Edge Trimming strange behaviour

[masterzone]

Dear All,

I'm trimming some large branch of flower from cosmos browser...I got a strange behaviour..
As you can see into the pictures below, if I put the edge trimming shader into the opacity some branch doesn't goes away...If I put the opacity value to 0%, all branch will be deleted...
Why? I missed something?
Alex

Cinema4D 2023 + Corona 9

[Beanzvision]

Hi there, perhaps this article will help under the ''how does it work'' section.
https://support.chaos.com/hc/en-us/articles/5244883674897-How-to-use-Chaos-Scatter-with-Corona-for-Cinema-4D-Advanced-Features#h_01GG6J1CWE8HJ5S0N5XV647AF9

[Beanzvision]

Does it work with a black color shader connected? I hope this helps. Actually, that will remove it altogether. :/

[masterzone]

mmm I don't understand how does it works and the manual doesn't help unfortunately...

Anyway...I'm try to understand why its needs a black color inside...I thought only edge trimming shader loaded into the opacity slot but a 0% of level of opacity was needed in order to works...🧐.
Why the black color shader is needed? you don't need only the edge trimming shader?

EDIT: maybe the problem is the kind of object? on leaf the edge trimming works as expected but on long brunches it doesn't works...to long extension of the brunch?

Try the scene attached...I have put edge trimming on all mats but the brunches still there...

[Beanzvision]

Thanks, I'll take a look and get back to you.

[masterzone]

however I found that I'm wrong in a point. On my first message Yes, the branches disappears but are disappeared into whole object and not only on trimmed area...so is useless to put the opacity level to 0%...sorry..

Anyway I would like to know how to trim branches but I think there are limitation based on where the object is placed...:)

[romullus]

It's important to understand that edge trimming is operating not on triangles level, but on mesh element level (mesh element is continuous group of polygons that are connected to each other by edges). It projects a ray from selection center of each mesh element towards distribution object, if the ray hits surface that is designated as trimming area, whole element gets culled, if it does not hit trimming area, the element gets rendered. Certain types of foliage, like bushes, doesn't work well with edge trimming, since they have long horizontal branches that are populated with small leaves. There's not much you can do about this, except... if you have lots of free time, you could break long branches into smaller chunks, so they could be better handled by edge trimming.

[masterzone]

It's important to understand that edge trimming is operating not on triangles level, but on mesh element level (mesh element is continuous group of polygons that are connected to each other by edges). It projects a ray from selection center of each mesh element towards distribution object, if the ray hits surface that is designated as trimming area, whole element gets culled, if it does not hit trimming area, the element gets rendered. Certain types of foliage, like bushes, doesn't work well with edge trimming, since they have long horizontal branches that are populated with small leaves. There's not much you can do about this, except... if you have lots of free time, you could break long branches into smaller chunks, so they could be better handled by edge trimming.

Please could you make the whole Corona Render help manual? 😄 when a picture says more than other things...

[TomG]

We don't do manuals :) Instead we have our YouTube https://www.youtube.com/channel/UC_5CpKKSI-YZhEWDt0fbhKA and our help documentation, this is the page with Edge Trimming https://support.chaos.com/hc/en-us/articles/5244883674897-How-to-use-Chaos-Scatter-with-Corona-for-Cinema-4D-Advanced-Features#h_01GG6J1CWE8HJ5S0N5XV647AF9 and does cover this topic "If a pivot of any integral part of the instance (any sub-mesh element of the instanced model) falls outside of the area specified by the scatter, such part is not rendered. This is a simplification, but you can imagine the pivot as a point on the distribute-on object from which an element is growing."

You can use a Slicer object instead if you want to "cut off" things like branches that stick out beyond the bounds of the scattering. This is often not a great approach though as it can cause things like "half a flower", as if everything was cut by a laser :) And that's where edge trimming comes in, but it does apply to whole objects so things can still "stick out" of the boundary of the scatter (by design).

[masterzone]

so..do you think that the branches pivot of the object shown in my render are inside of the scatter area? and not outside? maybe this is the reason...however I understand that the object is not suitable for edge trimming due to its extension...
Thanks anyway for make me understanding how it works...;)

[TomG]

I do think that is what is happening, the pivot is inside the scatter boundary and so the whole object gets rendered. You could try using a slicer for that, though it will cut off the branches... that said, that could be realistic, since it would look that way if trimmed with a chainsaw :)