Cross compile OpenCV to a RaspberryPI

[jsulm]

@hamzaelazizi How do you build?
Do you build in QtCreator?
How does your sysroot look like? Its structure should be: PATH_TO_SYSROOT/usr/* (* is lib and include)

[hamzaelazizi]

@jsulm yes i build in Qt Creator
i didn't understand your second question but here is my sysroot path : "/home/rpi-qt/sysroot/* (* is lib and include and local)
local is the file i copied with the OpenCV libraries in
when i build and compile another project, it works and i can deploy the application on the Raspberry but when i try to do something with OpenCV it shows that error "cannot find -lm"

[JonB]

@hamzaelazizi said in Cross compile OpenCV to a RaspberryPI:

error "cannot find -lm"

It is looking for file libm.a. Probably because OpenCV needs the math library. On Ubuntu, for example, that is in /usr/lib/x86_64-linux-gnu/libm.a. I don't know where yours is/is supposed to be, in your cross-compilation situation. Linux command find / -name libm.a -print 2>/dev/null on whichever machine, and wherever you need to look down from where mine specifies /, should find it if present.

[hamzaelazizi]

@JonB in case i was able to find its location, how can i tell Qt Creator to look for that library in that path ?

[JonB]

@hamzaelazizi
I have updated my post to give you a Linux find command to help you locate it.
Let's see where, if anywhere, it tells you it finds it before we know how to proceed.
I don't do cross-compilation so I don't know where you need to look or what to do about it when you find it, you may need @jsulm's help when you have told us where it is....

[hamzaelazizi]

@JonB it gave me all these locations:

/home/rnsaoberon/rpi-qt/tools/cross-pi-gcc-8.3.0-2/arm-linux-gnueabihf/libc/usr/lib/libm.a
/home/rnsaoberon/rpi-qt/sysroot/lib/arm-linux-gnueabihf/libm.a
/home/rnsaoberon/rpi-qt/sysroot/usr/lib/arm-linux-gnueabihf/libm.a
/usr/arm-linux-gnueabi/libhf/libm.a
/usr/lib/x86_64-linux-gnu/libm.a
/usr/arm-linux-gnueabihf/lib/libm.a

i used the one with the Bold writing

[hamzaelazizi]

@JonB after i included the one i told you about i started having other errors :

undefined reference to cv::imshow
undefined reference to cv::imread undefined reference to cv::wait
undefined reference to `cv::Mat

[JonB]

@hamzaelazizi
As I said, since I know nothing about cross-compilation or RPi you will have to await a comment from e.g. @jsulm as to what you are supposed to do....

[hamzaelazizi]

@JonB Okey i'll be waiting for him to reply back, hopefully he does

[jsulm]

@hamzaelazizi Try this (add it to your pro file):

LIBS += -L/usr/lib/arm-linux-gnueabihf -lm

[hamzaelazizi]

@jsulm yes i changed the path to the one i have and it worked but it then started showing these different errors :

undefined reference to cv::imshow
undefined reference to cv::imread
undefined reference to cv::wait
undefined reference to cv::Mat

[hamzaelazizi]

here's my .pro file code :

TEMPLATE = app
CONFIG += console c++11
CONFIG -= app_bundle
CONFIG -= qt

INCLUDEPATH += /home/rnsaoberon/rpi-qt/sysroot/usr/local/include/opencv4
#DEPENDPATH += /home/rnsaoberon/rpi-qt/sysroot/usr/local/include/opencv4

QT_CONFIG -= no-pkg-config

LIBS += `pkg-config --cflags --libs opencv4`

LIBS += -L/home/rnsaoberon/rpi-qt/sysroot/usr/lib/arm-linux-gnueabihf -lm
#CONFIG += link_pkgconfig
#PKGCONFIG += opencv4

SOURCES += \
        main.cpp

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /home/pi/$${TARGET}
!isEmpty(target.path): INSTALLS += target

and here's my main c++ code :

#include <opencv2/highgui.hpp>

int main()
{
    cv::Mat img = cv::imread("/home/pi/Pictures/drake.jpg");
    cv::imshow("Window",img);
    cv::waitKey(0);

    return 0;
}

[JonB]

@hamzaelazizi said in Cross compile OpenCV to a RaspberryPI:

LIBS += -L/home/rnsaoberon/rpi-qt/sysroot/usr/lib/arm-linux-gnueabihf -lm

While you await @jsulm who actually knows something about this...!

I believe you are saying:

• without -L/home/rnsaoberon/rpi-qt/sysroot/usr/lib/arm-linux-gnueabihf, although it cannot find the -lm library, you do not get errors undefined reference to cv::..., right?
• with -L/home/rnsaoberon/rpi-qt/sysroot/usr/lib/arm-linux-gnueabihf, it finds the math library, but you do get errors undefined reference to cv::..., right?

In that case, by putting /home/rnsaoberon/rpi-qt/sysroot/usr/lib/arm-linux-gnueabihf high on the library directory search list the cv::... stuff it also needs is no longer found, because it must find some different cv library archives (in that specified directory) instead of what it used to find, and those libraries do not include something which would resolve those references.

What you do/are supposed to do with that I do not know.....

UPDATE
Oh, maybe not. Those "undefined"s may always have been there. But when it cannot resolve the -lm on the link line it will stop there and won't get as far as the final step of linkage where it would notice undefineds. So the -L/home/rnsaoberon/rpi-qt/sysroot/usr/lib/arm-linux-gnueabihf may be nothing to do with the "undefined"s....

You might start by running pkg-config --cflags --libs opencv4 in a terminal, to at least see what that is producing. E.g. if that is not even outputting any library paths to put into your LIBS it's not right....

[jsulm]

@hamzaelazizi said in Cross compile OpenCV to a RaspberryPI:

INCLUDEPATH += /home/rnsaoberon/rpi-qt/sysroot/usr/local/include/opencv4

Please change all these paths!
You do NOT have to put the path of your sysroot!
It should look like this:

INCLUDEPATH += /usr/local/include/opencv4

Also, why /usr/local?! I doubt OpenCV packages from your OS install anything in /usr/local! Please fix this!

"LIBS += -L/home/rnsaoberon/rpi-qt/sysroot/usr/lib/arm-linux-gnueabihf -lm" - this is also not what I suggested! Why do you add sysroot path everywhere?

LIBS += -L/usr/lib/arm-linux-gnueabihf -lm

The errors you posted mean that OpenCV libs are not found. If you really install OpenCV using your OS packet manager on your RaspberryPi the libs should be somewhere in /usr/lib and it should be enough to add -lLIBNAME to pro file.

[hamzaelazizi]

@JonB i think i have got it to work, and i'm still in shock...

i added this code in .pro file : LIBS += -L/home/rnsaoberon/rpi-qt/sysroot/usr/local/lib -lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_imgcodecs

Apparently, Qt Creator wasn't able to find the libraries necessary for the application, so when i asked it to add them, it worked !
Tho, i'm sure i'll encounter more problems in the near futur, but that's enouh errors for one day.
i still have one question which is irrelevant to this topic : How can i use Qt to work with a thermal camera that uses SPI(GPIO) protocol to communicate. a FLIR Lepton thermal camera to be specific, Thank you !

[jsulm]

@hamzaelazizi said in Cross compile OpenCV to a RaspberryPI:

Apparently, Qt Creator wasn't able to find the libraries necessary for the application

Well, you have to tell the build system (not QtCreator) which libs your app needs, there is no magic.
Please also consider my previous post.

[hamzaelazizi]

@jsulm Thank you for your time and effort, sir ! much appreciated