So far, we've been looking at the sorts of things that make any digital image seem more realistic, whether the image is a single "still" picture or part of an animated sequence. But during an animated sequence, programmers and designers will use even more tricks to give the appearance of "live action" rather than of computer-generated images.
How many frames per second?
When you go to see a movie at the local theater, a sequence of images called frames runs in front of your eyes at a rate of 24 frames per second. Since your retina will retain an image for a bit longer than 1/24th of a second, most people's eyes will blend the frames into a single, continuous image of movement and action.
If you think of this from the other direction, it means that each frame of a motion picture is a photograph taken at an exposure of 1/24 of a second. That's much longer than the exposures taken for "stop action" photography, in which runners and other objects in motion seem frozen in flight. As a result, if you look at a single frame from a movie about racing, you see that some of the cars are "blurred" because they moved during the time that the camera shutter was open. This blurring of things that are moving fast is something that we're used to seeing, and it's part of what makes an image look real to us when we see it on a screen.
However, since digital 3-D images are not photographs at all, no blurring occurs when an object moves during a frame. To make images look more realistic, blurring has to be explicitly added by programmers. Some designers feel that "overcoming" this lack of natural blurring requires more than 30 frames per second, and have pushed their games to display 60 frames per second. While this allows each individual image to be rendered in great detail, and movements to be shown in smaller increments, it dramatically increases the number of frames that must be rendered for a given sequence of action. As an example, think of a chase that lasts six and one-half minutes. A motion picture would require 24 (frames per second) x 60 (seconds) x 6.5 (minutes) or 9,360 frames for the chase. A digital 3-D image at 60 frames per second would require 60 x 60 x 6.5, or 23,400 frames for the same length of time.
The blurring that programmers add to boost realism in a moving image is called "motion blur" or "spatial anti-aliasing." If you've ever turned on the "mouse trails" feature of Windows, you've used a very crude version of a portion of this technique. Copies of the moving object are left behind in its wake, with the copies growing ever less distinct and intense as the object moves farther away. The length of the trail of the object, how quickly the copies fade away and other details will vary depending on exactly how fast the object is supposed to be moving, how close to the viewer it is, and the extent to which it is the focus of attention. As you can see, there are a lot of decisions to be made and many details to be programmed in making an object appear to move realistically.
There are other parts of an image where the precise rendering of a computer must be sacrificed for the sake of realism. This applies both to still and moving images. Reflections are a good example. You've seen the images of chrome-surfaced cars and spaceships perfectly reflecting everything in the scene. While the chrome-covered images are tremendous demonstrations of ray-tracing, most of us don't live in chrome-plated worlds. Wooden furniture, marble floors and polished metal all reflect images, though not as perfectly as a smooth mirror. The reflections in these surfaces must be blurred -- with each surface receiving a different blur -- so that the surfaces surrounding the central players in a digital drama provide a realistic stage for the action.