Dynamic masking using the drawing API II - Page 4

This gives us the following movie:

First, a blank movie clip is created in which the line will be drawn. A setInterval action is called so that the animation will advance every 17 milliseconds. A variable is used to keep track of where we are in the animation: animIndex. In this case, the animation takes 60 steps. The time variable goes from 0 to 1 depending on the value of animIndex. The dist variable ('distance') is set to the value of time to create a linear animation (more on this later).

Since the dist variable takes all values between 0 and 1 an obvious set of formulas are used to move from point a to point b:

x = startX + dist*deltaX;
y = startY + dist*deltaY;

Here startX and startY are the starting coordinates and deltaX and deltaY are the x and y distance between the start and end points. So what we do in this script is apply this set of formulas to the start, control and end points. In this case, deltaX and deltaY are 0 for the start point, which further simplifies our code.