Projection mapping: correct perspective & crop a source (camera or screen) to map it onto a rectangular surface with corner pinning.
This extension is supposed to be used with a beamer/projector, preferably as a second monitor. You can aim the projector at a rectangular area, where you want your source feed to appear.
Use your webcam, your screen or any window open on your desktop as a source. You can correct the source (perspective, rotate, flip) by dragging each corner. Additionally you can crop out a rectangular area in case the source feed contains stuff you don't need.
Mappertje also integrates directly with my other extension: Pop-out Jitsi Meet:
When you've got both extensions installed you can also use all (remote) videos in the Jitsi conference as a source for projection mapping.