Your work flow is fouled up because you are expecting to have the mask on a layer stay put while the layer position is changed.
Try drawing your mask path on a solid that is the same size as the comp and make sure that the scale is set to 100%. Then:
- Set a single keyframe for the mask path by holding doen the Alt/Option key and pressing M
- Copy that keyframe
- Set a single keyframe for your null by selecting the null and pressing Alt/Option + m then paste.
- The null should exactly follow the path on the solid layer.
Turning a mask path to a position path will not work properly if the layer is scaled. Using the keyframe trick insures that you will always have success converting a mask path to a position path.
If you copy a mask from a layer and then paste the path as position data on the same layer the mask position will change and the shape layer will move. This is normal behavior. If you want the mask path to stay in the same place in the comp you cannot move the layer without making adjustments in the Anchor Point. or use the Pan Behind tool. To make your workflow work you would have to keep the Null in the same position by adding the path data to the position and using an expression to invert the position data for the anchor point. This would defeat the movement of the null.
the shortcut keys didn't work for me (maybe because i'm on mac), but your directions worked flawlessly on the first try. thanks a billion, because i was just about ready to throw my imac and extra monitor into my reef tank, then throw my tank out the window...
Those are the shortcut keys for both mac and windows. On a Mac keyboard the option key also says alt, on a Windows keyboard the key usually just says alt.
Alt/Option plus m, f, p, t, r, s or a will set a keyframe for a mask, mask feather, position, opacity, rotation, scale, or anchorPoint and reveal that property in the timeline. It's been that way for as many versions as I can remember. If you can reveal a property with a single keystroke adding the Alt/Option key will set or remove a keyframe for that property at the current time indicator position.
Glad I could help...