When compiled with video support ffmpeg licenses apply. Once you have dependencies installed, compile with cargo build -release -features=video or cargo build -release -features=video-static. If you're cross-compiling, try uncommenting section at the end of Cargo.toml, which includes some experimental fixes for ffmpeg. Monitor utilization of the IO subsystems, CPU and processor buses. Especially on macOS and Windows it takes expert knowledge to just get them installed without wasting several hours on endless stupid installation and compilation errors, which I can't help with. Please note that installation of these dependencies may be quite difficult.
Details depend on the platform and version, but you usually need to install packages such as libavformat-dev, libavfilter-dev, libavdevice-dev, libclang-dev, clang. You must have ffmpeg and libclang installed, both with their C headers intalled in default system include paths. The tool optionally supports decoding video directly, but unfortunately it relies on ffmpeg 4.x, which may be very hard to get working, so it's not enabled by default. Let me know if you'd like to use it in a product incompatible with this license. That is why I decided to use a paid API service like ImageOptim. It’s also not something you can do directly on a managed web server. Attempting to install all of the necessary command line tools for image optimization is really, really hard. With ImageOptim HTTP API, curl is the only prerequisite. I can offer alternative licensing options, including commercial licenses. That lead me to try out ImageOptim HTTP API. And link with target/release/libgifski.a.