Using Surface Tools

3D modeling methods, like most things, have seasons of popularity. One year NURBS modeling is all the rage, the next NURBS are out and subdivision surface modeling is in. Methods go in and out of fashion, make comebacks, and so forth. Currently MAX Surface Tools is enjoying its season in the sun, and with good reason. Surface Tools allow you to model in patches by building a spline cage and applying a Surface modifier that creates a patch surface over the cage. The beautiful thing about Surface Tools is that it is relatively easy to make detailed edits to the spline cage that would be difficult to add to an established patch model.

Understanding a Spline Cage

To understand how to work with Surface Tools, we need to think about splines some more. We’ve seen the similarities between Bezier splines and Bezier patch surfaces. The vertices and tangents are very similar and are based on the same math. It makes sense, then, that you could draw an armature out of splines to define a patch surface. Unfortunately, MAX does not allow “branching” splines—splines that have more than two edges meeting at a single vertex—so a welded armature is impossible. How, then, do we build a “spline cage”?

How the Surface Modifier Works

Peter Watje, who designed Surface Tools, wrote the Surface modifier to look at a single object with different spline sub-objects and, if the spline vertices are coincident or within a specified threshold, to create a patch surface with vertices corresponding to the meeting places of the splines. If a section of the spline cage has three sides, a Tri patch is created; if a section has four sides, a Quad patch is created; and if a section has more than four sides, there is a hole in the patch surface.

How the CrossSection Modifier Works

The other modifier that makes up Surface Tools is the CrossSection modifier. Cross-Section helps you build parts of your spline cage by taking splines and building a cage of splines around them, with the initial spline as cross sections. It’s easier to see than to describe in words, so let’s look at it.

1.  Reset MAX.

2.  In the top viewport create a circle.

3.  With the Move tool, Shift-drag the circle upwards in the front viewport.

4.  Check Copy and set the number to 2 in the dialog box, and click OK. What you have so far looks like Figure 5.12.

FIGURE 5.12  Circles to CrossSection

5.  Select the bottom circle, go to Modify Ø Edit Stack, and choose Convert to Editable Spline from the drop-down menu.

6.  Under the Geometry section of the Modify tab, click the Attach button. Click the next circle above it and then the top circle.

	WARNING It is very important to attach the splines to be CrossSectioned in the logical order of cross sections. Otherwise you will get chaos.

7.  Click the Attach button again to turn it off.

8.  Apply a CrossSection modifier (Modify Ø More Ø CrossSection). Now you have a spline cage between the circle cross-sections, as shown in Figure 5.13.

	NOTE CrossSection cannot be limited to a sub-object selection. A method for Cross-Sectioning parts of your spline cage is explained in “Detaching Splines to Cross-Section” later in this chapter.

FIGURE 5.13  Circles with CrossSection modifier applied

Surfacing a Spline Cage

We just created a simple spline cage, so let’s take a minute to see what it’s made of.

1.  With the spline object still selected, click the Edit Stack button and select Collapse All.

2.  Click Hold/Yes to “Are you sure you want to continue?” Then click OK to close the dialog box.

3. Click the Spline Sub-Object button.

4.  Click different places on the “spline cage.” Notice that the splines aren’t really connected where they meet.

	TIP It is important to remember that there is more than one coincident vertex at any branching point of a spline cage. These vertices need to be moved together and you do not want to Weld them, because this may distort your splines. There is no way of making a completely welded spline cage, because splines can’t branch.

5.  Click the Sub-Object button to get back to the object level.

6.  Apply a Surface modifier (Modify Ø More Ø Surface).

7.  Check Flip Normals and Remove Interior Patches on the Modify tab. You have a continuous patch surface (based on your non-continuous splines), as shown in Figure 5.14.

FIGURE 5.14  Surface applied to spline cage

Good Habits for Working with Surface Tools

Working with Surface Tools takes some getting used to. You’ll pick up the habits that are most useful for Surface Tools one way or another; why not try these from the start rather than learning through painful experience?

Make a Modifier Set On the Modifier Tab, configure a button set of the modifiers you will use for Surface Tools modeling: CrossSection, Surface, Relax, and perhaps any other modifiers appropriate to your plans for the model as well. This way you don’t have to keep going through the “More” list all the time.

	TIP Another option is to create a customized toolbar including buttons for these modifiers.

Surface a Reference Copy To see what result you’re getting in your surface, make a reference copy of your spline cage (Shift-move and check Reference in the dialog box) and apply a Surface modifier to the reference. Some people like to model half the model as a spline cage, mirror it as a reference, and apply Surface to that, so the changes in the spline cage are seen in the other side of the model. (This is not a good option if you have any problems telling your left from your right.) Some people like to keep the reference surface in the same place as the cage (Edit Ø Clone, check Reference, and apply Surface). In this case, you may want to view your reference surface in see-through mode by pressing Alt+X while it is selected.

Turn Weld Threshold to Zero As mentioned earlier, you don’t want to weld your vertices all the time. If you have any weld threshold set at the Vertex sub-object level, you will get an annoying screen every time you move vertices together or create a line, asking you if you want to weld coincident endpoints. If you get this, click No. To avoid getting this message all the time, turn the weld threshold to zero until you actually want to weld.

WARNING When asked if they want to weld coincident endpoints, some people like to try Yes, see if it’s what they want, and Undo if it’s not. They do this because welding may result in a smoother surface. This is not recommended for beginning Surface Tools modelers. If you’re not experienced in what to look for, you may accidentally weld vertices that completely screw up your model and may not notice it until you’re much further along. You can always weld the coincident vertices of a specific point in the cage to see if this helps you. This gives you more control over how and where the welding happens.

Marquee-Select Vertices While marquee-select is not entirely necessary with the Area Selection feature in MAX R3, it is a good habit to marquee-select vertices when you move them, in order to get all the coincident vertices. It also helps to remind you of what you’re in fact dealing with: coincident vertices of several splines, rather than the welded point of an armature.

Turn Off Axis Constraints When you are moving vertices around, you don’t want the snap settings to be using the axis constraints. Right-click the 3D Snap button and uncheck Use Axis Constraints on the Options tab.

Know When to Use Vertex Snap When building your spline cage, you will sometimes want to draw a new line starting and ending at two existing vertices or to move a vertex to be coincident with another vertex. In this case you want 3D Snap ON and the snap settings to be set to Vertex. (Right-click the 3D Snap button, uncheck Gridpoints, and check Vertex.) At other times, you want to draw a new spline, or to move a vertex to a place other than another vertex. In this case you want 3D Snap OFF. Pressing S toggles between snaps on and snaps off.

	NOTE When your vertices still aren’t coincident, even after using Vertex Snap, you need to fuse them, as covered in the troubleshooting section that follows.

Use Selection Center to Scale Vertices You can scale Bezier vertices together the same way as you would with polygonal vertices in an editable mesh. The trick is that you have to choose Use Selection Center from the Use Center flyout in order to scale the vertices rather than the handles.

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