building opencv 3.4.9 Windows 32 bit version with MinGW

I read a lot of confusing and contradictory posts on installing MinGW, MYS2, CMake etc on Windows and using them. Did not want to create any elaborate manually installed toolchain like
https://here-be-braces.com/a-windows-cpp-ecosystem-without-visual-studio/

Thought I could just run the powershell script from Mudlet, which gives a way of building it on Windows 7 - https://wiki.mudlet.org/w/Compiling_Mudlet#Compiling_on_Windows_7.2B

That worked quite well (on a VM running on GCP) upto installing QT. Before that, I had to enable running powershell scripts with
Set-ExecutionPolicy Unrestricted
or else the machine would not run unsigned powershell scripts. Also, I disabled Windows Defender Virus protection from Windows settings, since the virus scanner was running for every make, configure etc and slowing down things.

After installing QT, the installation script seemed to stop. Hitting Ctrl-C made it start again, and it installed a few more tools till luarocks. There, it gave some error and exited. But that was sufficient for me to try and build opencv.

Downloaded OpenCV 3.4.9 zip file, extracted it to C:\opencv3.4.9,
cd c:\opencv3.4.9
md build
cd build
cmake ..

But of course, that did not work so easily.

I had to set the CC environment variable, which I did temporarily with

$env:CC = "C:\Qt\Tools\mingw730_32\bin\gcc.exe"

since otherwise I would need to open another shell and all the env variables created by the mudlet install script would be lost. Then there were more errors about not specifying the MAKE and bitness, so finally the working cmake command was

cmake -D"CMAKE_MAKE_PROGRAM:PATH=C:\Qt\Tools\mingw730_32\bin\make.exe" -G "MinGW Makefiles" ..

That itself took a good five to ten minutes. Then make took another two or three hours! Looks like ffmpeg gets downloaded and installed during the build process.

At around 48%, the build exited with the sprintf error mentioned here. So I added the line
#define NO_DSHOW_STRSAFE
just above
#include "DShow.h"
in the file sources\modules\videoio\src\cap_dshow.cpp
went back and ran make again, and it started from where it left off (noticing that the earlier files had already been compiled.) I did not do multi-threaded compiling, since that also might give errors, as mentioned here.

Now the example files and the opencv test files created in the bin folder run, but OCVWarp still does not work. Silently exits on Windows 10, "Could not open video file input.mp4" error message on Windows 7 for OCVWarpcmd. Is it due to some codec missing issue? (Edit - it was a strange issue - my separate post on that is here.)

For building OCVWarp, I had to add some environment variables, since this was a new session after a restart. Paths to mingw's bin folder and cmake's bin folder added to the PATH variable,

the CC and CXX environment variables set to gcc.exe's path,

and the OpenCV_DIR variable set to the install folder, which in my case was C:\opencv3.4.9\build\install


For running on other machines, along with the opencv dlls, the runtime dlls from mingw's bin folder also need to be copied. libstdc++-6.dll, libwinpthread-1.dll, and libgcc_s_dw2-1.dll in this case.

Edit - 16th April - updated with link to building OCVWarp, and corrected the generator name - not NMake Makefiles, but MinGW Makefiles.