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I'm trying to make a grass cutting animation. A blade rotates and when it collides with grass, I want to replace the grass object with the sliced variant.

I tried the particle system with 'die on hit' ticked, and to the blade I added collision with 'kill particles' ticked, but that didn't work. But even if it worked, I still don't know how to replace them.

Sample video: https://youtu.be/YpSQlPAyW3g

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  • $\begingroup$ You might find the (free) "Molecular" add-on useful: blender.stackexchange.com/q/69836/111042 But the documentation is AWOL. $\endgroup$
    – james_t
    Commented May 1, 2021 at 15:15
  • $\begingroup$ What should happen with cuted part? Disappear from frame to frame or animate a fail? If fail where and what kind of motion? $\endgroup$
    – vklidu
    Commented May 1, 2021 at 15:33
  • $\begingroup$ be sure to follow this for any clues when someone comments or answers (How to cut hairs in two pieces in blender?): blender.stackexchange.com/questions/181530/… $\endgroup$
    – james_t
    Commented May 1, 2021 at 15:35
  • $\begingroup$ @vklidu Like a regular grass, it should fall down. I would like to apply physics to it. $\endgroup$
    – user14913400
    Commented May 1, 2021 at 15:45
  • $\begingroup$ There may be something like Hair and the python "bpy.ops.particle.brush_edit()" method to "cut" the "hair"? $\endgroup$
    – james_t
    Commented May 1, 2021 at 15:57

1 Answer 1

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version – Boolean

Some kind of illusion can be done with boolean operation. You can start with Geometry Nodes to create grass object ...

enter image description here

... but final "cut" is boolean operation (grass differenced with Circle object displaced by Radial Texture), that results in glitchy anim (doesn't matter if I use boolean operation inside GN or as modifier).

enter image description here

What solved this is Solidify modifier for grass object, but in cost of viewport playback (1 fps).

enter image description here

Falling parts are particles emitted from Blade object ...

version – Particles

Another illusion can be done with two exactly the same overlapping particle systems.

One with tall grass and hidden blade on plane surface level to kill particles ...

enter image description here

... in combination with second one identical Plane. The only difference is Render as Object with shorted grass model.

enter image description here

This gives you an illusion

enter image description here

Falling cute grass can be faked in the same way like in the first version.

Notes:

  • Particles works here only because I disabled Gravity
  • I wanted to use Hair system, but it didnt react on collision object Kill Particles.
  • Another thing that doesn't work is to use blend texture controlled by another object (that is known issue)
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  • $\begingroup$ That works great, thanks! I would also like the grass falling and possibly regrow it so it would make a nice loop, I thought about replacing the dead particle with the sliced part and the shortened grass, which would have an animation of regrowing, and after the blade hits it, it would repeat. But unfortunately I don't know how to do any of these, could you please point me it the right direction? $\endgroup$
    – user14913400
    Commented May 1, 2021 at 21:38
  • $\begingroup$ great idea man!!! $\endgroup$
    – Chris
    Commented May 2, 2021 at 8:07
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    $\begingroup$ I stacked today with idea make it much easier and "cut" grass just by material transparency ... But I could not figure out how to apply radial gradient that could handle "regrowing" grass ... To animate cut parts can be faked simply by emitting particles from blade ... Again illusion but it could work. $\endgroup$
    – vklidu
    Commented May 2, 2021 at 19:41
  • $\begingroup$ It hurts my mind that blender is incapable of such a simple task... never mind, thanks for your time! $\endgroup$
    – user14913400
    Commented May 3, 2021 at 7:16
  • $\begingroup$ I'm not saying blender is not capable ... I have a trouble :) For someone else it can be easy-peasy ... $\endgroup$
    – vklidu
    Commented May 3, 2021 at 8:09

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