While I frequently haunt the Creative Cow forums for tips and tricks, this is the first time I have actually asked for advice.
I imported paths from Illustrator, then converted them to shape layers, then pasted the path keyframes to achieve the start and end keyframes I need, but the frames in between are not animating in a straight forward way that I expected.
Perhaps an image would clarify what I'm trying to do:
The center circle is a screenshot from in-between the two keyframes.
I just want the line to extend the branches out without contortion.
I'm relatively new to After Effects, so maybe I'm missing something obvious...
To expand on what Roland said it will make it easier if you modify the shapes so that they have the same number of vertices and those vertices are all in the same relative position on the path from keyframe to keyframe.
The way the path animates is by looking at the index of each vertex on the first keyframe and moving that vertex to the position of the vertex with the same index on the second keyframe. So the "first vertex" on one keyframe will interpolate to the "first vertex" on the second keyframe.
If there is a mismatch in the number of vertices between keyframes then AE will automatically create new vertices that will likely not match the correct relative position of the vertex at that index on both keyframes. That's what's happening in your case.
There's also the Mask Interpolation tool accessible under Window > Mask Interpolation that will give you additional options for interpolating between Masks (not Shape Paths unfortunately, but you can copy the shape paths to masks, operate on them and then copy and paste them back). The downside is that this tool will create a mask keyframe on every frame.
When morphing, it is ideal that the start and end shapes have the same number of vertices. This often results in a more intuitive process as it helps you to align vertices more accurately when tweaking the result.
Watch this tutorial -
[Christopher Brockman]"Also, would I be right in asking if the point with the ring around it is the initial vertex?"
The First Vertex has TWO visual cues. When a Shape Layer is selected via the Direct Selection method - CTRL+click, the First Vertex appears with a ring around it. However, when a Shape Layer's path is selected by selecting the Path property in the Timeline, the First Vertex appears with an additional square around it.