How does copying-pasting a shape path to a motion path work?

[interesting_Gleam1519]

I got to know that you could copy a shape path and paste it on to the position property of a layer to have a motion path same as that of the shape path. I don't quite understand however, how this works? Like even when I scale up the shape layer, it still pastes the original path I made not the scaled up one. 

On a macro level I can say the shape path is copied and pasted as a motion path, but what is exactly being copied and pasted? As in something like shape path vertex coordinates? Where are they measured from? How during pasting these vertex coordinates get converted to position coordiantes? 

Essentially i'm looking for what data is copied and from which reference? And then during pasting what data is pasted and from what reference, to finally get that shape path to motion path conversion.

Thanks! 

[Rick Gerard]

The vectors in a shape layer path are above the layer/scale property. You can see it in the order of properties in a shape layer by pressing the 'u' key twice to reveal all modified properties of any layer. If the layer has been scaled up, the Scale option will also be displayed. If not, hold down the Shift key and press 's' to add Scale.

The vector path, and all of its properties, are above the Scale property, so when you copy them, the position of the path is in its original, unmodified position and shape relative to the composition center point.

If you want to modify the size of the original "Path" shape layer using scale, position, and rotation values you need to copy the path before you change any of the Layer/Transform properties (or set a keyframe and then reset), copy the Shape/Contents/Path, then select the Position of the layer you want to follow the path (follower), then Parent the "follower" to the Shape layer and make your position, scale and rotation adjustments. The last step is to turn off parenting, reset the scale position of the "follower" layer to 100% 100% and the layer will now follow the path.

Everything in After Effects is based on property order. The Shape Layer Path is always positioned above Path Transforms and Layer Transforms, so its properties are always based on the composition center. Only through parenting or a very complex expression can the "follower" layer position match the shape layer path.

I hope that makes sense.

[interesting_Gleam1519]

That did make sense! Thanks.

So, when I'm copying a path from the shape layer, I'm copying the coordinates of the vertices of that path as measured from the comps center (while also copying bezier handles info). 

When I paste this path on the position property of a layer, AE uses the copied info to calculate the distance of the vertices (now of the motion path) from the comps origin (that is the top left corner of the comp), because it is from the comps top left that the position of a layer is measured.

Am I right?

[Roland Kahlenberg]

As Rick correctly noted, the source and destination layer and composition sizes play a part. Things get messy if you want a 1 to 1 copy-paste result. On the bright side, I'm about 70% into completing a script that is aimed at making this workflow a one-click solution. I will share an update when it's done and ready for the public.

Best Wishes