Beam starting/ending points are offset from motion-track point

[lylephoto]

I'm using After Effects CC 2018 on Windows 7...

Short version: When I parent a beam's end point to a motion-track point on a null, it's offset a bit to the right and slightly lower (perhaps 30 to 40 pixels each way) in relation to the motion tracker. It follows the track uniformly as expected, but always with that offset, which of course kills the accuracy. Likewise, parenting a beam to a shape layer's position results in the same offset.

All the tutorials I've watched on how to animate beams via their starting/ending points don't seem to have that problem. They always seem to align accurately to whichever element they're being parented.

I can't figure out what I'm doing wrong to cause the offset.

Here's the longer, more detailed version of what I'm doing...

Project concept: pricetags move around slowly while lines (via the beam generator) connect those pricetags to corresponding items on a base-layer video (FWIW, the video track is a pre-composition, made necessary by the use of a warp stabilizer; resulting motion tracking is accurate). The pricetags reduce in scale as items get further away. Simple concept.

First bit: Each pricetag is contained in one shape layer, made with a horizontal rectangle and triangle at the side; a merge-path and stroke are applied to the entire shape. I snap the entire shape's anchor point to the tip of the triangle. The beam's starting point is supposed to be at the tip of the triangle. I then keyframe a slow, simple, straight-line movement across the duration of the composition. It all goes smoothly until I parent the starting point of the beam to the shape's keyframed position. Instead of connecting to the shape's anchor at the pointed tip (isn't that where it's supposed to connect??), the beam's starting point is offset by about 30 or 40 pixels to the right and down, so it doesn't actually touch the pricetag.

Second bit: I motion-track an object on the video layer, send it to a null, and then parent the beam's end point to that null. All pretty basic stuff. Except the beam's end point is... you guessed it... offset. It doesn't land on the motion track point. It's a bit to the right and down, roughly 30 to 40 pixels each way.

So I tried it a different way, customizing this script template on the beam's starting and end points:

StartPointLayer=thisComp.layer("Null 01");
StartPointLayer.toComp([0,0,0])

I adjusted the numerical values to get the beam's point precisely where it needed to be. It worked like a charm until the pricetag was scaled up or down, at which time the starting point no longer corresponded accurately.

I hope this all makes sense. Any ideas?

[Rick Gerard]

I think you mean that you attached the null and the beam start and end points with an expression because Parenting only works with layers. If you are parenting the beam layer to a moving layer, then the entire Beam is going to move, and for the coordinates of the beam to match the position of another layer, the beam layer must be in the home position or the expression tying the points to the position of another layer must be corrected using the toWorld or toComp method in the expression.

If the start and end of the beam do not line up with the nulls when you simply use the Pickwhip to attach the start point to the start null then there are a couple of things that can cause that problem.

Maybe the Beam layer is not at the default position in the comp frame. Move the layer the entire beam effect moves.

Maybe the length of the beam is not at 100%. My guess is that the layer is out of position.

As far as the scale issue, if you use the layer scale parameter and the layer doesn't scale around the point of the triangle then you have not got the anchor point properly setup. It's really easy to see exactly where the transform (scale) center is if you temporarily make the layer 3D.

For any further diagnosis, I'll need to see a screenshot with the modified properties of the layers that are giving you problems revealed. Just press UU with the layers selected, PrintScreen and Paste to the forum so we can see what's going on. This is not a bug, it is just user error.

[lylephoto · Accepted solution]

My face is red. I discovered my problem. The solid on which the beam was built wasn't centered in the frame. That's it. I must have bumped it slightly before generating the beam and didn't realize it. Ugh.

Hopefully my dimtwit moment can help someone else.

[Hajnal5E2B]

omg this was my exact problem as well! thank you for posting about your solution!

[Guest]

Well two years later and it did!  Thanks Lyle & Rick!