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I want to create the effect of a train traveling through a glass plane and becoming progressively invisible as it passes through. How would you set this up to animate it?

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Note: This answer is only convenient if your object is made of a single (or just a few) materials, and if the plane it intersects is parallel to two cartesian axes:

enter image description here

All you need to do is to Mix your material (in this case, a simple principled shader) with a fully white Transparent BDSF.

The Mix Factor must be controlled by a Math > (Greater/Less) than node, that compares your object's Geometry > Separate XYZ > [selected coordinate] with a fixed value.

In this example, Z is compared with 0:

enter image description here

Additional notes:

  • If you are using Cycles and you see some black spots, you may need to increase a bit the number of Transmission bounces under Render > Light Paths.

  • If you are using EEVEE, your material's setting should have Blend Mode and Shadow Mode set to Alpha Clip.

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  • $\begingroup$ I also want to do a billboard with multiple posters disappearing as they move up or down. Will this method work? $\endgroup$ – Franck Louis Nov 27 '19 at 10:59
  • $\begingroup$ This would work for everyting (in render/rendered mode: the objects always stay visible in solid mode), but the Mix must be applied to all the materials involved! $\endgroup$ – Nicola Sap Nov 27 '19 at 11:05

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