Flash Basics: Understanding Frames and the Timeline
A frame is one moment of the animation. Dictating
each segment of time and movement, frames are at the core of any
animation. The number of frames in your movie and the speed at
which they're played back will determine your movie's overall
length. A movie set to play at 12 fps will display 12 frames in
one second, 24 frames in 2 seconds and so on. A twenty second
animation would have 240 frames.
You can adjust the properties of your move by going
to Modify-->Movie
Here you can adjust the frame rate, (how
many frames your animation displays per second), and the dimensions
of your stage (dimensions of final swf file) as well as background
color.
The timeline is full of different kinds of frames,
that represent different kinds of animation
Placeholder Frames:
Placeholder frames are not really frames but spaces
where frames can be placed. They are indicated by the timeline
grid. Devoid of content, these frames make up the majoritiy of
the timeline when you begin your Flash project.
Because your movie needs real frames on at least
one layer of the timeline to keep playing, the animation will
end once it reaches a point where all layers contain only placeholder
frames.
This movie will stop playing at
frame 20
Keyframes:
Keyframes are moments when the animation is to undergo
a visual change, or as you'll learn later on, when you want an
action to occur, or want to add a label.
This example has 7 keyframes, demonstrating the
different uses of keyframes in the timeline.
You can define every frame of your animation by
hand. This is called frame-by-frame animation and it requires
numerous keyframes since each frame is edited individually.
frame-by-frame animation
A tweened animation on the other hand, requires
only two keyframes-- one begins the tween and one that ends the
tween. Changes that occur between the beginning and ending keyframes
are calculated by Flash and thus do not require additional keyframes.
Although most keyframes contain content that is
visible on the stage, they can also be blank-- usually the result
of removinga movie element from the animation. Every new project
you begin in Flash starts with a blank keyframe on Frame 1 of
Layer 1.
A regular keyframe is identified by a solid black
dot.
A blank keyframe is identified by a small black
vertical line
A keyframe with an attached action is identified
with a small a.
Regular Frames:
Regular frames always follow keyframes and contain
the same content as the last keyframe on the same layer.
A keyframe on the timeline denotes a change, regular
frames proceeding a keyframe denote the duration of that change.
In this example, after the motion tween, the content
will still be visible on the stage from the end of the tween at
frame 14 through the end of the animation at frame 20.
Tweened Frames:
As mentioned above, a tweened animation requires
only 2 keyframes and Flash calculates the changes in between them.
Tweened frames are always part of a tween sequence consisting
of two keyframes and any number of frames in between. The frames
between the two keyframes represent computer-calculated graphics.
There are two types of tweening:
motion tweening- used to tween the size,
position, rotation, alpha, etc. of symbols, groups or text blocks
in your animation.
This is the timeline for the above motion
tweening animation.
Important Note:
shape tweening- used to morph one simple
shape into another-- for example, smoothly transforming a blue
circle into a red square.
This is the timeline for the above shape
tweening animation.
Go
on to Symbols, Tweening and Properties: 
