Unable to include saliency module in custom OpenCV build (core,imgproc,objdetect)

[Akhil-Chaturvedi]

Hello,

I’ve been trying to build a minimal OpenCV library for my Android app. My app directly uses the following modules:

• From OpenCV core: core, imgproc, objdetect
• From opencv_contrib: saliency

Since these depend on other modules, I also include: features2d, calib3d, flann, java.

The problem: after ~10 build attempts, the saliency module never seems to get compiled or included in the resulting build. I’ve tried multiple approaches:

• Following YouTube tutorials
• Repeating the full build process several times (including step-by-step runs assisted by Gemini 2.5 Pro and other AIs)
• Adjusting CMake options to ensure OPENCV_EXTRA_MODULES_PATH points to opencv_contrib/modules

Despite that, I don’t see saliency in the final build output, while the other modules compile fine.

My questions:

1. Is there a restriction or special requirement that prevents saliency from being compiled in a minimal build like this?
2. Am I missing a simple step or flag that ensures saliency is included?
3. Has anyone successfully built OpenCV with exactly this module set (core, imgproc, objdetect, saliency, and required deps)?

Any guidance would be greatly appreciated, I’m likely overlooking something small, but after many attempts I haven’t been able to get saliency working.

Thanks!

[sturkmen72]

see https://github.com/opencv/opencv_contrib/blob/4.x/modules/saliency/CMakeLists.txt

try

ocv_define_module(saliency opencv_imgproc opencv_features2d WRAP python java)

[Akhil-Chaturvedi]

Thanks for replying! Here’s the last command I used to build:

py C:\opencv_build\opencv-4.12.0\platforms\android\build_sdk.py ^
--sdk_path "C:\Users\Electrobot\AppData\Local\Android\Sdk" ^
--ndk_path "C:\Users\Electrobot\AppData\Local\Android\Sdk\ndk\android-ndk-r27d" ^
--extra_modules_path "C:\opencv_build\opencv_contrib-4.12.0\modules" ^
--modules_list "core,imgproc,objdetect,saliency,features2d,calib3d,flann,java,videoio,imgcodecs" ^
--no_samples_build ^
--disable GAPI ^
.

I inspected the generated .aar but couldn’t find any indication that the saliency module was included. Would it be useful if I share the build logs or the AAR file contents for verification?

[Akhil-Chaturvedi]

Hi, I investigated this issue (Java bindings missing for saliency).
The root cause was the missing WRAP java flag in the saliency module's CMakeLists.txt file.
I have submitted a Pull Request containing the fix: [#4018]
This PR includes the necessary configuration change to ensure the Java wrappers are generated for Android builds. This issue should be resolved once the PR is merged.

[Akhil-Chaturvedi]

@sturkmen72 Thank you very much! I actually did not see your suggestion on the original issue before I created this PR. Your comment was precisely the correct fix (adding WRAP java). This resolved the final compilation issue for the Android SDK build on my end. Appreciate the help and the confirmation!