An ellipse or a shape is not created with the same calculations as a mask or motion path. You can copy a shape created with the pen tool, but the Rectangle, Ellipse, and all the rest of the shape tools are not bezier or rotospline paths. Spin down the properties and you won't find a path you can animate, you'll only find dimensions and things like points. These you can animate but mathematically, they are completely different than paths.
You can, however, use these tools on a solid layer to create a mask. Then you can past them to other layers or to a motion path.
I almost forgot to mention that you can use auto trace to create a mask from a shape layer ellipse, star, polygon, or rectangle. Now you'll have a path ath you can paste.
First, you'll need to understand the difference between the two kinds of shapes:
- parametric shapes
- Bezier path shapes
Any time that you create a shape with the Pen tool, it's a Bezier path shape.
By default, when you create a shape using the shape tools (like the Ellipse tool), it creates a parametric shape. You can create a Bezier path shape instead by holding the Alt or Option key.
The important point is that only Bezier paths can be copied to and from masks, because all mask paths are Bezier paths.
Todd, I didn't know about holding down the alt/option key. What a great little gem.
The After Effects CC (12.2) update makes creating Bezier paths easier and more obvious.
option for creating shape layers based on Bezier paths:
When a shape tool (Rectangle, Rounded Rectangle, Polygon, Star, or Ellipse tool) is active, you can use the new Bezier Path option in the Tools panel to create a new shape based on a Bezier path, as opposed to the default of creating a new shape based on a parametric path. Holding the Alt (Windows) or Option (Mac OS) key while drawing a shape causes the opposite behavior—i.e., if the Bezier Path option is enabled, holding the Alt or Option key causes the shape tool to create a parametric path; if the Bezier Path option is disabled, holding the Alt or Option key causes the shape tool to create a Bezier path.
command for converting a parametric shape layer path to a Bezier path:
You can convert a parametric path to a Bezier path after the parametric path has already been created by context-clicking (right-clicking or Control-clicking on Mac OS) the property group for the parametric path (e.g., Rectangle Path 1) and choosing the Convert To Bezier Path command from the context menu. If the parametric path is animated (keyframed), the converted Bezier path is a static path based on the parametric path at the current time; keyframes are lost.
IMPORTANT: When you use the Convert To Bezier Path command to convert a parametric shape path to a Bezier shape path, the Bezier path that is created does not animate well (i.e., interpolation between paths behaves strangely and unpredictably). This is related to path direction and how transformations are stored. For now, you should not use these converted paths for animated paths (interpolation between paths); but, if you do want to try, you may be able to work around the issues by reversing the path before conversion.