Hi,
As you can see in the image. Most circles were deformed after exporting .ai format. Has anyone gotten the same issue? Is the cause from Rhino or Adobe?
-Kev
To create a circle using NURBS definition, you need to use control point weights that are not 1.0. Normally, the control points of a Curve in Rhino have only weights of 1.0, but a quarter circle has three control points and the middle control point has a weight of sqrt(1/2)=0.707… (and a circle is made of four quarters of course)
Anyway, it is possible that Adobe Illustrator does not understand or use the weights, or that they get lost during export/import somehow.
What you could try is to use the Rebuild command to create a curve having only 1.0 weights. Rebuild the circle using a large-ish number of control points (maybe 10-20 or so) until the devitation is very small. Then export again. Hopefully, Illustrator now shows the (nearly) circular shapes correctly.
Thanks! That makes sense but I don’t understand why there is virtually perfect circles exist after exporting AI. Not a big deal just wonder why. (I just tested this looks fine) Degree=2, 10 control points VS 2, 8 points)
The curves are visually circular. The one with 8 control points has sqrt(1/2) weights on the corner control points, the other has only 1.0 weights. The rebuilt curve is not exactly a circle. You can visualize this with the curvature graph command:
Clearly, the two curves are not identical.
This thread inspired me to investigate this issue that has pained me in the past. I had a great time last night with curvature graphs and !what-ing a bunch of circles and curves. I now understand what was causing me the grief!
Thanks for this discussion folks.
Ernest
bump
This is definitely a Rhino exporting issue. I’ve noticed that if the drawing was exported as dwg and then imported to AI then circles are perfectly normal. The disadvantage of this method is that it can take ages for illustrator to import the file especially if it has hundreds of thousands of circles like my one.
Rebuilding circles with 1.0 weight is a possible workaround but it has some tolerance issues with radius which is inappropriate for me.
I believe that Rhino exporting algorithm simply needs to be corrected.
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The problem was the unit setting between Rhino and AI was not close enough. For instance, Rhino (feet) vs AI (mm) will create the distortion problem. So, be sure to make units like, for instance, Rhino (mm) AI (cm).
@KelvinC
I exported as DXF 2004 Polylines, everything looks fine/same in Illustrator.
Hope this helps!
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The next release of Rhino WIP will have a fix about this problem. Thanks.
https://mcneel.myjetbrains.com/youtrack/issue/RH-35480
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Still having this problem in 2022 
This may be why: From the myjetbrains issue, “The curves in the .3dm file are rational (weights not constant). The .ai format only accepts non-rational curves. There is no perfect conversion so we have approximate the the original curve with a fitting operation.”
both formats dxf and svg have circle / arc entities. (center, radius, start/end-angle.)
i would give svg a chance.
sorry i can t test it - i am using affinity-designer, not illustrator.
Hi -
I would strongly recommend not to use the .ai format. Both pdf and svg export have been tuned up to better support arcs and circles. As mentioned by others, dxf / dwg might be another alternative.
-wim
Hi Wim,
I tried with SVG and it does have round circles. The layer structure is not preserved however; it puts everything in one single layer.
.dwg seems to preserve circles and layers
Best,
Jeremy
Hi Jeremy -
When Rhino writes an SVG file, it takes the layer names to generate SVG groups.
How does Illustrator deal with layers when exporting to SVG?
-wim
if you need circles the layer-structure to be preserved i recommend:
→ export → pdf
Hi Wim,
Nested layers in Rhino; curves exported as .svg
Svg opened in Illustrator; all paths are circles, and on Layer 1. Hierarchy lost
Hi Tom,
How have you managed to get the layer hierarchy to remain when exporting to PDF?
When I exported from Rhino with the nested layers (export> PDF)
And open it in Illustrator; this is what happens
Clipping groups are made, and the layer hierarchy is lost/different
Cheers,
Jeremy
no adobe / illustrator on my machine. - demo/screenshots in affinity designer.
but layernames and one-dimensional (not sure about hierarchy) layerstructure stay. ( i think so in illustrator too)
i use it to control cutting order on a laser - on this computer there is illustrator - it is the same.
are you using export → pdf
which version of illustrator are you using ?
what import options ?
for which reason do you need the layer-structure ?
there is also Select → Same stroke Color in illustrator - this as backup will allow you to order / repair the document by colors.