How to configure Qt 6.5.1 QtMultimedia to use custom FFmpeg installation?

Martin Burchell

@eramne This is working for me (Ubuntu 20.04), in so much as Qt detects FFmpeg during configuration:

cd /home/martinb/workspace/qt6_local_build/ffmpeg_linux_x86_64_build/FFmpeg-n6.0

/home/martinb/workspace/qt6_local_build/ffmpeg_linux_x86_64_build/FFmpeg-n6.0/configure --disable-programs --disable-doc --disable-debug --enable-network --disable-lzma --enable-pic --prefix=/FFmpeg-n6.0
make
make install DESTDIR=/home/martinb/workspace/qt6_local_build/ffmpeg_linux_x86_64_build/FFmpeg-n6.0/installed/FFmpeg-n6.0

/home/martinb/workspace/qt6_local_build/src/qt6/qtbase/configure -top-level -prefix /home/martinb/workspace/qt6_local_build/qt_linux_x86_64_install -static -gstreamer -I /home/martinb/workspace/qt6_local_build/openssl_linux_x86_64_build/openssl-1.1.1s/include -L /home/martinb/workspace/qt6_local_build/openssl_linux_x86_64_build/openssl-1.1.1s -ccache -no-sql-sqlite -no-sql-db2 -no-sql-ibase -no-sql-mysql -no-sql-oci -no-sql-odbc -no-sql-psql -qt-doubleconversion -qt-libjpeg -qt-libpng -qt-zlib -no-warnings-are-errors -nomake examples -nomake tests -release -openssl-linked -- -DOPENSSL_USE_STATIC_LIBS=ON -DOPENSSL_ROOT_DIR=/home/martinb/workspace/qt6_local_build/openssl_linux_x86_64_build/openssl-1.1.1s -DFFMPEG_DIR=/home/martinb/workspace/qt6_local_build/ffmpeg_linux_x86_64_build/FFmpeg-n6.0/installed/FFmpeg-n6.0

I have a problem further down the line whereby the linker is trying to link the FFmpeg libraries both with and without the lib prefix. I'm currently investigating this.

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nevermind the gstreamer backend is not working
giving me "Internal data stream error." when i try to play most files, other files it just crashes
back to trying to get the ffmpeg backend to work i guess

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that's weird, since i have all the libav/ffmpeg libraries built and installed at /usr/local/FFmpeg-n6.0, including libavdevice, libavfilter, and libpostproc

this is after running:
./configure -prefix "/home/eramne/Qt6.5.1/" -release -openssl -openssl-linked -no-widgets -submodules qt5compat,qtbase,qtdeclarative,qtimageformats,qtmultimedia,qtshadertools,qtsvg,qttranslations -DFFMPEG_DIR=/usr/local/FFmpeg-n6.0/

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maybe its just trying to use the ffmpeg packages i have installed via apt instead of the ones i've built and linked to with -DFFMPEG_DIR=/usr/local/FFmpeg-n6.0/? If so, how do i stop it from doing that?

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yeah, it's doing that. I tried building/installing ffmpeg 5.1, using the -DFFMPEG_DIR thing to point to that, and in the output of the configure script it still showed that it's using the same versions of the av/ffmpeg libraries

./configure -prefix "/home/eramne/Qt6.5.1/" -top-level -release -openssl -openssl-linked -no-widgets -submodules qt5compat,qtbase,qtdeclarative,qtimageformats,qtmultimedia,qtshadertools,qtsvg,qttranslations -DFFMPEG_DIR=/home/eramne/Desktop/ffmpeg/installed/ffmpeg-5.1
./configure -redo

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i modified the https://github.com/qt/qtmultimedia/blob/6.5.1/cmake/FindFFmpeg.cmake file like this

### Macro: set_component_found
#
# Marks the given component as found if both *_LIBRARIES AND *_INCLUDE_DIRS is present.
#
macro(set_component_found _component )
  if (${_component}_LIBRARIES AND ${_component}_INCLUDE_DIRS)
    # message(STATUS "  - ${_component} found.")
    set(${_component}_FOUND TRUE)
    set(${CMAKE_FIND_PACKAGE_NAME}_${_component}_FOUND TRUE)
  else ()
    # message(STATUS "  - ${_component} not found.")
  endif ()
endmacro()

find_package(PkgConfig QUIET)
#if (NOT PKG_CONFIG_FOUND AND NOT FFMPEG_DIR)
#    set(FFMPEG_DIR "/usr/local")
#endif()
set(FFMPEG_DIR "/home/eramne/Desktop/ffmpeg/installed/ffmpeg-6.0")

to explicitly force it to use my own ffmpeg installation
and that worked

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i guess cmake variables are separate from environment variables
since i tried using export FFMPEG_DIR=/home/eramne/Desktop/ffmpeg/installed/ffmpeg-6.0, but that didn't work
i guess i probably could've used the configure.cmake file in some way to set that variable? but i don't use cmake and i don't know how to do that, and i didn't feel like figuring out how

Martin Burchell

@eramne You need the -- in the Qt configure line to separate the CMake options from the Qt ones:

./configure -prefix "/home/eramne/Qt6.5.1/" -top-level -release -openssl -openssl-linked -no-widgets -submodules qt5compat,qtbase,qtdeclarative,qtimageformats,qtmultimedia,qtshadertools,qtsvg,qttranslations -- -DFFMPEG_DIR=/home/eramne/Desktop/ffmpeg/installed/ffmpeg-5.1

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@Martin-Burchell oh! thanks for this

Martin Burchell

@eramne Is it working for you now? I've got a problem linking FFmpeg with static Qt, which I've logged at https://bugreports.qt.io/browse/QTBUG-115052 but if it's working for you, feel free to mark this as Solved