Consider using Android CMake toolchain file instead (it shipped with NDK 15+, so it is supported natively and should be well tested):

android-ndk-r15c/build/cmake/android.toolchain.cmake

Options are similar, but not exact. Something like this:

cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_TOOLCHAIN_FILE=${ANDROID_NDK}/build/cmake/android.toolchain.cmake -DANDROID_NATIVE_API_LEVEL=android-23 -DBUILD_SHARED_LIBS=OFF -DENABLE_CXX11=ON -DANDROID_STL=c++_static -DANDROID_ABI="armeabi-v7a" -DANDROID_TOOLCHAIN=clang -DANDROID_ARM_NEON=ON <opencv_src>

Lets leave OpenCV's Android toolchain "as is" for legacy NDKs (and old devices).

Thanks @alalek, that sounds like a good idea, I'll have a look

In my personal Android point of view I want to see it merged !

With this spirit "let things untouched although a improvement/modernization is there" let OpenCV midterm (current) unusable.
And why ? Because of 3% of all users !
Please tell them "Sorry 3.4.0 is your last version support api 9, we have to got to the future. Urgent !"

Look at current Android situation, it's not usable out of the box:

• outdated Eclipse sample project, support ended one year ago. since 2015 we know Android Studio is the future
• you can only build artifacts with archeological build tools, it's not specified which one
• nowhere any adaption to recent version is done, new user needs days to use it
• ... and a lot of more reasons

@hannesa2 We can't merge any incoming patches, because who would maintain this a few months later?

The first two commits just hides "android" tool problem, without resolving them (creates .apk files with zero size, including Android tests - do we really need this?). Only complete migration to gradle builds can help here (see #8460). Also ANDROID_SDK variable is not in recommended set of environment variables: https://developer.android.com/studio/command-line/variables.html

The third 90cc3f6 commit is useful, but it is not complete: CMake is not able to properly call "find_file()" on headers.

I'm not able to reproduce problem resolved by 3135af8 commit. It should not break anything, but I have no idea that problem related to NDK16 is resolved here. @erikman Is it a build problem or runtime issue (.apk crash on launch)?

@alalek The problem is a build problem, I ran into this when I needed to recompile due to #10229 so my only goal was to be able to build libraries with r16 with my own build flags. I did not need the .apk at all so I was fine with not building it. So yes this hides the android tool problem, but will allow the build to produce libraries that can be used.

I was not aware of ANDROID_SDK_ROOT being available, I'll update to using that if available.

3135af8 and 90cc3f6 are not needed when using android.toolchain.cmake from ndk.

With the two first patches I'm able to build with the toolchain from ndk. Still doing some final testing.

I looked into ANDROID_SDK_ROOT and it is not set as an environment variable when gradle runs cmake (I find it strange that we only get the ndk location and not the sdk location, see https://developer.android.com/ndk/guides/cmake.html ).

Locally I get around this by passing it from my gradle.build:

android {
    defaultConfig {
        externalNativeBuild {
            cmake {
                arguments "-DANDROID_SDK=${project.android.sdkDirectory}"
            }
       }
   }
}

my point is that I would like to pass the path as a cmake variable and not just an environment variable (which is harder to pass into cmake from gradle). So what I was doing in my patch was only to expose the ANDROID_SDK that already was used by OpenCV to also be exposed as a cmake variable.

Now I'm a bit hesitant to rename it since it is used by many of the support scripts: test_cmake_build.py, samples/android/hello-android/run.cmd, test_ant_build.py, android/build_sdk.py and modules/ts/misc/run_android.py and modules/ts/misc/run.py. @alalek: What do you think?

@erikman, thank you! Some parts of your patch have been already re-used in #10515. Let's close this one