Looking at the Flash timeline interface, you can see that it derives from traditional cel animation. You see layers, numbered frames, and what look like spreadsheet cells, analogous to the cels traditionally used to separate characters and scenery.
In the traditional Disney and Warner Brothers style, characters are inked and painted onto celluloid, then photographed over an illustrated scene — frame by labor- intensive frame. Cels were see-through layers of celluloid that could provide individual control of different elements composed within a scene. Backgrounds were drawn as needed.
From Cels to Layers: Lifts and separates
In Flash animation, layers on the bottom usually hold a stationary background, while other layers above hold the pieces of action. Each plane, however, can serve several functions, moving a graphic along a path, revealing its content over time by taking away masking, playing audio, even holding the animator’s production notes. Layers can be named, locked, and organized into folders. Keeping everything separate means greater freedom to build and edit a scene.
24 Drawings per second? Not likely.
You could animate frame by frame in Flash, but I doubt anyone does. Cel animation means 24 hand-drawn and painted characters per second, a technique that can keep artists employed, but that few want to pay for any more. The closing of the last Disney 2D animation studio was the subject of a discussion on Slashdot.
Creating animation with Flash has more in common with the stop motion paper cutouts of Monty Python’s Terry Gilliam than it has with Bambi and Donald Duck. In these paper-puppet productions, one cutout drawing of an arm or head is used on camera throughout a scene, sliding and rotating between frames to create the action.
The character functions a lot like the flat pose-able scarecrows with the hinged limbs that you see taped to picture windows on Halloween. Flash’s use of Symbols and Instances deftly exploits this idea of “one image throughout.” Beyond sliding or rotating an image, you can have multiple copies of the same image on stage at one time, and change their properties of size, color, and transparency, all over time. In other words, you can create a snowstorm with one snowflake.
But it would be pretty tedious to create a layer for every snowflake and add randomness to its downward movement. For that sort of thing, you can use Flash’s programming language, ActionScript. Here’s a tutorial on ActionScripting snow.
From optical to digital: The art of motion
Flash allows you to incorporate traditional rules of thumb for animated motion. Like easing in and easing out on an action–that’s slow to start, fast in the middle, slow to finish. Put a little stretch and squash to your objects during that action to give them a sense of weight and gravity response. These techniques, as the pros who defined and perfected them learned decades ago, are more felt than seen yet give the motion zest and punch. Most importantly, it will keep your animations from looking robotic and PowerPointy.
Watch out for bitmaps
Most Flash images are vector drawings, meaning they’re stored as instructions for drawing the screen rather than as bitmaps containing a set number of pixels. This means they can be scaled to any size and will still look crisp. In our work, we use a fair number of bitmaps, typically screens depicting a software interface. These images look their best at the size they were captured at, which is something that should be taken into account when you’re planning to use them with a web page or PowerPoint presentation.
— AC