In his book (Digital) Lighting & Rendering, Jeremy Birn (2000, p63) states that “shadows serve a practical purpose in most scenes by showing the spatial relationship between objects. They show where an object is planted on the ground or how far an object is located above the ground”. Without shadows it is difficult to interpret the spatial relationship between objects in a scene. Looking at the image below, it is difficult to tell if the balls are floating in space or resting on the cabinet.
Birn (p66) goes on to say that “when asking the audience to accept a scene that would otherwise strain its credibility, convincing shadow interaction can add an important piece of reality to help sell the illusion. If a production is supposed to be completely photorealistic, a single element such as a missing shadow could be all it takes to make your work look ‘wrong’ to the audience. Shadows serve the interest of adding realism and believability, even if there is no other reason for them in the composition”.
In the image below, although the lighting on the spider matches the rest of the scene, the absence of a shadow being cast onto the ground spoils the illusion.
In the following image, the ground shadow below the spider helps cement the relationship between the environment and the 3D model.
The practice of adding a shadow to support the illusion is something I had commented on previously through my own observations, however, something that I hadn’t previously considered but is demonstrated by Birn is the use of casting shadows by objects not visible on-screen. In the following image, a cookie has been created to cast shadows over both the spider and the environment thus cementing the real and virtual elements together even further.