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Retain relative position values while pick whipping two objects within a layer to other layers
Hi,
I’m sure this is a simple thing, but I can’t figure it out.
I’m trying to “parent” two objects (polygons) within a shape layer, each to a separate other shape layer (lines), for a simple horizontal movement. When the line moves, I want the polygon to move with it. Simple.
Because I’m using the polygons as mattes on a background image, I have to keep them in the same layer. So I can’t parent them. I’m pick whipping the position expression of each polygon to its designated line’s position. However, as soon as I pick whip, the polygons snap out of position — their position values become the same absolute values as the line; I need them to be relative.
Thanks for the help!
-Tom
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4 Replies
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Thanks very much Dave. That was my backup plan, which I ended up using.
Would be nice to know if there’s a way around this for the future, though … I can imagine more complex situations where going old school might not be feasible, with more objects in the matte layer, strange shapes on crazy paths, etc. (e.g. keyframing this was a little bit of a pain with getting the speed ramps just right, and it was a very simple movement.)
Anyone know?
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When you pick whip one position to another you get something like in expression window:
thisComp.layer("Shape Layer 1").content("Ellipse 1").transform.position;What you probably need is:
pos = thisComp.layer("Shape Layer 1").content("Ellipse 1").transform.position;
value + pos - pos.valueAtTime(CERTAIN_TIME_POINT);This captures relative change in parent-shape’s position and adds it to the child-shape’s position. That change is relative to a CERTAIN_TIME_POINT (in seconds) which you need to pick up. In the end it works like usual layer parenting.
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Hey man,
Oleg’s answer is definitely the way to go, it’s the most versatile way, if you make his (CERTAIN_TIME_POINT) just 0 it should work, so that effectively zeros out the animation transformations. Then you’re just adding values to it’s current value rather than adding two unknown values together.
It’s good to keep shape layer positions clean for this type of thing. So when you make a new shape, make sure all the positions are 0,0 and all the scales are 100,100, particularly the group position as that tends to default to something non zero when you use the shape tool. Starting at 0,0 and 100,100 is always much less of a headache than some random numbers that you have to cancel out later.
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Yes! That is exactly what I was looking for. Thanks guys, that’s perfect. Much appreciated.
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