You can add another null and scale that null. You will then need to work out an expression that eliminates the scale change from anything parented to the first null.
Let me make that clearer. Name your first null positionNull, name your second null scaleNull, make the parent of the positionNull the scaleNull, then parent any other layers you want to follow the positionNull to the position null. To each of those layers you will need to add an expression that divides the layers original scale by the change in scale of the scale null so the layer does not change size.
This expression should do the trick:
temp = thisComp.layer("scale null").transform.scale;
Things would be soooooo much easier if shape values had a method like "positionFromRelativeArcLength(pourcent)", available to expressions, which would return the position on the shape at some pourcentage of its total arclength.
All stroke effects have such a parameter and calculates it in no time...
Then, no need to copy/paste shapes into motion paths, everything is real time, you can scale the motion path by scaling the transform group of the shape, + you can make actually animated trails (ones for which the underlying curve does vary over time).
Far too static at the moment.
That would bring so much.
you can make actually animated trails (ones for which the underlying curve does vary over time)
This is what repeaters do... EZ as pie.
I'm not sure I follow the rest of your post. I don't know how you would get or what an arclength is.
The total arclength is the length of the path curve in pixels (once it has been flattened). The suggestion is to make an expression like this one possible (in the position property of a solid for instance)
which would put the solid at 12% of the path (from the start) without having to copy/paste the path in the motion path of the solid.
Exactly like the Start and End property of the stroke effect work, but to animate a solid, not to plot the pixels of the stroke.
This can be done with a valueAtTime function looking at the position and the time combined with a comparison operator in an expression. If I get the time I'll build an animation preset for the same and post it. This would solve the scaling down a motion path problem but may be probmematic when scaling up a motion path.
EZPZ if you approach the problem from the right angle.
Thank you a lot Rick Gerard! That helped me a lot! Amazingly today i just discovered my thanks did not send after your helping hand. Highly appreciated your answer. Big bow!