Using Reference Images

Although we won’t be using them for actual modeling in this chapter, let’s take a quick look at how to work with reference images. One option is to create three planes and map each with an orthogonal reference image in the diffuse channel. Another option is to put a different reference image in the background of each viewport. Let’s do the latter.

1.  Reset MAX.

2.  Choose Views Ø Viewport Background to bring up the dialog box in Figure 5.1.

3.  Select Front in the Viewport drop-down list.

4.  Click the Files button.

5.  Choose \Maps\Backgrounds\House.jpg and click Open.

6.  Under Aspect Ratio, check Match Bitmap.

7.  Check Display Background, Lock Zoom/Pan, and Active Only.

8.  Click OK to close the dialog box. Your front viewport now has the house image in the background. You can zoom in to the photo as necessary this way.

9.  Repeat steps 3 through 8, loading reference images into the top and left viewports for modeling.

FIGURE 5.1  The Viewport Background dialog box

Understanding Splines

In Chapter 4, we used splines in different compound objects. Patch and NURBS modeling are spline-based systems, so we need to discuss splines in more depth.

The term spline originated in ship-building, where a piece of wood would be shaped into a curve by distorting it with two pegs. Mathematicians borrowed the word to describe curves in terms of mathematical functions. In computer graphics, a spline is a curve defined by mathematical functions rather than a straight line segment defined solely by its two vertices.

Bezier Splines

We have a mathematician named Pierre Bézier to thank for the math behind patch surfaces as well as the splines created with the Shape tools in MAX. Bezier splines are also fundamental to vector drawing programs like Illustrator, FreeHand, and CorelDraw, as well as to the Pen tool in Photoshop. You need to understand them if you are going to work with computer graphics.

Editing a Bezier Spline

The vertices of an editable spline in MAX have four options for interpolating the tangents of the curve between them: Smooth, Corner, Bezier, and Bezier Corner. Let’s look at what these do to a spline.

1.  Reset MAX.

2.  Drag out a circle in the front viewport (Create Ø Shapes Ø Circle).

3. Go to the Modify tab, click the Edit Stack button, and select Convert to Editable Spline from the drop-down menu.

4.  Click the Sub-Object button and stay at the Vertex level.

5.  In the front viewport, drag a marquee around the whole circle to select all its vertices. You should see four vertices with handles, as shown in Figure 5.2. These are the default Bezier tangents.

FIGURE 5.2  Vertices with Bezier handles

6.  Select just the top vertex and move one of its handles. Notice that the handles on either side of the vertex are dependent, and therefore always move together.

7.  Right-click the vertex on the right side of the circle. In the shortcut menu you will see the four vertex options, with Bezier checked. Select Smooth from the menu. The handles disappear, and you have a smooth curve through the point determined entirely by MAX.

8.  Right-click the vertex on the bottom of the circle and choose Corner from the shortcut menu. This changes the tangents to straight lines coming in and out of the point.

9.  Right-click the vertex on the left side of the circle and choose Bezier Corner from the shortcut menu. This gives you handles that look exactly like those we saw with Bezier.

10.  Move one of the tangent handles. This time the handles are independent, so you can shape the curve more precisely. Your circle should look something like Figure 5.3 when you are finished.

FIGURE 5.3  Circle with tangents edited

Non-Uniform Rational B-Splines (NURBS)

B-spline is short for basis-spline, a mathematical concept including Bezier splines and NURBS, the two types of splines available in MAX. A rational B-spline is one that is defined mathematically as the ratio of two polynomial functions. A non-uniform B-spline is one in which the influence of a curvature can be varied. A non-uniform rational B-spline is called NURBS for short. Fortunately for those of us who don’t breathe the rarefied air of higher mathematics, we never have to understand any of the math to use NURBS.

	NOTE Another type of spline, called H-spline for “hierarchical spline,” is available through a plug-in for MAX called Rodin from Digimation.

With NURBS, the curve of a surface is shaped by control vertices (CVs) that do not lie on the NURBS curve. (This is true even of point curves, except in this case, the CVs are not accessible and points on the curve are accessible, in order to provide an alternate interface for modeling. Point curves and surfaces can be converted to their underlying CV form.) Each CV has a weight which determines the extent of its influence over the curve.

Shaping a NURBS Curve

Let’s look at how CVs shape a NURBS curve.

1.  Reset MAX.

2.  Choose Create Ø Shapes Ø NURBS Curves.

3.  Click the CV Curve button.

4.  In the front viewport, click once to create one endpoint, click in another place to create a CV, click in a third place to create the other endpoint, and then right-click to end the curve. Your curve should look something like Figure 5.4.

FIGURE 5.4  NURBS curve with control vertex

5.  Go to the Modify tab and click the Sub-Object button to get back to the Curve CV level.

6.  Select the CV that is shaping the middle of the curve.

7.  In the CV section of the Modify tab, change the weight to 5. Notice how the CV pulls the curve towards itself more strongly, as in Figure 5.5.

FIGURE 5.5  The weight of the CV has been increased here.

8.  Refine the curve with another CV by clicking the Refine button and clicking the curve. Click the Refine button again to turn it off.

9.  Move the new CV around to change the shape of the curve, as in Figure 5.6. Notice that if you move it so it is coincident with the first CV, you get a sharp edge.

	TIP Making two or three CVs coincident gives you sharper edges in NURBS.

FIGURE 5.6  Shaping the NURBS curve with a new CV

Modeling with Patches

A Bezier patch is a surface defined by vertices and the tangents of the edges between the vertices. A patch can have either three sides (Tri patches) or four sides (Quad patches). Quad patches tend to be preferred when modeling smooth surfaces, except when a Tri patch is necessary (to fill a hole in a model) or when it is helpful for modeling a particular detail. The vertices of patches can be coplanar (giving you Bezier handles that move together across the vertex) or corner (with independent handles).

MAX gives you several ways to model in patches. The traditional way is to start with a patch, subdivide it, add patches, and adjust their curvature. In MAX, primitives, meshes, and loft objects can also be converted to patches and then edited in patch form. Patch modeling has recently enjoyed a rennaisance with MAX users due to the addition of MAX Surface Tools as an additional method of modeling in patches.

Building and Editing Patch Surfaces

The old method of working with patches has been called “knitting a house.” The advantage of learning this way of modeling is that it applies to most 3D applications, so you will be able to transfer your skills easily to a job that requires you to model in patches in a different program. The disadvantage is it requires a great deal of patience and practice, but that’s true of most things in 3D.

Creating Patches

Let’s start by creating a patch and subdividing it.

1.  Reset MAX.

2.  Select Patch Grids from the Create drop-down menu (Create Ø Geometry Ø Patch Grids).

3.  Click the Quad Patch button and drag out a patch in the top viewport.

4.  Go to Modify Ø Edit Stack and choose Convert to Editable Patch from the drop-down menu.

	NOTE You can also use the Edit Patch modifier, analogous to the Edit Mesh modifier for meshes.

	5. In the Selection rollout, click the Patch Sub-Object button and then select the patch object in the viewport. You can see that the whole object is selected; we have a single patch.
	6. Under the Geometry section of the Modify tab, click the Subdivide button. Now select the patch object again. Only a quarter of the object is selected; there are now four patches.

TIP You can subdivide patches at the Patch or Edge sub-object levels. Checking Propagate when subdividing propagates the subdivision through adjoining patches, all the way through the model. The only time you would not check Propagate is when you want to make a “hole” in your surface. If you have two patch edges adjacent to a single patch edge, the surfaces are discontinuous. You can then move the extra vertex of the side with two edges, shaping a hole in the surface of the object.

© 2000, Frol (selection, edition, publication)