I have a particle system where the top and bottom of the particle are different colors. The particle should rotate when a field is applied and show the other color when it's upside down.

This is not happening. Below I've shown the rotation progression.

hair system showing all orange on top hairs start rotating

They just..swap colors?

once hairs pass 90 degrees (i.e. the blue side should be up) they start swapping top/bottom hairs are now showing all orange on top even though they have rotated almost 180 degrees

How do I stop this from happening?

Instance object and appearance of top and bottom of particles:

Top side showing orange enter image description here bottom side showing blue enter image description here

  • $\begingroup$ drive.google.com/file/d/18r91klJSHk3fBrrRuODgpjT5Bedg9M8B/… here's the blend file $\endgroup$ May 24, 2020 at 23:32
  • $\begingroup$ Weird. It seems the rectangular particles can only bend over backwards. As soon as they the start going face first towards the ground, they quickly turn around (on the Z-axis) and continue in the same direction, but now bending over backwards again. (Blue side being the back and orange being the front.) They don't arrive at the tipping point at the same time, so at certain moment some are turned while other haven't yet. It seems to be a inherent property of the Orientation Axis (in Particle Properties > Rotation) which is set to "Velocity / Hair" . I wish I had an idea how to fix it. $\endgroup$ May 25, 2020 at 13:54
  • $\begingroup$ Is this a bug then? I can't imagine that auto-flipping things in a physics simulation is desirable behavior $\endgroup$ May 25, 2020 at 15:29
  • $\begingroup$ I suspect it is by design. Imagine an upside down (somewhat rigid) L-shape. If that L was a hair on a head, you would want to have it point in the same direction as the wind, and not bend over backwards in the wind. So it would have to turn. It seems the particles behave a bit similar to a wind vane. I must say though, I have very little knowledge of the inner workings of Blender and I'm not qualified to say with any certainty if it is a bug, feature, or anything else. $\endgroup$ May 25, 2020 at 16:06


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