algorithmic modeling for Rhino

Mirrored geometry displayed darker, even with custom preview??

Hello everyone!

I am still fairly new to Grasshopper, but once again I have come across some behaviour that just doesn't seem like a feature.

I have a geometry that I am placing multiple times in a grid, basically they are bricks with a hole in them. I use cull pattern to delete some which are then mirrored and placed with the inverse cull pattern.

For some reason the mirrored bricks are displayed darker in Grasshopper though. This happens with a custom preview and one material for everything or without it when it is just the standart red colour (see pictures). Everything else in the patch has preview turned off of course.

Any Explanation for this behaviour? How do I get them all to display the same way. As soon as they are not mirrored they display as expected (in the lighter colour).

Thank you.

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Here is the last image.


can you post the definition?

Oh hey again Danny.

The definition of the cull pattern? Its just based on largen than and smaller than of a list of values.

I just baked everything though and it looks like the actual geometry gets mangled when mirroring. Was thinking before that when mirroring maybe UVs get flipped too? like being turned inside out?


My suspicions are that you have twice the geometry on the mirrored ones, but I cannot tell by pictures alone. Although that last shot would suggest my theory is correct

If I bake and click and drag to select a mirrored and unmirrored one, it shows as "2 closed meshes" though?

It seems like its messing up the geometry though. Do you know how to correct the geometry issue? Like is there a flip UV command in GH? I couldn't find anything..

Upps, forgot image.


If I use rotate instead of mirror, which in my case works, as the front and back is the same, so i can rotate them 180 degress "front to back", then everything works as expected and it renders fine.

So mirror is definetly at fault here.

Was this ever solved? I have the same issue in 2017, mirrored geometry displaying darker, in grasshopper display and when baked into rhino and rendered in Vray, as image shows

Is it a mesh?

I'm having the same problem. It is indeed a mesh.

I've fixed it by recomputing, then unifying the normals. You can recompute normals by baking the mesh the running the rhino command "rebuildmesh"

Or in C# script editor you can make a little component to do it:


In my plugin Pufferfish(

I made a Rebuild Mesh component which has a bunch of mesh options I found useful for cleaning this stuff. Try it out to see if it works. (the first option on the component is rebuild normals)

Works like a charm! Thank you Michael.





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