Page 2 of 2 Every movie clip has its own timeline, which is a measure of the length of time a movie clip could possibly run. Every frame on the timeline (represented by the space between 'ticks' on the timeline ruler) is a unit of time. A keyframe is a "container in time" for objects to be present on the stage at that frame. If the object changes later in the timeline, we add a keyframe to indicate a change in the object. For instance, a motion tween for a graphic might be 10 frames long. We put a keyframe at the start, and a keyframe at the end of the tween to indicate the start and end times of the animation. Flash adds the intermediary keyframes of the animation when we publish to a SWF. If a keyframe has no objects on the stage at that frame, it is called a blank keyframe.
The frames per second (fps) value of the FLA document is defined as the measure of how many units of time, or frames, will elapse every second the SWF is run. A Hollywood movie reel, for example, will run at approximately 24 frames per second, or 24fps.
This can be set by selecting the Size button in the Property Inspector when nothing on the stage is selected, or by selecting Modify > Document > Frame rate, or Ctr-J (PC) / Cmd-J (Mac). Setting the fps of your FLA will determine the speed of your tweens and animations. All movie clips run at the fps rate of the FLA document.
About magic frame rate numbers: Recommended fps values for your FLA document are either 21 or 31, otherwise known as the "magic frame rate numbers". The reason for these values is that the default document setting of 12 fps is more often than not too slow for tween animations, resulting in "jerky" motion. So most of the time it needs to be higher. In addition, it has been found that two SWFs published to the same fps, will run slower on the Mac Flash Player than on the PC Flash Player, given equivalent processing power. Sometimes the lag time is quite noticeable. To counteract this "bug", we must set our FLA to either 21, 31, 41, 51 or 61 fps to negate this effect. But of course most computer processors cannot keep up with a frame rate higher than 31 fps, or performance issues in the Flash Player may occur. Thus it is advised that only frame rates of either 21 or 31 be used for most FLA documents. Only if you are running an FLV video player in your browser with the video at high resolution (larger than 320x240) would it be advisable to drop the frame rate of your FLV down to 12-15 in order to conserve performance. See Tom Green's CMX article for more details on video performance in Flash.
Within a movie clip we have what are known as layers. This should be immediately apparent upon looking at the timeline interface of the Flash authoring environment, in much the same way other programs such as After Effects, Photoshop and Fireworks have layers. Like these other applications, layers are a convention within the FLA to help us organize our assets so that we have a visual representation for which assets on the stage will be 'in front' and which will be 'behind' each other in the stacking order within a movie clip.
Every movie clip has an asset stacking order. When you export your FLA, under the hood of every SWF, every asset is given a depth number, whether they be Video, Text, Symbol, Button or Movie Clip objects. This exists so that, internally, the Flash SWF can keep track of the stacking order of all objects within a movie clip. Each child movie clip in turn has its own stacking order, its own depth organization. The organization of depths within a SWF is sometimes referred to as z-sorting, z-layering or depth management. We will explore the depth stack at greater length later in this series.
Now you have been exposed to some of the fundamental concepts of the movie clip architecture: movie clips, the timeline, layers and depths. Follow along in this series as we examine these and other concepts in greater detail.
Keywords
fla, swf, level, layer, frame, label, depth, architecture, movie clip, depth stack, drawing api, 5d, scenes, screens, slides, runtime, authortime, application
