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Activity › Forums › Adobe After Effects Expressions › Trying to figure out how to have a cylinder move in relation to a semi circle

  • Trying to figure out how to have a cylinder move in relation to a semi circle

    Posted by Geordie Strike on February 2, 2022 at 1:40 pm

    What I’m wanting to do is have a cylindrical shape move in relation to a rotating semi circle beneath it. Essentially as the semi circle moves below, the cylinder stays in contact and moves how it would if they were both solid shapes in real life.
    Super confused as it involves rotation as well as positional movement and I’m really bad at math… I feel like there is some sort of mathematical equation out there that could solve the problem for me if somebody is kind enough to share it with me!

    Filip Vandueren replied 4 years, 7 months ago 2 Members · 5 Replies
  • 5 Replies
  • Filip Vandueren

    February 2, 2022 at 1:53 pm

    Could you post a still image of what you’re starting from ? 3D ? 2D ?

  • Geordie Strike

    February 3, 2022 at 3:35 am

    Hi Filip, I attached the image below, So as the semi circle like rotates below the cylinder moves in the opposite direction but stays attached to the surface of the semi circle, hope that makes sense!

  • Filip Vandueren

    February 3, 2022 at 8:08 am

    I’ll see if I can figure it out.

    Maybe renderTom’s roll-it script on aescripts can help you? But kind of expensive for an expression you might only need once.

  • Geordie Strike

    February 3, 2022 at 8:26 am

    I’m pretty sure that’s exactly what I’m after! The only thing is the semi circle isn’t a flat surface, so when the cylinder rolls I feel like it wouldn’t roll along the edges of the semi circle?
    Thanks very much for sharing that script with me though and would love to know if you ever manage to figure it out 🙂

  • Filip Vandueren

    February 3, 2022 at 10:55 am

    Yes, the problem is that there’s no real mechanical way in which this would make sense if they were gears. Where is the point of rotation ? Are their center points “flexible”, so they always move close together ?

    Which of the two shapes is “driving” the rotation (so that’s the one that has a constant revolution speed, and the other shape follows). If it’s the semicircle: where is it’s axis: in the middle of the shape, or in the middle of the flat side (which would not work IRL)

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