The Smoothing Threshold Concept

The

The MSA affects the ability of the smoothing algorithm to build a smoother/flatter boundary between connected *Polygons*. The mechanism only works on *Polygons *sharing a common edge and two *Points *along that edge. *Smoothing *also affects the ability of Layout's *Surface *functions to render a smooth looking boundary that would otherwise appear angular. This ability is known as *Phong Shading*. The MSA in Modeler pops up within the *Subdiv* (Subdivide/Smooth) command in the *Polygon *menu. When you wish to apply the
*Subdivide/Smooth *command, you are given the option of using a more appropriate
MSA than the 89.5degrees default, assuming you actually know a more appropriate value! This is where a clear understanding of the MSA principle is needed. Unfortunately, the conventional approach requires a degree of lateral thinking and can make an interesting Modeler task into a chore.

NewTek's version 5 definition of the MSA describes how it *....determines the maximum angle between any two adjacent polygons that will be rendered smoothly. Any angle greater than this value will be rendered with the edges between the polygons showing.* So, the MSA is the maximum angle between adjoining polygons that can be smoothed. Well no, actually, it's not. That interpretation suggests only very angular seams can be smoothed, when the opposite is the case. So, understanding how it, *'determines the maximum angle'*, is the clue to understanding the problem. A more accurate definition of the MSA is:

*The maximum angle subtended by the projected Normals of adjoining Polygons, up to which the seam will be smoothed.*

It may be located anywhere within the surface, though LightWave places it at the geometric

centre of the Polygon. When you select a polygon or group of polygons, LightWave

automatically draws in the Normals. Normals are automatic features and cannot be manipulated

in any way. They provide a visual indication of the direction a polygon faces.

Double-sided polygons show two normals.

So, if the *Normals *meet the criterion, then the *Polygons *will be *Smoothed*. Confusing isn't it! The best thing here is a diagram. The following shows two pairs of adjoining polygons seen 'edge on' so their seam angle and their *Normals *can be clearly visualised. The polygons may be single- or double-sided, the principle is unaffected.

In this diagram, the first pair of

B = 180 - SA2

The proof of this relationship is a simple exercise in trigonometry and will not be considered further here. The rule to remember is that the opposite angles of any quadrilateral add up to 180degrees. So, what we need to determine is which seam will be smoothed under the default setting. In the first pair of

(*) In the WaveGuide manual.

After making a brief reference to the significance of the

*A default MSA of 89.5degrees assures that any surfaces at right angles (90degrees) or greater to each other will not be smoothed* (!) (My exclamation mark)

In mid-2001, NewTek's release of LightWave v6.5 and subsequently v7 for the PC and Mac platforms, included a new approach to the Maximum Smoothing Angle. They've called it the 'Smooth Threshold'. Now this may be purely coincidental, but it's inclusion in their first upgrade since the publication of WaveGuide looks suspiciously like plagerism. Having said that, NewTek's description of the smoothing principle is no more lucid than it was in version 3.5!

**Go to Tutorial 2
Tutorial 3
Tutorial 4
Tutorial 5
Tutorial 6
Go back to the Tutorials Page
Home Page
Introduction
The Gallery
Downloads
WaveGuide Manual
Other LightWave Sites
My Amiga
**