OpenCV 3似乎不适用于Qt

Question

I followed this tutorial to install OpenCV to work with Qt. I used OpenCV with Qt before, a couple of years ago, and I had to build both OpenCV and Qt myself. I was now happy that a solution exists by building only OpenCV.

CMake, as usual, had a lot of errors. I had to disable a bunch of extras, like opencv_apps and examples, but then it built the libraries correctly. The dynamic libs didn't work, but I planned to use it statically anyway.

After linking the OpenCV libraries in Qt, not forgetting that the opening of images has been moved to "imgcodecs"

LIBS += -lopencv_core300 -lopencv_highgui300 -lopencv_imgproc300
LIBS += -lopencv_imgcodecs300

I tried a simple program:

#include <opencv.hpp>

// ...

cv::Mat image = cv::imread("testimage.png");
cv::namedWindow("Test image");
cv::imshow("Test image", image);
cv::waitKey(1000);

It didn't work.

matrix.cpp:-1: error: undefined reference to `ippicviSum_8u_C1R@20'

Searching for "ippicviSum" on Google returns absolutely nothing (well, until this question will be crawled by them).

I know that the libraries at least somewhat work, or are at least found , because if I remove lopencv_imgcodecs300 from the project file, I get an additional error, which wasn't there when the library was included:

error: undefined reference to `cv::imread(cv::String const&, int)'

---

I gave up on the C++ API, and tried to use the classical C API. (In my trial 2 years ago, the C++ API also had some problems - the basic opening and displaying worked but the more complicated algorithms gave linker errors, so I resorted to the C API, which worked correctly)

#include <cv.h>
#include <cvaux.h>
#include <cvwimage.h>
#include <cxcore.h>
#include <highgui.h>

// ...

IplImage *image = cvLoadImage("testimage.png");

The result:

loadsave.cpp:-1: error: undefined reference to `cv::String::deallocate()'

Answer 1

'ippicviSum_8u_C1R' is the intel performance primitives (ipp) library that now comes with opencv for free. It looks like you are missing the download, I normally build opencv from source but if you got an installer you might need to get the Intel library separately.

There is no need to use the cv::String type, just use regular std::string. The cv types are there to support old embedded platforms with obsolete c++ compilers. It is possible that some Qt macro is redefining "String" and breaking the code ?

edit: The problem might also be that you are using the legacy C api. instead do:

cv::Mat image = cv::imread("testimage.png");

or if you still have an issue,

cv::Mat image = cv::imread(std::string("testimage.png"));

Answer 2

The problem seemed to be that OpenCV 3.0 is incompatible with the mingw version (491) shipped with Qt 5.4.

Upgrading to Qt 5.5, which uses mingw 492, solved the problem.