This will come in handy when you build your first Qt based OpenCV application.
# Create an OpenCV image.
Mat image(320, 240, CV_8UC3, Scalar(0,0,0));
# Convert it to QImage
QImage qImage = QImage((const unsigned char*)(image.data),
image.cols,image.rows,
QImage::Format_RGB888).rgbSwapped());
I have encountered this problem : “dyld: Symbol not found: __cg_jpeg_resync_to_restart”, and solved this problem by the method above.
But when I running it in debugging mode, it receive the same runtime error. How can I solve this problem? Thank you for your selfless help!
Did you solve the problem when running in debugging mode?
Hi ,Tnx for the video,I wrote all that you said in .pro and I added #include
cv::Mat inputImage = cv::imread(“address of my pic”);
cv::imshow(“Display Image”,inputImage);
cv:waitkey(5000); To my mainwindow.cpp when I push build button I get many errors what should I do?
Thank you for this post. It saved me a lot of time. In my case, the paths were different. Maybe because i’m using MacPorts instead of homebrew.
The difference: /opt/local/bin vs /usr/local/bin
Thanks. Yes in case of MacPorts the path will be different. Thanks for pointing it out.
I have problems when i want to debug my application. When debugging, the dynamic libs can’t be load. Can this be because the DYLD_LIBRARY_PATH is unset?
Thanks a bunch. I am sure someone will greatly benefit from this comment.
Hi I have installed the opencv3 using homebrew and qt5.8 in a macOS Sierra. Its very strange cause when I try to use the wich command to find pkg-config I have no answer …, so I try to use the path that you suggest in the video but if I try to build the project I found the error “opencv development package not found”.
Are you sure OpenCV3 is linked ? By default it is not. You can link it by using
brew link opencv3 --force
Yeh, the error persists..
The process “/Users/charlie/Qt5.8.0/5.8/clang_64/bin/qmake” exited with code 3.
Error while building/deploying project CameraTest (kit: Desktop Qt 5.8.0 clang 64bit)
When executing step “qmake”
I find the error, I just forget a / in the build environment!!
Yay! Thanks for letting us know. I am sure someone in future will save a lot of time because of this thread.
I already subscribe for the OpenCV books and this script and do note have any answers..
Hi, I’m trying to follow this post with the example code downloaded and the camviewer widget is not showing under the central widget. Any explanation for this? Thanks.
Sorry for the late reply. I can’t think of an explanation. I hope you were able to solve it.
I am using openCV3 but I didn’t get the error `ld: library not found for -lippicv`
May be its not needed now.
Then you are all good. It depends on your installation.
Hi, great tutorial, I finally could installed opencv with QT thanks to this tutorial. I have a question, How can I get the code from the example that is in the video? I am already subscribed to learnopencv.com
Thanks. When you subscribe, we send send you access to all code in this blog including this post.
Thanks
HI, first, thank you for all the tutorials and thx for helping me understanding the basic and advance notions of computer vision.
I followed every step to link opencv3 with Qt5 but I get always the same issue no matter what I do: Iib -llibopencv_dnn.3.3.1.dyllib not found but I specified the path to that library in the *pro.file. hope you can help. https://uploads.disquscdn.com/images/7244a1e9bc563c1d4c052ffb5b5299548c6bc7f401a7d2f67f00c92b9bff31dc.jpg
I am trying to run a Qt application that my colleague is building, utilizing opencv. I keep getting an error:
ld: library not found for -lopencv_contrib
I have scoured the internet trying to find a solution for this problem, as well as uninstalling and reinstalling opencv to no avail. I’m on a Mac running OSX 10.13.1. Any advice you have would be so helpful!
Hi, thanks for your help . I tried to add Dlib to Qt to your opencv explanation above couldn’t succeed !
Can you show us a direction to use both opencv and Dlib with QT creator ? It will be very helpful..
Best..
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