Fast Render 3D Animation by using 2D Textures masks.
This example uses an image from a 3D demo in the Blender web site (Classroom by Christophe Seux CC0).
Uses Blender 3.X but should work in 2.93 as well.
Rendering with a 2D environment is much much faster. In this example the original 3D background scene (100MB) rendered one frame in 2 minutes but using this one frame as a 2D environment the entire animation of 600 frames rendered in 2 minutes.
The high speed is useful for storyboards, pre-visualization, and drafts before committing to long render times of a full 3D version (or maybe instead of full 3D). Add additional effects like multiple camera views, zoom/pan in a video editor or use HDRi backgrounds, be creative.
One aim was to make some parts of the 2D background image appear in front of 3D rendered objects. An advantage of a mask texture is that it is easy to deal with foreground objects that have holes in them (for example, chairs).
A second aim was to manage the 3D shadows eg prevent 3D shadows from making existing 2D shadows darker.
The story: A teacher sees something outside the classroom and goes to investigate.
One challenge here was to animate an 'actor' that has no moving parts at all. The walk animation is done with some Sine waves generated by F-curve modifiers. The eye blink is done just by changing the color of the eyes.
Lots of room here for further experimentation. It can work with HDRi backgrounds but that needs a second viewlayer to get a clean background and a slightly different compositor set up.
Seeblend 25/1/2022 CC-BY