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flip books

A flip book contains a sequence of pictures that change gradually from one page to the next, so that when the pages are turned rapidly, they seem to produce a continuous animation. Flip books have been around since before the 1870's, when they became very popular in Europe and America. One of my favorite modern flip books is shown in the video above by Matthew Schlian. Movies work in much the same way, except that they close a shutter in between frames so that we don't see the frame changes.

visual persistence

Both movies and flip books depend on a feature of human vision called "visual persistence". When light shines on the retina, chemical and electrical signals are sent to the brain, providing the basis for our sense of vision. When the light is turned off, the brain continues to receive signals from the retina while the processes that produce these signals shut down, and as the signals already produced complete their journey to the brain. This continued visual signal, which causes a positive afterimage even after our eyes are closed, is known as persistence.

Visual persistence in the retina explains why we don't see the dark intervals between movie frames, but the physiological basis for our sense of motion is rather more subtle. When we look at a flip book we can still get a sense of motion, even though the "frame changes" are not hidden from us. Apparently, there are processes in our brains that try to interpret a sequence of discrete images as continuous motion even when we can see the intervals between images. The physiological mechanisms that accomplish this feat are still the subject of some speculation and research.

A common rule of thumb in the movie and animation industries states that in order to make a movie appear truly continuous, it needs to play at least 16 frames per second. Accordingly, most movie projectors play at either 24 or 30 frames per second. The animation in figure 1 plays at 12 frames per second, and is visibly jerky.

make your own flip book

activity — print attached flip book
Print out the attached flip book to view. There are files for several different paper sizes, including 5" by 8" and 3" by 5" cards. A flip book works best on fairly stiff paper; card stock is good. Line up all the pages carefully and bind one side of the book with binder clips. Flip away.

activity — make your own flip book
It's pretty easy to make your own flip book, though it may require some patience. Follow the guidelines at wikiHow to make a flip book from your own drawings, or the photoshop instructions at photojojo to make a flip book from a video.

question to ponder

To provide a sense of continuous motion in a flip book, is it more important to make image frames that differ only a little bit, or is it more important to play the images quickly? Does one effect compensate for the other? Try speeding up and slowing down the flip rate to see how it affects your sense of continuous motion.

further resources

For a really slick example of a flip book, check out the video by TheIllustrationArt of the rock group Winter Gloves.

Pascal Fouché has put together a comprehensive history of the flip book at http://www.flipbook.info/index_en.php.