Flipped normals when i mirror a block

I am constructing a block nativly in rhino and i use it (rotated mirrored … ) when i exported to obj or bake the mesh of the block inside rhino the normals of the mesh of the block that have been mirrored are inverted…
Any thoughts …

Hi Yannis- I see that, thanks- I’ll see if that can be tuned up.

Bug track item-


Mirroring is an operation that is orientation reversing. Normally, to compensate for this, Rhino reverses the normal of the mirrored geometry - this can be seen using the Dir command. The reversed normals on a surface show a small, purple line to indicate that its normals have been reversed.

Probably, the reversal has not propagated in your block somehow, giving flipped normals on your meshes. This may be a bug in Rhino, because when I test this with a simple block definition of just one surface, the mirrored block has a correctly reversed normal.

The Block is ok . But when you try to extract mesh or export to obj without exploding the block the normal is inversed.

Ah, yes I see now.

As you see above pascal ( from mc neel ) has tag it as a bug … so we wait …

Hi, any news about this bug?
I currently experience a very similar issue, except the normals look flipped in the in-place block definition editor also.

The following picture shows the issue : two instances of the same block definition, one is mirrored. When editing one the glass normal points outwards, in the other one the glass normal points inwards. And of course, if I flip it in one instance, it also flips the other one (block definition edit) so it’s impossible to have both normals correct at the same time.

This is not a mesh, the definition is made of surfaces and polysurfaces


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I noted this when I first started using ORCA3D in Rhino5. My weight tallies on a yacht project were getting weird when adding blocks such as cleats, winches, or items like that for fully outfitting a design in order to determine the center of gravity. Mirrored blocks would have a negative value in the Orca report so a winch that was inserted on port would get cancelled out by its mirror to starboard. I let the developers of Orca know about it and they came up with a fix and had a new release in just a day or so.

I experience this behaviour on a regular basis, and it is really a big problem when I am dealing with blocks (and I have to use them massively …)
Right now I have a project with a huge number of block types e.g. lights with emitter planes in front of them … which I cannot mirror !

Please mcNeel fix this old bug!



Curves also get flipped - if I change the mirrored curve to match the parent then history is broken.


Is this been fixed ? I’m using Version 5 SR12 64-bit
(5.12.50810.13095, 8/10/2015)

It is a big problem for me too … any news?

Hi all - Sorry, no - this is not fixed as far as I can see.


Any idea when is a new SR due ?

@gmillas ??
This is unrelated to this topic …

Dear Pascal

Is there anything new regarding this issue?
Could it be fixed in Rhino 6?


Hi Pierre - it is not fixed as far as I can see - I hope it can be fixed in V6, but I don’t know.


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Thank you Pascal for your answer
Have a nice day

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Hi, @pascal I am working on a big master plan where blocks are essential and when we export the blocks to Twinmotion for replacing them for trees, suddenly a bunch of those trees (too many to fix manually) are upside down. So I came here and discovered that the bug has some history already.

Any plan to fix this in the coming future? Many thanks!

Hi Jorge -

That sounds like a different issue. Can you post a picture?

Sounds similar to the Revit Family issue with Mirrored Rhino blocks.