Drawing a line with a path?

[Liam_Wiebe]

Recently I watched a video by Vsauce (The Brachistochrone - YouTube )

There's an effect in the video that I would like to try but I'm not sure how to do it...

As you can see, a point on the smaller circle draws a line as it rolls around the larger circle.

How could I replicate this effect? I thought of manually drawing a path and using the "stroke" effect to draw the line as the point on the circle goes by, but I'm sure there's a better way to do this. Does anyone happen to know a better way to do this?

[P.M.B · Accepted solution]

You are right. Draw or create the shape.  You can do it with a mask and apply the stroke effect if you want but it's probably easier to

just make the shape with the default stroke on it then use the Trim Paths effect to have the stroke draw on.

Next creat the circle and,  select the stroke path in the layer controls & CTRL-Copy then CTRL-Vpaste onto the position attribute

of the circle.  This will paste the keyframe info of the stroke onto the position of the circle.  Finally select all those keyframes & Alt-Drag

them out to match the timespan of trim path key fames.

Let me know if you have any questions.  I made a video but accidently rendered it at half resolution and don't have time at the moment to re-render it.

Screw it: Here's the video even though the resolution is crud

[Liam_Wiebe]

Thank you for the detailed video!

I think I get what you are going with, I'll have to try it out myself, but I think that answers my question

[P.M.B]

You're welcome,

The part where I creat the trim path is confusing because the "fill" is turned on and the stroke is black against a black background.

Where the video skips all I did was turn off the fill and then change the stroke color.

The reason the video cuts is because it took me about 30 seconds to figure out why I couldn't see the stroke.  LOL