y=value[1]-sourceRectAtTime(time).height/2;[0,y]
y=value[1]-sourceRectAtTime(time).height/2; [0,y]
Scale a layer to fit a pixel width. Apply to scale property
Apply this expression to the property you want to control (like opacity) and make sure it is referencing the correct control layer.
Delay property by certain number of frames
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