Unexpected attribute values

 
 
 

As you work with expressions, you’ll sometimes see attribute values you didn’t expect. The following topics describe a few common causes of confusion.

Important

Always examine the Script Editor for error messages after you edit an expression and click the Create button. If you alter a previously successful expression and a syntax error occurs, Maya executes the previous successful expression when you play the animation. This might lead you to believe your editing changes took effect.

Values after rewinding

When you rewind a scene, an expression executes with the last settings made for attribute values. This sometimes gives unexpected results.

Example

Ball.tx = $distance;
$distance = time;

Assume for this example you’ve set the starting frame of the animation to frame 0.

The first statement sets Ball.tx to the variable $distance. The second statement sets $distance to the value of time.

When you play the animation, Ball moves along the X-axis with the increase in time. Ball’s X-axis position is 4 grid units, for example, when animation time equals 4 seconds.

When you rewind the animation, Ball’s position along the X-axis doesn’t return to 0 as you might assume. The previous execution of the expression at time equals 4 set the $distance variable to 4. So rewinding sets Ball.tx to 4, then sets the value of $distance to 0, the value of time upon rewinding.

If you rewind again, Ball’s position along the X-axis returns to 0 as desired. Because the previous execution of the expression upon rewinding set the $distance to 0, the expression now correctly sets Ball.tx to 0.

To fix this problem, reverse the order of the statements and compile the expression:

$distance = time;
Ball.tx = $distance;

After you play and rewind the expression, the first statement executes and assigns the time to $distance. The next statement assigns Ball.tx the value of $distance, which the first statement set to the value of time. Because $distance is set to 0 as the first statement after rewinding, Ball returns to the desired translateX position.

Increment operations

If you increment an attribute or variable during animation, you might be confused by its behavior.

Example

Ball.ty = 0;
Ball.ty = Ball.ty + 1;

Ball’s translateY position stays at 1 unit along the Y-axis. Ball’s translateY position doesn’t increase by 1 each frame as the animation plays.

Example

Ball.ty = Ball.ty + 1;

Ball’s translateY position increases by 1 each frame as you play the animation. When you rewind the animation, translateY increases by 1 again.

When you play the animation again, the translateY position increases by 1 each frame. If you rewind the animation or drag the current time indicator, the translateY position continues to move up the Y-axis. The attribute never returns to its original position.

To return Ball to a starting position each time you rewind, you must initialize the attribute to a starting value. For example, you could use the following expression:

Ball.ty = Ball.ty + 1;
if (frame == 1)
	Ball.translateY = 0;

This returns Ball to a Y position of 0 when you rewind to frame 1. When you drag the current time indicator, though, Ball doesn’t return to its Y position of 0.

The if statement resets the value of translateY to 0 only when frame 1 plays. Frame 1 is the default frame that plays when you rewind an animation. You would need to use a different frame number in the if statement if you’ve set your animation to start at a different frame.

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