Clean Scattering borders using cycles?

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1

Hello,

In forest pack for 3dsmax, they use vray to cut out part of the scattered assets going out of the emitter mesh boundaries. (if i understand correctly, by checking each mesh-island lowest vertex on the z-axis, if out of from emitter boundary, island is removed)

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It’s possible to do the same effect thing with cycles transparency but only with a texture projected on a global space

Capture d’écran 2020-12-02 160646
Capture d’écran 2020-12-02 160646548×929 256 KB

I found no way to make such boundary filter work with a procedural method, that would for example, analyse the emitter geometry. Ideally we would be able to get vcolor information from the emitter.

any thoughts on this?
is this truly impossible to find a non destructive method that don’t rely on projected uv-images?

*forgot to mention, the technique should work on non-squared shaped emitter, and of course a same instance shader for multiple emitter

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2

Is forest pack using this VRay shader in some automated way?

Corona render has something similar.

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3

yes, their scattering clean border option is only for vray.
here’s the doc

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4

VRayDistanceTex looks like an extraordinarily useful procedural texture shader node, for clean scattering as well as a host of other use cases. The examples are the bottom of the doc are very helpful.