It looks like you already know how to use the point tracker. I'm assuming that's how you got the position keyframes for the null. You just need to copy those keyframes and paste them to a solid to create a path that can be used for the Stroke effect.
For the benefit to others:
- Use AE's standard point tracker to track the persons head
- Add a null to the comp
- Apply the tracking to the null
- Select the position property of the null in the timeline to select all keyframes
- Create a new solid the size of the tracked movie or comp and leave it at the default position.
- Use the pen tool to create a single mask point on the solid then paste to create a mask path exactly the same shape as the position path
- Now add Effects>Generate>Stroke and set to paint on transparent
- Edit the start and/or end point of the stroke to make the line follow the climber
Did you follow that? There's not much to it unless you don't know anything at all about tracking in AE or copying motion paths to vector paths. Let us know. It's pretty simple.
I am trying to do something simular with facing fracking so i can see where the nose moves in a scene and leave a line behind it. I have got you all the way up to step 6 and then im getting a bit lost. I am fairly new to AE so please bear with me. I created the new solid and used the pen tool to create a mask point. When you say "paste to create a mask path exactly the same shape as the position path" paste what, where? Is there any way you can use video to demonstrate? I would really love to learn how to do this but all my searching is coming up pretty empty. Thanks!
It looks like I forgot to include the copy position keyframes in step 4. Just select the position property and use the keyboard shortcut Ctrl/Cmnd + v to copy all of the keyframes.
If you have copied the position keyframes from your null and you simply use the keyboard shortcut Cmnd/Ctrl +v a mask path Will be created that exactly follows the motion of the null.
This has more than you need and it deals with 3-D layers but the principle is exactly the same. It is just in reverse. In this example I copied a path and pasted it to position. You will do just the opposite.