How to compile OpenCV iOS with ENABLE_BITCODE

Question

When I tried to compile my XCode project with OpenCV 2.4 iOS using XCode 7 + iOS SDK 9, XCode complained that

ld: \'opencv2.framework/opencv2(alloc.

Answer 1

After some search and trial, I figured out a way to compile OpenCV iOS from the source with Bitcode. A compiled binary is also provided here: [v3.0] [v2.4]. [Disclaimer: I am not responsible for the integrity of the compiled binary. Use at your own risk.]

The steps of compilation is basically the same as the official document, with only one extra step.

1. Download the code with git:

   cd ~/<my_working_directory>

   git clone https://github.com/Itseez/opencv.git

2. Make symbolic link for Xcode to let OpenCV build scripts find the compiler, header files etc.

   cd /

   sudo ln -s /Applications/Xcode.app/Contents/Developer Developer

3. [Key Step] Change the compilation script to add the extra option for Bitcode: edit ~/<my_working_directory>/opencv/platform/ios/build_framework.py, and locate the line containing -DCMAKE_C_FLAGS. Add a flag of -fembed-bitcode. For example, in the source I got, it's line 55, and will look like

   "-DCMAKE_C_FLAGS=\"-Wno-implicit-function-declaration -fembed-bitcode\" " +

   after the change. [ref]

4. Build OpenCV framework:

   cd ~/<my_working_directory>

   python opencv/platforms/ios/build_framework.py ios

   If everything’s fine, a few minutes later you will get ~/<my_working_directory>/ios/opencv2.framework. You can add this framework to your Xcode projects.

P.S. Ask a question, even when you already know the answer is encouraged according to this post on Meta Stackchange.

Answer 2

OpenCV is precisely the kind of software (along with audio and video Codecs) that is likely to have hand-rolled ARM NEON optimisations. The documentation suggests that ~40 functions have had this treatment in OpenCV3.0.

If compiling for LLVM bit-code you'll get the generic (less optimised, implemented in C or C++) versions instead.

Use of Bitcode is optional - except when compiling for Apple watch, where it's hard to imagine you'd run computationally complex image processing anyway. If you're bundling a watch app, override the build setting for bitcode on it only.