The SafeFrame feature was previously available as a separate plugin on the [niklasrosenstein.com] online store and is now integrated into NR Workflow Tools.
Original Plugin Description
It is a common problem with Cinema 4D when doing renders in the Viewport and not the Picture Viewer, that more area of the final image is visible than in the production renders. SafeFrame approaches this problem by masking out these areas!
Usually, the Cinema 4D Viewport Rendering would fill all the available space in the Viewport.
With SafeFrame enabled, the superfluous space will be rendered black.
You can also choose to add a border, change the color and alpha.
Disclaimer
The SafeFrame plugin does not make rendering faster. Unfortunately, it is not possible to tell Cinema 4D to skip certain parts of the image. Also note that this plugin only works with the Standard and Physical renderer.