Why after linking the OpenCV libraries, Mat objects are still 'undefined'?

[georgiav]

Hi, I'm new to QT and I'm currently trying to create GUI for my C++ program that uses OpenCV to create an image collage from multiple images. The program compiles and runs as it should from a shell and I want to figure out how to change it to work on Qt as well. I defined the libraries and include path in the .pro file :

         INCLUDEPATH += -I\usr\include\opencv4
         
         LIBS += -L\usr\local\lib\libopencv_core.so \
                 \usr\local\lib\libopencv_highgui.so \
                 \usr\local\lib\libopencv_imgproc.so

and in the main.cpp:

         #include "mainwindow.h"
         #include <QApplication>
         #include <QPushButton>
         #include<math.h>
         #include<stdlib.h>
         #include <stdio.h>
         #include <cmath>
         #include <opencv4/opencv2/core.hpp>
         #include <opencv4/opencv2/highgui.hpp>
         #include <opencv4/opencv2/imgproc.hpp>
         using namespace cv;
         #include<iostream>
         using namespace std;

When I try to run the code I get errors like <unknown type name 'Mat' > How can I fix this ?
This is a function from the code for example:

       struct RGBvalues{
         long R;
         long G;
         long B;
       };
       
       //splits the input image into tiles and stores them in a vector
       vector<Mat> split ( Mat & image, int M, int N )
       {
         // All images should be the same size ...
         int width  = image.cols / M;
         int height = image.rows / N;
         // ... except for the Mth column and the Nth row
         int width_last_column = width  + ( image.cols % width  );
         int height_last_row   = height + ( image.rows % height );
       
         vector<Mat> result;
       
         for( int i = 0; i < N; ++i )
         {
           for( int j = 0; j < M; ++j )
           {
             // Compute the region to crop from
             Rect roi( width  * j,
                           height * i,
                           ( j == ( M - 1 ) ) ? width_last_column : width,
                           ( i == ( N - 1 ) ) ? height_last_row   : height );
       
             result.push_back( image( roi ) );
           }
         }
       
         return result;
       }

[SGaist]

Hi and welcome to devnet,

Because you are not linking to the OpenCV libraries. The "-L" option is to add search paths for the linker. You are looking for the "-l" as well. This is the one that instructs to link to a library.

[mrjj]

Hi
Have you tried with
INCLUDEPATH += -I\usr\include

as the includes already have the opencv4 name.
#include <opencv4/opencv2/core.hpp>

also if you look inside the include folder, does it have the opencv2 sub folder ?

ahh. captial L. i need glasses :)

[georgiav]

@mrjj I tried to change it the way you suggested and using -l instead of -L but I still get those errors.

[mrjj]

@georgiav
Hi
make sure you do a build -> clean
and then run qmake
then recompile
or even better delete the build folder. qmake caches stuff so often a clean is needed.

[SGaist]

Your paths are invalid, use forward slashes for them.

[georgiav]

@SGaist @mrjj I changed the paths to this

INCLUDEPATH += -I/usr/include

LIBS += -l/usr/local/lib/libopencv_core.so
LIBS+= -l/usr/local/lib/libopencv_highgui.so
LIBS+= -l/usr/local/lib/libopencv_imgproc.so
Should it work now?
and did the cleaning process and now I have more errors but all because the libraries still seem to be unrecognised.

[SGaist]

How did you install OpenCV ?
Did you check that it is from the correct architecture ?

[georgiav]

@SGaist I was checking now, I'm using it from the virtual machine provided by my university. The path seemed fine when I was compiling the program from a shell so I'm not sure what's wrong. What should I look for ?

[SGaist]

Call "file /usr/local/lib/libopencv_core.so". What do you get ?

As for your .pro file:

LIBS += -L/usr/local/lib \
        -lopencv_core \ 
        -lopencv_highgui \
        -lopencv_imgproc

should be enough to link to these OpenCV libraries.

[georgiav]

@SGaist It was a 'symbolic link' to libopencv_imgproc.so.4.5.0, so I called it again for that and got:
file /usr/local/lib/libopencv_imgproc.so.4.5.0
/usr/local/lib/libopencv_imgproc.so.4.5.0: ELF 64-bit LSB shared object, x86-64, version 1 (GNU/Linux), dynamically linked, BuildID[sha1]=a9e4463d8bcc7f5051819e1ab34a131be7685cb3, not stripped

[SGaist]

Can you show your current .pro file content ?

[georgiav]

@SGaist yes :
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

   CONFIG += c++11
   
   # You can make your code fail to compile if it uses deprecated APIs.
   # In order to do so, uncomment the following line.
   #DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0
   
   SOURCES += \
       core.cpp \
       main.cpp \
       mainwindow.cpp
   
   HEADERS += \
       mainwindow.h
   
   FORMS += \
       mainwindow.ui
   
   INCLUDEPATH += -I/usr/local/include
   LIBS += -L/usr/local/lib \
           -lopencv_core \
           -lopencv_highgui \
           -lopencv_imgproc
   
   # Default rules for deployment.
   qnx: target.path = /tmp/$${TARGET}/bin
   else: unix:!android: target.path = /opt/$${TARGET}/bin
   !isEmpty(target.path): INSTALLS += target

[SGaist]

That looks good. What exact error are you getting now, after doing a full rebuild ?

[georgiav]

[SGaist]

That's not a linking problem you have there.

Where is opencv2/core.hpp located exactly in your system ?

[georgiav]

it should be the one on top left right? given the path

[SGaist]

Did you notice the "opencv4" folder that is missing from your INCLUDEPATH statement ?

Either add it there or to your include statements.

[georgiav]

I tried that and it still doesn't work apparently

[SGaist]

Please provide the information directly.
What exactly did you try ?
What is the error your get ?