FFmpeg filter_complex: How to Handle Multi-Input Video Processing

FFmpeg's filter_complex flag extends the tool's capabilities beyond the single-stream limitation of the standard -vf flag, enabling developers to work with multiple inputs and outputs in one command. It uses bracket-based stream labels such as [0:v] and [1:v] to route video and audio streams through a processing graph. Filters can be chained with commas for sequential operations, while semicolons separate independent filter chains within the same command. Common use cases include picture-in-picture overlays, text rendering after scaling, and combining footage from separate source files. Understanding the label-routing syntax is central to using filter_complex effectively, as it dictates how each stream is directed through the processing pipeline.