I want to create an educational introductory video, where the tutor stands and explain a subject, and the background is an illustration of that subject. This is done using a technique called Chroma Key, where a blue or green curtain is placed in the video background, and replaced with another image or video at post-production.
Is there a free tool that can help me merge my videos using Chroma key?
Clarification: I prefer open-source solutions which work well under Ubuntu.
Reference: This question deals with Chroma key, but it's about real-time video (which I don't need) and the answers suggest proprietary software.