It seems to be more and more common to use a graphic technique in which shapes, specifically one end of a line, tracks something moving in a video clip. It is very easy to do in Motion and I'll show you how.
I'll start a new generic Motion project the frame size, rate, duration of my source video clip. If you don't have a video clip to use, follow along by creating a small circle in Motion, animate it from one corner to the opposite one, export as a QuickTime movie, and use that. I'll import my clip into the default Group.
With the source video clip layer selected, playhead at the start of the timeline, Command-2 for the Library. Go to Behaviors, Motion Tracking, apply the Analyze Motion behavior to this layer. F2 for the Behaviors section. Drag the Tracker, crosshairs inside a circle in the Canvas, to a point you want to track. The more visually clear, the higher contrast it is, the better tracking performance will be. Click the Analyze button in the Inspector. Tweak as necessary. For tracking basics see my Hub article 'Tracking Essentials In Motion 5'.
From the Shape menu in the Toolbar, the fourth button from the left, select the Line drawing tool and draw your lines. It is very important to be aware that the first mouse click is 'Control Point 1', when you drag and let off the mouse to finish the line, that is 'Control Point 2'. I'll make my Brush Color red, Start Cap Bevel, End Cap Arrow, and increase the width to taste. When I draw my second line, I'll do the same thing, but make it orange.
To make these lines stand out, select both at once, right-click either, from the pop-up menu chose 'Group'. With the Group layer selected, F1 to the Properties tab, turn on and tweak the Drop Shadow section.
Now for the really fun part! Select the first line, the red one. Command-2 for the Library tab. Go to Behaviors, to Shape, select Tracking Points and apply it to our line. F2 for the Behaviors Inspector. Drag the Analyze Motion behavior from the source video layer, into the 'Source' well of the Track Points inspector. You will see 'Controls Point 1' and 'Control Point 2'. These are the Control Points 1 & 2 of which I asked you to remember in the previous section. Set Control Point 1 to 'Unassigned' so that this end of the line is unaffected. Set Control Point 2 to 'Track 1' so it follows Tracking Point 1 of the Analyze Motion behavior of the original video layer. If you play your project now you should see our first targeting line's arrowhead stuck to the tracker, and its rear end stuck in place.
Option-drag the Track Point behavior from our current Line layer to our second line layer. Select it and in the Inspector set it up exactly the same way our first one is. Now play your project and it should have both lines following our tracking point in the Analyze Motion behavior. Not too much work for such an impressive trick, is it?
Create a generic Motion project, draw two small circles, one top right, one bottom left, keyframe them to move in horizontally different directions from the start to the end of your timeline. Export this as a ProRes movie file. Delete all your layers and import that same ProRes movie file. Apply an Analyze Motion behavior to it, have one tracker follow one circle, a second one follow the other.
Now build a rectangle, red outline, no fill. Adjust the control points so the opposing corners are touching the two circles. Apply the Track Point behavior to it and assign Track 1 to Control Point 2, and Track 2 to Controls Point 4. Watch what happens!
This is a really good technique to know for creating many interesting and robust effects. It is also very simple to do. I want to acknowledge Mark Spencer for his contributions to my own journeys learning Motion. Now go out and create some amazing graphic effects!