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AECbytes Tips and Tricks Issue #28 (March 20, 2008)

Rounding Edges at Render Time in 3ds Max

Scott Onstott
Book & Video Author

There are few perfectly sharp edges in the real world but 3D modeling programs tend to render all corners and edges with absolute precision as if they came out of a machine shop as reference surfaces. Overblown precision like this often detracts from the realism of computer generated images, making them look "too perfect."

It is possible to geometrically model small radius fillets between surfaces, especially with NURBS. However, you probably don't have the time or the inclination to model fine detail like this (unless you are an industrial designer). And even if you did, adding this kind of small-scale detail to furnishings and other entourage would likely inflate your polygon count so much that rendering times would be excessive.

This tutorial is about an extremely useful and easy to use 3D shading effect that appears at render time in 3ds Max. Rounded Corners gives the illusion of smooth fillet curves on all corners and edges and is built into the arch+design material. It's an illusion very much like bump maps which have been around since the dawn of computer graphics. Both bump maps and rounded corners are shading effects that are calculated at render time—so they render very quickly.

The arch+design material becomes available when you set mental ray as your production renderer in 3ds Max 9 or later. Download the sample files and let's dive right in. Extract the contents of the zip archive into a folder and open Armchair.max from that folder.

Open the Render Scene dialog by pressing the F12 key. Select the Common tab and scroll down to expand the Assign Renderer rollout. Click the ellipsis button next to the Production Renderer, select mental ray from the list that pops up, and click OK.

Click the large Render button in the lower right corner of the Render Scene dialog box to do a test render. The optimized chair only has 2388 polygons and it renders quickly. All the curves in this rendering are geometrically modeled.

Press M to open the Material Editor. Select the third material slot on the top row and name it Wood. Click the Standard material type button and change it to Arch & Design (mi). Click on the wooden Chair object (not the leather seat). Assign the new Wood material to the Chair object. Select Pearl Finish from the template drop-down list. Drag the Cherry map from its slot in the Material Editor into the Color map slot (circled below). Choose Instance rather than Copy when prompted.

Scroll down in the Material Editor and expand the Special Purpose Maps rollout. Instance the Cherry map from its slot in the Material Editor into the Bump slot. Increase the Bump amount to 1.0. Do another test render and compare. You may have to zoom in on a detail of the chair to perceive the slight surface relief that the bump map simulates (see three-part image below for comparison).

Scroll up in the Material Editor and expand the Special Effects rollout. Check the box to enable Round Corners. Set the Fillet Radius to a quarter inch if it's not already. Render again and this time the difference will be more striking. All the edges and corners of the wooden chair frame will appear to be rounded off—all without adding any polygons!

Below is the comparison image—please click it to get a closer look. The bump map is helpful in giving more texture to the wood and the rounded corners effect is fantastic in how it softens the hard edges. If you look closely you'll see it isn't perfect, however. All edges in the mesh are rounded—including some inner edges I'd rather not have smoothed out. Remember that these are shading illusions so they'll never be as accurate as geometrically modeled detail—but close enough for most work.

click to enlarge

In the end, the chair looks more realistic and aside from some minor set up, it didn't take any more time to model or render.

A video showing this technique is available on my video podcast, The Digital Architect.

About the Author

Scott Onstott is a book and video author of AEC software tutorials. He has a degree in architecture from UC Berkeley and has served as an instructor there, in addition to working in several prominent engineering, architecture, and interiors firms in San Francisco. He has also worked as a technical editor and technology consultant.

Scott has contributed to over two dozen books and videos on AutoCAD, Architectural Desktop, VIZ Render, Revit, 3ds Max, VIZ, Photoshop, Illustrator, Painter, Fireworks, and Dreamweaver. He can be reached via:

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