Configure and Build OpenCV to Custom FFMPEG Install

It's years after the question was asked, but I recently ran into the kinds of issues described in this question! I'd like to add my input.

First: I only built the static ffmpeg libraries (for reasons), which the opencv build process is somewhat hostile towards. There have been posts by opencv developers stating that they don't support an opencv build against static ffmpeg libraries, but my thinking was "what if you're also building static opencv libraries? Certainly that should be supported?"

And the answer is yes, you can compile static opencv libraries against static ffmpeg libraries! I did this with opencv 4.1.1 against ffmpeg 4.2. I had to use the following cmake options:

cmake3 \
-D BUILD_opencv_apps=OFF \
-D BUILD_opencv_python2=OFF \
-D BUILD_SHARED_LIBS=OFF \
-D WITH_FFMPEG=ON \
-D OPENCV_FFMPEG_SKIP_BUILD_CHECK=ON \

These options result in a set of opencv static libraries that include whatever ffmpeg static libraries that you've also built, assuming your PKG_CONFIG_PATH is setup to correctly find those libraries (or if you've installed them to some default system location).

Q/A:

  • Why can't we build apps?
    • The opencv build process assumes that you are building and linking against shared libraries, even if you only build static libraries. This seems like a shortcoming in the build process rather than a technical limitation; perhaps some day this will be fixed.
  • What is the OPENCV_FFMPEG_SKIP_BUILD_CHECK flag?
    • There is a step in ffmpeg's cmake process where an application is built and linked using the same faulty build process as opencv's other applications. It's faulty in the sense that the process assumes that you're linking against shared libraries when it doesn't need to; you can perform the same build steps yourself while not making that assumption and create a working application. Thus, this build check needs to be turned off in order for opencv to accept that yes, in fact, any static libraries it finds are okay to use. This is
  • And why do you disable python support?
    • The opencv python build process explicitly produces a shared library loadable by Python.
  • Does this imply that you're still building the tests suite even with the static libraries?
    • Yes. But I haven't tried running them

Something like

export LD_LIBRARY_PATH=/ffmpeg_install_path/lib/
export PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/ffmpeg_install_path/lib/pkgconfig
export PKG_CONFIG_LIBDIR=$PKG_CONFIG_LIBDIR:/ffmpeg_install_path/lib/

should work. At least it works for OpenCV 2.4.x on my Ubuntu.


For OpenCV 3.x and ffmpeg 3.x, I have to apply the following patch

diff --git a/cmake/OpenCVFindLibsVideo.cmake b/cmake/OpenCVFindLibsVideo.cmake
index 13b62ac..bab9df3 100644
--- a/cmake/OpenCVFindLibsVideo.cmake
+++ b/cmake/OpenCVFindLibsVideo.cmake
@@ -228,6 +228,12 @@ if(WITH_FFMPEG)
     if(FFMPEG_libavresample_FOUND)
       ocv_append_build_options(FFMPEG FFMPEG_libavresample)
     endif()
+   CHECK_MODULE(libavcodec HAVE_FFMPEG)
+   CHECK_MODULE(libavformat HAVE_FFMPEG)
+   CHECK_MODULE(libavutil HAVE_FFMPEG)
+   CHECK_MODULE(libswscale HAVE_FFMPEG)
+   CHECK_MODULE(libswresample HAVE_FFMPEG)
+   CHECK_MODULE(libavresample HAVE_FFMPEG)
     if(HAVE_FFMPEG)
       try_compile(__VALID_FFMPEG
           "${OpenCV_BINARY_DIR}"

And with the following script to build

#!/bin/bash

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$HOME/Applications/ffmpeg/lib
export PKG_CONFIG_PATH=$PKG_CONFIG_PATH:$HOME/Applications/ffmpeg/lib/pkgconfig
export PKG_CONFIG_LIBDIR=$PKG_CONFIG_LIBDIR:$HOME/Applications/ffmpeg/lib

cmake \
    -D BUILD_EXAMPLES=ON \
    -D BUILD_TESTS=OFF \
    -D OPENCV_EXTRA_EXE_LINKER_FLAGS="-Wl,-rpath,$HOME/Applications/ffmpeg/lib" \
    -D CMAKE_BUILD_TYPE=RELEASE \
    -D CMAKE_INSTALL_PREFIX=$HOME/Applications/opencv \
    /path/to/opencv

Also, when build ffmpeg, I have to enable flags --enable-avresample.

Tags:

Linux

Linker

Ffmpeg

Shared Libraries

Opencv

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