Shaping Patches

Adjusting patches to shape them is very similar to shaping Bezier splines, as we did earlier in the chapter.

	1. Click the Vertex Sub-Object button and select the vertex in the center of the four patches.

2.  In the front viewport, move the vertex upwards to form a hill.

3.  Move one of the Bezier handles on the vertex. Notice that the default is the Corner type, like the Bezier Corner tangents we used with splines.

4.  Right-click the vertex and choose Coplanar from the shortcut menu.

5.  Move one of the handles again. This time the handles are dependent.

	TIP Coplanar tangents make it easier to create a smoothly continuous curve; Corner tangents make it easier to create a precise change in curvature.

Taking Advantage of Patch Surface Options

Let’s take a look at the beauty of working in patches.

1.  In the Modifier Stack rollout, click the Sub-Object button to get out of the Sub-Object level.

2.  Scroll down to the Surface section of the Modify tab. Notice that there are separate settings for View Steps and Render Steps.

3.  Right-click the label of the perspective viewport and choose Edged Faces from the shortcut menu. This shows us the effect of surface steps.

	TIP Edged Faces displays patch steps and polygons in viewports set to Smooth+Highlight.

4.  Change View Steps to 15. Increasing the steps makes the patch surface much smoother, as can be seen in the before and after shots in Figure 5.7. The power of the two settings is that you can model with low steps in the viewport to keep your speed fast, render trials with slightly higher steps, and use high steps for the final render to get very smooth curves.

5.  Change the View Steps to 5 and the Render Steps to 12.

6.  Uncheck Show Interior Edges to show just the edges of the patches.

FIGURE 5.7  Top, a patch surface with 5 steps; bottom, 15 steps

Adding New Patches

To continue to build in patches, you need to add patches at the edge level.

	1. Click the Edge Sub-Object button.

2.  Select an outer edge of one of the patches.

3.  Click the Add Quad button in the Geometry rollout. MAX adds a new four-sided patch to that edge.

Attaching and Welding Patch Surfaces

As with mesh, spline and NURBS objects, you can attach separate patches as sub-objects of one object. You can then “sew” the pieces together by welding vertices.

1.  Click the Sub-Object button to get out of the sub-object level.

2.  In the top viewport, create another patch grid near the newly added patch, as in Figure 5.8.

FIGURE 5.8  Create a new patch.

3.  Using one of the select tools, select the first patch object.

4.  Go to the Modify tab and click the Attach button in the Geometry rollout.

5.  Click the newly created patch. The new patch changes color to match the first patch and is now part of the same object.

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

7.  Click the Vertex Sub-Object button.

8.  In the top viewport, marquee-select the 2 vertices near the edge.

9.  Turn the weld threshold spinner (next to the Weld button in the Geometry rollout) up to about 30 and click the Weld button. The 2 vertices should weld into one, connecting the surfaces at one point.

10.  Select the other 2 adjacent vertices and click Weld again. You should now have one smoothly continuous patch surface, as in Figure 5.9, which can of course be further shaped by the methods covered earlier.

FIGURE 5.9  Newly welded patches

Using Interior Vertices

The default Bezier handles control the curvature of the vertices on the corners of the patch. Additional control of the curvature is available through interior vertices. You can access these by choosing Manual Interior from the right-click shortcut menu on the Patch level, and then going to the Vertex sub-object level. Yellow squares appear that allow you to change the curvature of the interior of the patch.

Converting a Loft Object to Patches

Another way of patch modeling is to get part of the way there through the techniques of mesh modeling we’ve already covered, convert to patches, and continue to edit using the patch modeling methods above. All the mesh primitives can be converted to patches and, in an especially useful feature, loft objects can also be converted to patches.

Shaping a Loft Using Scale Deformation

First we need to create a loft object to convert to patches.

1.  Reset MAX.

2.  Create a small circle in the left viewport (radius about 20).

3.  Create a line across the top viewport, clicking once on the left, once on the right, and then right-clicking to complete.

4.  With the line selected, make a loft object (Create Ø Geometry Ø Compound Objects Ø Loft).

5.  Click Get Shape and then select the circle.

6.  Select your loft object and go to the Modify tab, scrolling down to open the Deformations rollout at the bottom. Click the Scale Deformation button. This brings up the dialog box seen in Figure 5.10.

FIGURE 5.10  The Scale Deformation dialog box

	7. Click the Insert Corner Point button and click three places between the endpoints of the line in the dialog box.
	8. Click the Move Control Point button to move the first point down. Notice that the loft is scaled proportionally as you do this.

9.  Marquee-select the second and third points (of five total points) and right-click one of them. Choose Bezier Smooth from the shortcut menu.

10.  Right-click the fourth point and choose Bezier Corner from the shortcut menu.

11.  Using the Move Control Point tool to move the points and the handles, create a graph that looks about like Figure 5.11.

12.  Close the Scale Deformation dialog box.

Converting to Patches

When converting a mesh to patches, MAX assigns patches based on the number of faces of the mesh. To use patches efficiently, we want as few vertices as possible. Let’s optimize the loft before converting it to patches.

1.  With the loft still selected, open up the Skin Parameters section of the Modify tab.

2.  Change the Shape Steps and the Path Steps to 2. This reduces complexity of the loft a great deal.

FIGURE 5.11  Create a scale deformation like this one.

NOTE You can output a loft object directly to patches by checking Patch instead of Mesh under Output in the Surface Parameters rollout of the loft. The only problem with this is that it gives you no control over the number of patches you get. By adjusting our shape and path steps, outputting to mesh, and then applying an Edit Patch, we get fairly precise control over the subdivision of the resulting patch object.

3.  Apply an Edit Patch modifier to the loft. Since we are only using this as a starting point, it doesn’t matter if our model is distorted at this point.

4.  Click the Vertex Sub-Object button.

5.  Now you can move around the vertices and their tangents to shape the object however you like.

6.  To smooth things out when you are done, apply a Relax modifier, and set the Relax Value to 1.

	NOTE A simple example of this exercise is available on the CD as loft_patch.max.

© 2000, Frol (selection, edition, publication)