Imagine Moving head is a few feet behind the path the “talent” walks. (See Examples below. 'o' are to fill space for editing. Yes I will use CAD next time.)
Theoretically, (in radial coordinates) A and B are same distance from light and have symmetrically opposite angles. What this means is that tilt at A and B are identical and Pan is the only parameter that would change from point A to B. However, we know that if we record position A and B then cross-fade, that the beam will swing way out from the line in a circular arc.
The solution for this and the way to make the light travel in a linear fashion on the line is to introduce a third location C which is on the line at a location directly in front of the light. Basically, you bisect the triangle between A B and the light. Then if you program a cue at point C, the head tracks the line to C and then the next cue tracks the line to point B.
I understand this is a complex explanation but this is the only effective way to track a line with any fixture but a a head specifically. Also note that this only works with one light tracking as the light reaching point C must be exactly perpendicular to the line you are tracking. You can however use multiple fixtures but remember that if proportionally the distance between the lights is small compared to the distance to the line and the distance to the walk path then the error goes down but if the distance from the light to the line is small and the distance between fixtures is great, you will see the arc movement instead of the linear line track.
oooooo X oooooo X oooooo ooooooo \oooooo/oooooooo oooooooo \oooo/ oooooooo ooooooooo \oo/ ooooooooo A————– \/ ————–B – oo | oo – – C –
As a solution, you can create subsequent points on the line respective to each lighting fixture that are perpendicular to each light. For example, if you have 4 lights that track the line, you have 4 points between A and B. Oh and for this kind of cue, I recommend building a cue-list (as a chase) that grabs the position only (or focus, zoom, Iris too depending). Make a guess on your timing, and then hold on to the Rate wheel when you run the cue. oooooo X ooo X ooo X ooo X ooo X oooooo oooooo | ooo | ooo | ooo | ooo | oooooo oooooo | ooo | ooo | ooo | ooo | oooooo A——-1—–2—–3—–4—–5——-B
I have done this with as many as a dozen lights, tracking a line 60' long and it took 13 cues and about a hour to build and test with the artist. It didn't look “Autopilot” smooth but it did look better than a “spot opp” smooth.