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I describe the problem in this video

But the basic summary is this: I'm trying to get a camera and a grease pencil object to travel along the same path, with the camera offset slightly so that it can watch the object. I'm trying to have each of them track each other, so that the camera is always looking at the GP and the DP is always facing the camera.

I did something similar previously using Track To constraints for this video, but in that one I used two paths, and the necessary Track Axis and Up settings seemed to be different for this one. After some finagling, I got the constraint settings aligned so that they were facing each other.

The problem is now that for some reason, when I actually have them move along the path, the grease pencil object will rotate, tilting slightly on the y axis and then doing a full z-axis rotation. This persists if I use multiple paths along the same line, if I remove offset. There are no transform keyframes on either the camera or the GP.

I tried starting over by deleting all paths and follow constraints and setting up the dual tracking constraints first, confirming that that was working properly, then adding the follow path constraints onto two different paths; the result seems to just be that it now rotates less, and only on the x-axis, which is an improvement but not a fix or an explanation.

I am pretty lost on this, doesn't make sense that this happens for two objects traveling at the same speed in a straight line. Anyone else have insights?

Blender file here

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  • $\begingroup$ hi! it's hard to say what's going on here cos the example file doesn't include the path nor GP object. would it work to parent the GP obj to the camera and just animate the camera to follow the path? $\endgroup$
    – pevinkinel
    Commented Jun 20, 2023 at 10:42
  • $\begingroup$ @pevinkinel Thanks for having a look! If I could animate the parenting property, that would work, but because the paths are different lengths - I'd be trying to get a bird animation to reach a branch, and then for the camera to continue pulling out to a wider scene. And yeah, sorry, I was editing the file and forgot to save an alternate version of what I was doing; I went through the backups and it should be viewable now. $\endgroup$
    – Michael Weston
    Commented Jun 21, 2023 at 15:57

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Just want to update this to say I figured it out: For anyone at my early level who is having a problem similar to this, the order in which Object Constraints are placed matters. My GP Object was calculating Track To first, then Follow Path, so its calculated orientation towards the camera was being thrown off by the tilt of the path it was on. As soon as I switched their order - so that it figured out the path and then tracked towards the camera - it worked perfectly. Lesson learned.

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