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Solving The Too-Many-Points Problem
By Jeremy Birn

A vexing problem with many NURBS models is that extra curves accumulate in areas of high detail, and then extend into areas where they are not needed. After you have added enough curves to define a detailed area such as an eye or ear in your surface, you will have created bands of extra points running all the way across the model, making the surface harder to manipulate and animate.

If you are stitching together multi-surface NURBS models with the requirement that all of the surfaces need to be fitted together point-to-point along the edges, then extra curves could still be transmitted over your whole model, increasing the complexity of other NURBS objects that each need a matching vertex count. Instead of helping the situation, SurfaceContinuityManager+ can actually make things worse, because you are now transmitting both extra curves and also extra "cuts" along the surface of your model. With the curves increasing the vertex count, and the cuts increasing the object count, models can quickly become unmanageable.

The following tutorial shows one strategy that allows high-detail and low-detail regions to co-exist in different parts of the same multi-surface model, and may help alleviate the situation in some cases.

 

1. Get > Primitive > Grid. Choose Cubic NURBS in both U and V. Change the X Cell Count and Z Cell Count to 8. (Leave the X Cell Size and Z Cell Size at 1.) Click OK.

2. Click the in the frame of the Top view, and turn off the window's grid, so that you can see the NURBS surface better.

3. In the top window, Draw > Cut the surface in half horizontally. (The cut line with be at the location 4 in the U Direction.) Cut the left half in half again, at the location 2 in the U direction. Select the right half, and cut it in half at the location 6 in the U Direction. As with all the other times you use Cut, go into the Schematic window, figure out which objects are which, and delete the uncut or partially cut surfaces that are left behind. There should now be only four surfaces in your scene.

4. Draw > Cut (how did you know that command was coming? ;) the four columns in half vertically, so that they are each cut at the location 4 in the V direction. As always, find and delete the original full-height pieces in the schematic window.

5. Delete the outer two surfaces on the top of the grid, as shown below.

6. Multi-Select the top two surfaces, and numerically enter in a new Scale X value of 2 in the Scale X gadget on the right of the screen.

Stop and think about the two areas you have created. The lower area has twice the resolution, and twice the number of objects, as the top area. They certainly could not be merged with Draw > Merge Surfaces while keeping their different resolutions, but an intelligent multi-surface assembly with SurfaceContinuityManager+ can make it possible to unite them. The next steps create a situation in which the number of points match-up along all shared edges, without changing the top or bottom resolutions.

7. Multi-Select the center two of the lower surfaces. Tag one point on each surface (as shown below) at the top of the center column where they overlap.

8. Make sure in the lower-right of the screen that you are in TAG mode.

9. Select Edit > Proportional Setup. Click the Linear button to make sure the curve is linear, and enter the value 2 in the Distance Limit. Click OK.

10. Click the small triangle in the upper-left corner of a trans gadget on the right side of the screen, to call up the numeric Translation dialog. Enter a value of 1 in the Z translation, and click Add.

The result of steps 7-10 should be a triangular gap in your surfaces, as shown below.

11. One last time, use Draw > Cut to divide the upper two surfaces vertically, cutting each at the location of 6 in the V direction. In the schematic view, find and delete the two uncut pieces.

12. Choose Show > Tag(Unselected). Make sure no other objects have points tagged, and then tag the upper row of points on the two outside lower surfaces, as shown below.

13. Also activate the Magnet function, by clicking on the in the frame of the Top window, and click On Tag in the Magnet section of the dialog. Unselected Objects should also be picked. Click OK. NOTE: Proportional Modeling should still be activated.

14. Select the middle-left surface. Hold down the m key, and drag the center point along the bottom of the surface, until it snaps to the center tagged point of the surface below it.

15. Hold down m again and click the next point to the right, and drag it until it aligns with the next tag to the right on the surface below.

16. Hold down m again, and drag the corner point to align with the last tag below.

17. Repeat the last three steps on the other side.

18. Turn off Edit > Proportional Modeling. Move each of the points along the lower edge so that they are each exactly magneted to the tag below them.

Now that all of the points are aligned, only a Four Sided surface is needed to complete the continuity, which can be made in the same way that it was made at the top of the hemisphere of the last tutorial.

19. Draw > Extract four curves from the interior edges of the surfaces around the opening.

20. Show > Edge Flags, and turn off Show > Line to get rid of the blue lines. Make sure that the red tips of all of the curves point to the left. If you have followed these instructions exactly, only the curve in the upper left will need to be inverted. Select whichever curves need to be inverted, and choose Effect > Inverse. Make sure that the red tips of all of the curves point to the left.

21. Choose Surface > Four Sided, and click each of the four curves running clockwise. Choose "Uniform" for the type of parameterization.

23. You can hide or delete the four curves. Select all 7 surfaces that you have created, and choose Deformation > SurfaceContinuityManager+. The high-resolution and low-resolution regions are now seamlessly united into the same surface.

This basic resolution-shift pattern can be reused in many places in many different models. Load the NURBANA|SI model multihead1 (below), and you will see this same configuration of surfaces used to seamlessly unite a low-detail scalp, a high-detail ear, and a low-detail neck, achieving a high-res ear that does not cause banding over the rest of the head. (See other angles and NURBANA wireframe configurations in the NURBANA|SI section.)

This tutorial copyright © 1999 by Jeremy Birn.
Please do not duplicate any part fo this tutorial without written permission.