This iExpression computes the rotation of a wheel based on the movement of the layer. This can be used to rotate the wheels or a car or any other kinds of wheels that roll on the ground. This iExpression has the following limitations: (a) the wheel may only move along a straight line (for example horizontally, but any other direction is also possible). On this line it may move both forwards and backwards. (b) the size of the wheel may not change over time. If you have more complex animations where either (a) or (b) is not satisfied, use the iExpression "Rotating Wheel Curvaceous Motion" that can handle these cases but computes a bit slower.
the diameter of the wheel (in number of pixels)
Use Movement From
Choose if the rotation should be computed according to the movement of the layer itself or according to the movement of another point.
Link to Other Point
link here the other point, to whose movment the according rotation should be computed.
use ground layer
if the ground itself moves relative to the composition, use this option.
Ground Layer (Name or Index)
is the name or index of the ground layer relative to which the wheel should rotate.
Specify Diameter in
Specifies whether die diameter is given in composition or in layer space. Example: Say the wheel is a solid of size 100 pixels and is scaled to 200%. With the option 'layer coordinates' you should set the diameter to 100 but with the option 'comp coordinates' you should set it to 200, since from the comp's perspective, the solid has a size of 200 pixels.
Activate this option if the wheel rotates in the wrong direction. This may happen when the ground is above the wheel instead of below it.