HTML and CSS Reference
In-Depth Information
Frames are a series of discrete images shown in rapid succession to simulate motion or change in an
object. Frames are the basis for anything you see on a computer, television, or movie screen. This idea
goes back to the earliest cartoons. Animators drew individual pictures on sheets of cellophane (known as
cells)
, and the earliest motion pictures used a similar technique with multiple photographs.
The concept is simple: You show a bunch of images that vary slightly from one to the next, and the mind
blurs them together as a single, moving image. But why do we insist on calling it an
illusion of motion
? If
you see a man walk across the room on a movie screen, is that not motion? Of course it's only an image of
a man, not a real person, but that's not the reason we don't consider it motion.
Remember, moving objects travel from a point here to a point there by passing through the intervening
space. That is real motion; objects move through space smoothly, not in several jumps. But frame-based
motion does just that. It doesn't move from spot to spot, it disappears and reappears in another location in
the next frame. The faster it's moving, the bigger jump it takes.
If you were shown a picture of a man on the left side of a room and then a few seconds later another
picture of the same man on the right side of the room, you'd infer that there are two pictures, not an
animation. If you were shown a half dozen pictures of the man in the process of crossing the room, you'd
still interpret these as a series of individual photographs. (See Figure 1-1 for an example of this.) If the
images were presented fast enough, it wouldn't change the fact they remain a bunch of still photos, but,
you would no longer see it that way. Your mind will process it all as a man moving across the room. It is no
more real motion than the original two photos were, but at some point, the mind gives up and buys into the
illusion.
Figure 1-1.
A series of still photographs by Eadweard Muybridge
This point has been extensively examined by the film industry. Researchers have found that at a rate of 24
frames per second, people will accept these frames as a single moving image. Go much slower than that,
and the jumpiness gets annoying and starts to break the illusion. And it seems that the human eye can't
distinguish frame rates higher than that—showing 100 frames per second won't make your animation
seem any more realistic (although higher frame rates in a programmed animation can result in more
responsive interaction and will seem smoother).
