A timeline is used to display and animate symbols. A timeline consists of one or more frames, each frame representing the content of the display at a particular time. Each SWF has a single main timeline. Additionally, each MovieClip symbol also has an associated timeline.
The timeline is used to add, remove and alter the position of symbols on an abstract stage. The stage is an unbounded coordinate space with an origin (0,0) and the positive y-axis below the origin (contrasting with the mathematical y-axis in which the positive y-axis is above the origin). The coordinate space of the main timeline is absolute with the origin always displayed in the top left-hand corner. As MovieClip symbols can be placed anywhere on a stage, the origin of the MovieClip stage is always set to the location where the symbol was placed.
All objects placed on the stage have an associated unique z-order depth associated with the operation. Symbols with a higher depth will be displayed on top of objects with a lower depth when these symbols overlap.
Frames are used to represent animations. Frames are visited at a rate per second defined for the overall SWF. If the rate is too small then animations may seem slow and jerky. On the other hand, if the frame rate is set too high, animations may occur too quickly to be seen, or may use too much CPU the client computer. A timeline consisting of a single frame can also be used - this is useful for grouping modular elements ad animation is still possible through ActionScript.
When multiple frames are used, symbols can be added, removed and altered on the stage at each frame. The contents of a single frame is the contents of the previous frame combined with all the frame operations defined on the frame itself. That is, once a symbol is added to the stage it remains on the stage for all subsequent frames until explicitly removed.
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