How do you rotate left and right expressions in After Effects?

To create rotation expressions in After Effects that control left and right rotation, you typically use conditional logic based on another property's value, or incorporate the time variable with mathematical functions.

Here's a breakdown of how you can achieve both constant rotation and rotation controlled by another parameter:

1. Constant Rotation using the time Variable (Left or Right):

This is the basic approach described in the reference, but clarifying left/right is key. After Effects interprets positive values as counter-clockwise (typically seen as "left") and negative values as clockwise (typically seen as "right").

• Select the Layer: Choose the layer you want to rotate.

• Access the Rotation Property: Press R on your keyboard to reveal the Rotation property.

• Add the Expression: Alt-click (Option-click on Mac) the stopwatch icon next to "Rotation" to enable expressions.

• Enter the Expression: Type the following expression into the expression editor:

  time * 300; // Rotates counter-clockwise (left) at 300 degrees per second

  To rotate the other direction, use a negative value:

  time * -300; // Rotates clockwise (right) at 300 degrees per second

  • time: Represents the current time in seconds of the composition.
  • 300: This is the speed of rotation in degrees per second. Adjust this value to change the rotation speed.

2. Rotation Controlled by Another Parameter (Left or Right Based on Slider):

This method uses a Slider Control to determine the direction and speed of rotation.

• Add a Slider Control: Select your layer. Go to Effect > Expression Controls > Slider Control.

• Add the Expression to Rotation: Alt-click the stopwatch next to "Rotation."

• Enter the Expression:

  sliderValue = thisComp.layer("Your_Control_Layer_Name").effect("Slider Control")("Slider");
  time * sliderValue;

  • Replace "Your_Control_Layer_Name" with the actual name of the layer containing the Slider Control.
  • sliderValue now stores the value from your Slider Control. Positive values will rotate counter-clockwise, negative values clockwise, and 0 will stop the rotation. You can keyframe the slider to change rotation direction over time.

3. Using Math.sin or Math.cos for Oscillating Rotation (Left and Right):

This creates a back-and-forth rotating motion.

• Add the Expression: Alt-click the stopwatch next to "Rotation."

• Enter the Expression:

  amplitude = 45; // Maximum rotation angle
  frequency = 1;   // Speed of oscillation (cycles per second)
  
  amplitude * Math.sin(time * frequency * 2 * Math.PI);

  • amplitude: Determines the maximum angle of rotation in degrees. In this example, it will rotate +/- 45 degrees.
  • frequency: Controls how fast the object oscillates back and forth. A value of 1 means one complete cycle per second.
  • Math.sin(): A trigonometric function that produces a smooth, oscillating value between -1 and 1.
  • 2 * Math.PI: Converts the frequency to radians per second.

4. Combining Expressions:

You can combine these techniques. For example, you could use a Slider Control to set the overall speed and direction of rotation, and then use Math.sin() to add a subtle wobble.

Important Considerations:

• Anchor Point: The position of the layer's anchor point significantly impacts the visual result of the rotation.
• Units: Rotation values are in degrees.
• Troubleshooting: If your expression isn't working, double-check your syntax, layer names, and property names. The Expression Editor often highlights errors.

These techniques give you precise control over rotational animation within After Effects. By understanding the underlying principles of time-based expressions and conditional logic, you can create complex and dynamic motion graphics.