I'm working on a project where in order to get the right animation on this flying vehicle I've had to animate my HDA on a null that is attached to a camera. There's a trail SOP in my hDA to compute velocity but I figure since the animation is happening at the object level this isn't helpful. To circumvent that I've object merged my car HDA with transform set to “into this object”, deleted my old v attribute for good measure, cached it out and created a new trail SOP. I'm still getting oddball v vectors that look kind of perpendicular to what they should be. The vehicle is swooping in left to right but the vectors are aiming straight down. I know these should look correct in viewport regardless but I am using Redshift if it matters.

I've attached an image of network.

Did a little bit more digging and after turning off integer frame values in the global animation options to inspect my sub-frames I noticed that it was doing some crazy erratic vertical jittering inbetween the full frames. If I turn off the camera's animation in the dopesheet by unchecking the “all displayed” checkbox (essentially muting the keys) the sub-frames are smooth and my car's velocity vectors look normal.

Now one solution would be to not parent my vehicle to the camera but I am having a hell of a time figuring out a decent workflow to have my camera move and my vehicle's path sync up in such a way that keeps the vehicle framed nicely. I've tried motion paths but I am constantly fighting the follow path position value to make sure it's still in frame and I feel like i'm always counter animating.

I ended up re-animating in Maya using similar techniques. Motion Path animation parented to the camera. I exported as an alembic and brought it into Houdini via an alembic archive and the sub-frames look perfect! I dropped in a trail SOP and the vectors look the same incorrect way they did before. Vertical vector trails that do not match the trajectory of the car. What's going on here? Are there other ways to calculate velocity vectors?