Rendering in Revit - Circles Appear as Polygons, But Only in Imported Families

[tommyleeh]

Hello,

I am trying to render small details in Revit, and I am experiencing an issue where small curved faces render as polygons. However, this behavior seems to only occur when the small curved faces exist as part of an imported family. When I model-in-place an even smaller element, the render achieves the appropriate curvature.

Two images are attached for reference - the first shows a non-render screenshot depicting a modeled-in-place cylinder (left) and a family with a hole cutout (right). Both are of the variety "specialty equipment." In this image, both features appear with the appropriate level of detail. The second image is a render of the same view, but in this render, the cylinder face remains curved as desired, while the family with the hole cutout becomes polygonal. This behavior is curious to me, and I cannot determine what is causing the discrepancy.

What could be causing this behavior, and how can I render a small family with the desired level of detail in the curved faces? This is a simple example to illustrate the problem, so while I could use a workaround and model the entire family in-place, I would like to understand how to avoid this in the future.

Thank you for your time!

EDIT: I have discovered the difference between the family and the model-in-place geometry. When a cylinder is extruded from a circular profile, it maintains its curved faces when rendered. However, if this same cylinder is created from a revolved rectangle, it will render as a series of polygons. Why is this the case, and how can I make revolved shapes render as smooth curves?

[barthbradley]

More render time is needed is my first thought, but post the file for testing.  

[tommyleeh]

Hello,

This happens even when rendered at high quality. As I learned, it is a difference between how the geometry was created. A cylinder extruded from a circular profile will render with a curved face, but a cylinder revolved from a rectangle will render with polygonal faces. This is part of a much larger file, so I will try to create a separate, isolated file that illustrates the difference.

[barthbradley]

Just Group the relevant geometry and save the Group out of the Project as a separate RVT. 

[tommyleeh]

Here is the specialty equipment family I was using to compare the two methods of making a cylinder. Both have an equal diameter, but one is extruded and the other is revolved. For me, rendering them yields different results. I look forward to seeing if you have a similar experience.

[barthbradley]

Did you use a Polygon for the Extrusion Sketch?  

[tommyleeh]

The extrusion was generated from a circular profile. The revolved solid was generated from a rectangular profile.

[barthbradley]

I seem to remember a discussion about this before. It boils down to "faceting" I believe.  Don't recall solution or even if there was or is one.  

Here's the Rendered result of making the Revolve Barrel-Shaped.  Weird, huh? 

Here's a quickie Ray Trace of a typical-sized wine barrel.

So this could be related to the geometry's size.  Revit has a 1/32" minimum line segment it can compute. 

[tommyleeh]

Fascinating, or should I say "facet-nating" (I'm sorry). Thanks for your time investigating this. I am surprised that the results are that different! It looks like I will be avoiding revolved solids for the time being. Do you think this is something that would be worth bringing up to Autodesk's technical team? And if so, how would I go about doing that? I would like to one day be able to use revolved solids in Revit, as they are incredibly useful.

[barthbradley]

They will probably say they are BIM software - not nut and bolts software.  But open a support case anyways and let me know what they tell you. 

[tommyleeh]

Makes sense. Thanks again for the insight, and I will keep you updated if I learn more.

[barthbradley]

@ToanDN: Any thoughts on this one? 

[tommyleeh]

I reached out to Autodesk support, and this was their response:

"Yes, the way revolves are processed and rendered is different from extrusions. Unfortunately there is no user setting to control the faceting of revolved surface.

The Revit development team has improved this issue a couple of times in the past releases, where in Revit 2011 a 2" diameter would show edges.
https://forums.autodesk.com/t5/revit-architectu...
For small diameters such as 1/2", the limits are visible on render close-ups.

However, if you need revolves with less than 1/2" diameter to show smoothly on very close-up renders, please submit an wish in the Revit Ideas Forum.

This is the most effective way to request a new feature and/or a product enahcment. The Revit developer and management team monitor this forum regularly.
https://knowledge.autodesk.com/support/revit-pr...

In the near future, as you mentioned, please avoid revolves and use extrusions instead."

It looks like it is a limit of Revit's rendering capabilities at this time. I'll be following their suggestion and posting this as a wish in the Revit Ideas forum. Thanks again for your time investigating this!

[barthbradley]

AH! So my suspicions were correct! It is related to Revit's inability to compute very small line segments.  Well, thanks @tommyleeh for the official boiler-plate explanation. This is likely what I was remembering before.  

...Still, all is good. You now know how to make empty beer cans.   😁

[tommyleeh]

Haha this is true, that's a good use for this bug!

Happy to help! I requested "improved small-line rendering for revolved solids" in the ideas forum, so we will see what happens there.

Thanks again!

[ToanDN]

If you must model small objects using revolve, create a smaller slice of pie and array/joint to create a full pie.

From left to right: a cylinder extrusion, a 360 degree revolve, 12 x 30 degree revolves

[tommyleeh]

This is a clever workaround for my issue - thank you.

I hope someday Autodesk will improve the fidelity of small-scale renders when it comes to revolved solids, but in the meanwhile I'll be adopting this workaround.

Thanks!