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I have an animation of a needle going from left to right. So as the needle Animates in a semi circle, the Mask follows the animation of the needle revealing the background image. The problem I'm having is, the mask is awkwardly animating, stalling, & having parts of it delayed, the transition is not correct or smooth. I have a file to share which could make it easier to understand if someone can have a look at it.

enter image description here

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Another way to do it would be to use the Screw modifier:

  • Create a simple edge, move it away from its origin:

enter image description here

  • Give it a Screw modifier, play with the Angle value:

enter image description here

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  • $\begingroup$ Even better! That's the one, I reckon.. :) $\endgroup$ Sep 10 '20 at 11:49
  • $\begingroup$ Hey guys, all work well, but I went with moon boots option since i had already started. The only confusion was the screw modifier I had to enter at frame 1: 360 and make the screw modifier go all around to 540 degree angle to get the right type of masking. Here's the video animation: streamable.com/gcqkpw and if someone would like to see the Screw modifier from curiosity, Im happy to upload the file. $\endgroup$ Sep 10 '20 at 13:11
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You could use a curve instead of a mesh to make the mask.. then the keyable Bevel Start and Bevel End properties become available:

enter image description here

Make a 2D curve semicircle, with a Curve line-segment as Bevel Object, and animate the bevel.

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  • $\begingroup$ .. or you do the whole thing with a shader, if the context allows.. $\endgroup$ Sep 10 '20 at 10:47
  • $\begingroup$ so with this solution how does it hide what's behind? do you mean it has the same color as the background? I think we lack informations... $\endgroup$
    – moonboots
    Sep 10 '20 at 11:15
  • $\begingroup$ You mean the shader option? That would be a procedural transparency mask on the original dial, no separate masking object? Or a transparency mask on the masking object? Have I missed the whole point? I was just imitating the effect in the .blend file, but smoothly.. (I thought :) ) $\endgroup$ Sep 10 '20 at 11:20
  • $\begingroup$ As I said I think we lack some informations from Blender Breath ;) $\endgroup$
    – moonboots
    Sep 10 '20 at 11:32
  • $\begingroup$ @moonboots Looking back at the file, it does seem to be weight-painting, which partly explains his problem.. the gradient is not truly along the curve. It would have been better to paint the gradient on a straight band, and then deform it to a semicircle. $\endgroup$ Sep 10 '20 at 11:36
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My 2 cents, using shaders... not sure if the rainbow was part of the question...

enter image description here

The quadrant is made from a 1cm height rectangle, array modifier and simple deform (bend) modifier.

Array is 285 steps. 285 is tuned after the bend modifier has been set up so that the arc has the good size.

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The transparency is driven by the height in Z (from zero to 285). So the value node is keyframed appropriately. Don't forget to activate transparency if using Eevee.

The (optional) rainbow is the height (Z) clamped between 0 and 1. Z divided by 285.

enter image description here

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