If the observer was not registered, nothing happens. If the observer was registered for multiple notifications, it will now be registered for one fewer. The WritableRenderedImage is responsible for notifying all of its TileObservers when a tile goes from having no writers to having one writer. If the caller continues to write to the tile, the results are undefined. The WritableRenderedImage is responsible for notifying all of its TileObservers when a tile goes from having one writer to having no writers.
Returns null if none are checked out.
The operation is clipped to the bounds of the WritableRenderedImage.