Cross compile OpenCV to a RaspberryPI

[hamzaelazizi]

Hello, i've been trying to make an application that combines Qt and OpenCV, and then deploy this application on a RaspberryPi.
i managed to cross compile Qt to run applications i built on my host machine on the RaspberryPi but whenever i want to use OpenCV with it, i fail to get to work. The first time i had incompatible libraries with ARM (they were x86-64) and now when i installed ARM version of OpenCV i get this error in Qt Creator (cannot find -lm).
Can u please tell me how to cross compile OpenCV as well ? Thank you

[Kent-Dorfman]

This post is deleted!

[jsulm]

@hamzaelazizi said in Cross compile OpenCV to a RaspberryPI:

Can u please tell me how to cross compile OpenCV as well ?

Why don't you simply install OpenCV packages (including dev packages) on your RaspberryPi (using apt) and then symply sync your sysroot?

[hamzaelazizi]

@jsulm May i ask how do i sync my sysroot exactly because i've already installed OpenCV in my RaspberryPi

[jsulm]

@hamzaelazizi Depends on how you created it. There are different ways. For example you can simply connect the SD card from your RaspberryPi to your development machine. Or you can copy the /usr content (at least lib and include subfolders) to your development machine.

[hamzaelazizi]

@jsulm do i need to install OpenCV On the host machine as well ?

[jsulm]

@hamzaelazizi No, why should you if you want to cross compile? Host libraries have nothing to do with RaspberryPi.

[hamzaelazizi]

@jsulm then what path should i include in my .pro file ? i've created a small qt program on my host machine and i simply want to open an image and display it on the raspberry but i don't know what to include in my .pro file and when i compile it i get the error saying " :-1: error: cannot find -lm "

[jsulm]

@hamzaelazizi said in Cross compile OpenCV to a RaspberryPI:

then what path should i include in my .pro file ?

The libraries you need I guess? Or what exactly are you asking?
In this case OpenCV libs you're using.
In QtCreator you should have a Kit for cross compilation where you set your cross compiled Qt and also path to your sysroot.
You need to understand the difference between compiling for host and cross compalation. If you compile for the host then libraries/headers installed on your host are used. But if you're doing cross compilation libraries and headers are taken from the sysroot.
You can find here information about QtCreator set-up for cross compilation: https://www.ics.com/blog/configuring-qt-creator-raspberry-pi

[hamzaelazizi]

@jsulm i copied my folders "lib" and "include" where most of my OpenCV libraries are from the raspberry to the sysroot folder on my host machine but when i try to compile it keeps saying ":-1: error: cannot find -lm" i don't know why ?? i know it's an issue related to the math library but i don't know why it says this

[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