True 2D curves for accuracy



I know all the background of why 3d objects are faceted, but after having spend 4 hours this morning cleaning up an autocad nightmare mess [500 layers , blocks in blocks, nothing in the correct layer. etc etc ] if I could only draw real circles and arcs in SU I would dump the whole thing into SU, clean it up in a fraction of the time and export back out again… only issue is curves would be faceted…

Now I will give you another reason - [Accuracy] how many of you have tried setting out roads in SU, with lots of graceful curves… of course it gets converted into facets and all the geometric accuracy is lost, intersection points shift, centrepoints get lost,

If I could do my setout in 2d true curves I dont have to worry how SU approximates in 3D.


simply use a CAD system for doing this.


You are missing the point !

I do not want to open another CAD package just to represent a curve accurately

SU, Apart from being an exceptional 3D software is also great for

2D work as it is also exceptionally easy to draft, quick, intuitive, flexible, easy to manage


I cannot represent curves accurately !

If I could do that I could ditch Autocad [aka Draftsight ] entirely.

I am not expecting to represent 3d curves, just 2d curves mathematically correct.

All my orthogonal lines are mm accurate, but when I draw a curve it is immediately compromised

The world needs a simple intuitive 2D software package and SU is almost there

An no LAYOUT is not it!

LAYOUT does not know what is is and suffers accordingly!

Have you never exported or imported a curved line to or from a DWG ?


I’m surely not missing the point… SU is a polygon modeler regardless if used for 2D or 3D purposes, every entity output to the display is approximated/interpolated by edges and facettes, nothing will change this behaviour because it’s integral part ot the SU modeling kernel.


@gsharp I do agree that this is a limitation of SU but I also understand why it is (when you acknowledge that SU is a surface modeler and not really intended for 2D work). I presume you know that you can set the number of facets to approach a true curve? I also use a plugin (Circle Intersect) that works out the correct position of any intersections. I find that the combination of these two makes it possible to achieve accuracy sufficient for most real world purposes.

Like you, I am a transferee from the Autodesk world. Naturally, we wish that we could have all the advantages of SU with all those of a traditional 2D drafting package like AC. Sadly, you can’t. Not yet, anyway.


So, you know it is technically impossible for SU to represent a 2D curve?

As that is all I am suggesting as a request?

Or are you telling me you don’t want this feature because SU is a 3D modeler

and you have some inherent objection to it being used for 2D?


Don’t know if it helps with what you want to do, but if you export curves as 3dmodels to .dwg/.dxf you get true curves.


SU can handle internally analytic circles/arcs (which is good for being exact as long as possible) but needs to interpolate them as soon as e.g. modeling operations based on these entities do occur. See the element info tab for evaluating the current state of an entity.


The use of edge approximations is so fundamental to the way SU represents geometry that changing it would be incredibly difficult if not quite impossible.

There are already two workarounds suggested - one is to use the Circle Intersect plugin, which calculates exact intersections with a circle. The other, if you want to export your model with exact circles and circular arcs, is to export to a 3D DWG format. (You can do this even for a drawing with all z values constant - i.e., the geometry is only 2D.)

For whole circles or arcs (but not for ones that have been disrupted by joining edges, and hence converted to edges or curves) SU preserves in the object meta data information about the centre, radius and cardinal points or endpoints, and exports this to 3D DWG formats (but not to 2D DWG), if I understand it correctly.


Thanks for that hint Box…

I had forgotten that capability and that helps for export,

Unfortunately, it is very easy to break the curve into individual segments

with intersections or other SU commands

What I guess I was hoping for is a 2d curve entity that stays as a 2d curve entity both ways…

and does not get segmented.

Thanks sketch3d / John for your inputs

Simon thanks - I use the plugin you mentioned - only gotcha there is you have to be

careful to pick equal length segments


There is also a Plugin, Lines to Arcs, that reinstates Circles and Arcs rather than just weld the segments together.


Yep , got that one thx…

Unfortunately, If I draw a line from the center to cross the perimeter mid segment

the distance will not be a true radius…

This is the inherent inaccuracy of curves in SU that I would like to avoid

Usually I have one layer in SU called SETO [setout in AIA convention] that I put all geometric setouts,

it is grouped and locked and l identify curve centrepoints and radia etc…

it is only 2D and usually hidden unless needed to check my model…

But I can’t avoid the error of a mid segment measure…



It’s a good feature request but since it is unlikely to be implemented any time soon, perhaps you need to employ some options that are available. One option with native tools would be to draw the line from the center slightly beyond the edge of the existing arc. Then erase one side of the arc and replace it with a new arc drawn with the Arc tool. (Not the 2-Point Arc tool) so that you have end points in the right location. You can then trim the line from the center if needed.


An old request is to implement GuideArcs, GuideCircles and GuideCurves (ie, made of construction lines allowing all the various stipples.) These being guides would never need to be segmented.


The 2D vs 3D distinction is irrelevant.

Circles are planar regardless of whether they are drawn in a 2D environment or in a 3D environment. All 3D adds is the ability to have the circle arbitrarily tilted instead of restricted to a fixed drawing plane. The 3D math is a bit messier, but that is hardly enough to prevent true export. It seems more likely to me that the 2D export and import were originally conceived as image-oriented WYSIWYG, sort of like screen shots, even when the destination format is vector. For consistency with what you see when exporting to a raster format, the programmers just pushed out the Edges and Faces. Provided that the destination format supports real circles and curves, I can’t imagine any reason the 2D vector exporters and importers couldn’t be reworked to support real circles.

But a deeper issue is whether it would be practical to modify SketchUp so that it constantly recalculates the locus of a circle based on the mathematical formula for the circle vs based on concrete representation as a regular polygon. Without this fundamental change, many of the things one would expect from circles would be impossible. The smoothness of a circle couldn’t increase when we zoom in. We would still face the situation that some kinds of edits would force SketchUp to abandon the metadata identifying a sequence of Edges as coming from a circle. So, except in relatively unedited models we wouldn’t have circles to export anyway! Likewise, we would continue to see intersections happen along the polygon Edges, not at the true perimeter of the circle. This would cause inconsistencies between the model and the exported representation. Exporting/importing true circles today causes gaps or overlaps in 3D export and import because intersecting Edges end on the polygon, not on the mathematically exact circle!

I think this change would go very deep into the core design of the SketchUp model database and all the operations on it. Today, finding intersections between Edges is relatively simple and fast. With true mathematical circles it would be more complicated and inevitably slower. How much slower? I don’t know. And I despise those who say “it would be easy” or “it would be hard” without access to the SketchUp source code (which I don’t have) and without attempting to program the change, so I’ll stop there.


That would be a good start Dan…

its actually exactly what I wanted ,

downside is guides don’t roll into LO in vector format

sigh !


There is a plugin “Tangent Tools” that gives true intersections and tangents for circles. I don’t know if that will help you. It has solved a number of problems for me created by the segmented circles used by SU.


PS. I tend to save the center point of circles and arcs that I am using.


Hi Rick?

Yes I usually do the same saving centerpoint and radials [straight lines]

Re the plugin tip… thanks I will look at it…


PS I think you mean the amazing TIG and his “True Tangents” plugin [v3]


In 3DS max I remember the segmentation of a curve was still available to increase or decrease quite far into modification of the objective. Maybe this kind of non-greyed out sides in entity panel can be hopefully?