The Strategy

Instead of manually downloading dependencies and setting environment variables, I instruct antigravity to designed a CMakeLists.txt that automates everything:

1. Fetch FFmpeg: Download prebuilt shared binaries (DLLs) from a reliable source.

2. Fetch OpenCV: Download the official OpenCV source code.

3. Configure: Force OpenCV to use our downloaded FFmpeg and build shared libraries.

4. Install: Package everything (OpenCV DLLs + FFmpeg DLLs) into a single dist folder.

Key Build Files

build.bat - The build script that runs cmake

This script initializes the environment and runs the build. A critical detail I fixed was ensuring it runs from the correct directory to allow relative paths in CMake.

@echo off
setlocal

# Critical fix: Change directory to script location to avoid path issues
cd /d "%~dp0"
set BUILD_DIR=build
set INSTALL_DIR=dist

echo Configuring...
cmake -G "Visual Studio 18 2026" -B "%BUILD_DIR%" -S . -DCMAKE_INSTALL_PREFIX="%INSTALL_DIR%"

echo Building...
cmake --build "%BUILD_DIR%" --config Release

echo Installing...
cmake --install "%BUILD_DIR%" --config Release

cmake/FFmpeg.cmake - Handling Dependencies

I used FetchContent to download prebuilt FFmpeg binaries. The key here was installing the FFmpeg DLLs directly alongside the OpenCV binaries so the final application runs without "DLL not found" errors.

FetchContent_Declare(
  ffmpeg_prebuilt
  URL "https://github.com/BtbN/FFmpeg-Builds/releases/.../ffmpeg-master-latest-win64-gpl-shared.zip"
)
FetchContent_MakeAvailable(ffmpeg_prebuilt)

# Important: Install FFmpeg DLLs to the bin folder
file(GLOB FFMPEG_DLLS "${ffmpeg_prebuilt_SOURCE_DIR}/bin/*.dll")
install(FILES ${FFMPEG_DLLS} DESTINATION bin COMPONENT libs)

cmake/BuildOpenCV.cmake - Resolving Conflicts

During the build, I encountered two major issues:

1. vcpkg Conflict: The system had vcpkg installed, and its Protobuf library conflicted with OpenCV's internal Protobuf.

2. IPP Errors: The Intel Performance Primitives (IPP) module in OpenCV 4.12.0 caused build failures.

I solved both by disabling the conflicting/unnecessary modules in our CMake configuration:

# Fix for vcpkg/protobuf conflict
set(WITH_PROTOBUF OFF CACHE BOOL "Disable Protobuf" FORCE)
set(BUILD_opencv_dnn OFF CACHE BOOL "Disable DNN" FORCE)

# Fix for IPP build failures
set(WITH_IPP OFF CACHE BOOL "Disable IPP" FORCE)
set(WITH_OPENCL OFF CACHE BOOL "Disable OpenCL" FORCE)

# Point OpenCV to our FFmpeg
set(ENV{FFMPEG_DIR} "${FFMPEG_ROOT}")
set(FFMPEG_DIR "${FFMPEG_ROOT}" CACHE PATH "Path to FFmpeg" FORCE)

3. Verification

I created a separate test project (OpenCV_FFmpeg_Test) to confirm the build works. Works

• The Test: A simple C++ program that prints CV_VERSION and avcodec_version().

• The Link: I had to point find_package(OpenCV) specifically to the dist/lib folder because the root dist/OpenCVConfig.cmake was a dispatcher that failed with our custom folder structure.

cmake_minimum_required(VERSION 3.24)
project(OpenCV_FFmpeg_Test)

set(CMAKE_CXX_STANDARD 17)

# Point to our build artifact directory
set(OPENCV_DIST_DIR "c:/projects/opencv/dist")

# 1. Find OpenCV
# We tell CMake where to look for OpenCVConfig.cmake
# Pointing to the 'lib' folder because the root dispatcher might fail
find_package(OpenCV REQUIRED PATHS "${OPENCV_DIST_DIR}/lib" NO_DEFAULT_PATH)

message(STATUS "Found OpenCV: ${OpenCV_VERSION}")
message(STATUS "OpenCV Include Dirs: ${OpenCV_INCLUDE_DIRS}")

# 2. Setup FFmpeg (Manual setup since we just copied DLLs/headers)
set(FFMPEG_INCLUDE_DIR "${OPENCV_DIST_DIR}/include/ffmpeg")
set(FFMPEG_LIB_DIR "${OPENCV_DIST_DIR}/lib/ffmpeg")

# Helper to find FFmpeg libs in our dist folder
file(GLOB FFMPEG_LIBS "${FFMPEG_LIB_DIR}/*.lib")

add_executable(version_test main.cpp)

# Header includes
target_include_directories(version_test PRIVATE 
    ${OpenCV_INCLUDE_DIRS}
    ${FFMPEG_INCLUDE_DIR}
)

# Linking
target_link_libraries(version_test PRIVATE 
    ${OpenCV_LIBS}
    ${FFMPEG_LIBS}
)

# Copy DLLs to output directory for running
add_custom_command(TARGET version_test POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E copy_directory
        "${OPENCV_DIST_DIR}/bin"
        "$<TARGET_FILE_DIR:version_test>"
    COMMENT "Copying DLLs to executable directory..."
)

Conclusion

By using a scripted CMake approach, we created a reproducible build process that:

1. Is independent of system-wide installations.

2. Automatically handles FFmpeg dependencies.

3. Produces a clean dist folder ready for distribution.

The Antigravity agent helped identify the build errors (protobuf conflicts, IPP issues) and rapidly iterated on the CMake scripts to produce a working solution.