Using Virtual Reality Concepts - Flash 3D: Animation, Interactivity, and Games - page 285

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15
16

// define the total frames in the object movie clip
var totalFrames:Number = object_mc._totalframes;

Step 5: Rotate the object on its circular path

It's time to get the object rotating about its own axis as it follows the circular path.
Let's begin with line 72 even though no changes are necessary here. We need to
update the angle increment angleChange , which is the amount that the angle changes
as it moves in its circular path. This amount is based on how far the cursor is from the
center of the Stage. This difference is divided by speedfactor to control how fast the
object moves. The value of angleChange is then added to startAngle to determine the
location of the object on the circular path.

70
71
72

// update the angle increment based on how far
// _xmouse is from the center of the Stage
var angleChange:Number = Math.round((_xmouse - xo)/
speedfactor);
startAngle += angleChange; 73

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75
76

// use the angle to set the frame position
thisFrame = 1 + Math.round(startAngle*totalFrames/360)%
totalFrames;

Line 76 is a bit tricky. We want to create a relationship between startAngle and a
corresponding frame in object_mc , which we'll call thisFrame . What we need is that
the ratio of startAngle to 360 degrees is the same as the ratio of thisFrame is to
totalFrames . Expressed as an equation, we have

startAngle/360 = thisFrame/totalFrames

from which we get

thisFrame = startAngle*totalFrames/360

We can apply the modulo (%) function to the quantity on the right above so that only
the numbers in the range -( totalFrames-1 ) through +( totalFrames-1 ) are gener-
ated. In our specific case, this means the numbers -35 through +35, including 0.
Since our object has 36 frames, we need to add 1 to thisFrame .