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Temporary Parenting

Expression Fri, 08/29/2008 - 18:12 — FredLewis
Special thanks to: 
Dan Wilk, David Simons and the entire AE team for pointers and suggestions

You can use this expression instead of parenting, to temporarily attach one object to another during only part of an animation.

expression: 

Position Expression:
Apply this expression to the position channel for the null. Set "layertofollow" to the name of the parent layer. Set pickup and dropoff time values to times for pickup and dropoff, in seconds.

pickuptime = 2.5;
dropofftime = 6.0; layertofollow = "eagle" ticker =
Math.min(Math.max(pickuptime, time), dropofftime);
this_comp.layer(layertofollow).position.value_at_time(ticker)

Rotation Expression:
Apply this expression to the rotation channel for the null. Set "layertofollow" to the name of the parent layer. Set pickup and dropoff time values to times for pickup and dropoff, in seconds.

pickuptime = 2.5;
dropofftime = 6.0;
layertofollow = "parent_layer_name";

ticker = Math.min(Math.max(pickuptime, time), dropofftime);
this_comp.layer(layertofollow).rotation.value_at_time(ticker)

Instructions: 

1. Create a parent layer, a child layer, and a null layer.
2. Apply the expressions to a null layer's position and orientation respectively.
3. Set the "pickuptime" and "dropofftime" variables in both expressions to the times in seconds when you want parenting to begin and end.
4. With the time slider set to a time before the pickuptime, position the child layer where you want it to be before parenting occurs. Then parent the child layer to the null layer using normal parenting.

Before the pickup time, the null's expressions hold the null in the place where it will be picked up by the parent. After the drop off time, the null's expressions hold the null in the place where it will be dropped off. Once these expressions have been applied to the null, you can simply parent your child to the null using normal parenting. Then animate both the parent and child normally with keyframes before before, during and after the temporary parenting.

Note: The two expressions are identical except for the word "position" or "rotation" in the last line of the expression. A similar expression could be used for other channels, such as "scale", simply by changing the word again.