Render Nurbs Slower Than Not Rendering Nurbs?

[Robb63]

I had to render a fairly complex set of data yesterday for a client at approx 5500 pixels tall. I was working with the data in Rhino so I used the plugin to bring it into Keyshot, and had the "Render Nurbs" check in Rhino and KS.

I let the rendering start and when I can back about 10 minutes later it was still at 2%. I was up against a deadline so I had t figure out how to get it done much faster than that. I went back into Rhino, and meshed all of the data (using a very fine mesh), then switched off the "import Nurbs" check-boxes and updated the KS scene. This time, the whole thing rendered with exactly the same setting as before in under 10 minutes. Is this some bizarro thing that is only happening to me? I thought one of the features of "Nurbs Rendering" was faster renders?

Again, my mesh settings in Rhino were pretty fine, they are the same Mesh settings I use for 3D printing small parts to an Objet. I was just glad to have found a work around, and the client was very happy with the renders (which is ultimately what matters). Just wanted to find out for my own education.

Any ideas?

[soren]

KeyShot supports direct NURBS rendering using advanced algorithms, which is a very high accuracy rendering technique, where there is no approximation made to the geometry when rendering it. In other rendering software, NURBS surfaces are usually tessellated into an approximation using triangles which is then rendered and this is also the default behaviour when you import models in KeyShot.

You can see the difference if you import a highly curved object and zoom in. When rendering triangles you will eventually see jagged edges coming from the triangles, whereas in NURBS mode the surface will remain perfectly smooth. Springs and 3d text are good models to see the difference (see below - first image is triangles, second is NURBS which took 7 times longer to render).

Since NURBS rendering is so much more accurate it is also about 2-5 times slower than triangle renderings, but it depends on the scene. We are of course continually improving NURBS rendering performance and KeyShot 5.1 is going to be faster than 5.0 for NURBS rendering.

[Robb63]

I knew it took care of the faceted edges, but for some reason I was thinking using NURBS was faster than using mesh settings. That would explain why a lot of my scenes have seemed slower to move the camera, and update recently.

If I import the data using the Rhino plugin set to "Render Nurbs", then uncheck the "Render Nurbs" check-box within KS, does KS then stop using the Nurbs which would speed things up if a scene is running slowly? If it does, then is it using the mesh settings fro Rhino, or using a default KS mesh config?

[soren]

When you import a Rhino model with a mesh, that mesh is used for rendering. KeyShot does not support tessellation of NURBS surfaces at the moment, so you have to create the mesh in Rhino. Thus, if you import a file with a mesh you can switch back and forth between NURBS and triangles by unchecking "Render NURBS" or pressing the N key.

A useful workflow is to create a low quality mesh in Rhino and use this when you set up lights and materials. Then you can switch to NURBS rendering when doing the final render to achieve absolute geometric quality. If the original mesh is good for distance images you can of course render such images with the triangles and then use NURBS for closeup shots for example.