NURBS Modeling
January 24, 2003
By Brian J. Immel
Edit NURBS Menu
Projecting curves on surfaces
This function is dependant on the camera angle for how the curve will be projected onto the NURBS surface.
1. Create a curve and a sphere.
2. Move the curve so that it is sitting in front of the sphere in the side view.
3. Select both the curve and sphere.
4. Hit Edit Surfaces > Project Curve on Surface. Maya creates a curve on the surface of the sphere according the projection angle.
- There is a live link between the original curve and the projected curve. You can move the original curve to change the position of the projected curve but if the original curve goes beyond the projection radius of the sphere, then you’ll get an “Error: projectCurve1 (Project Curve Node): failed to compute the result” message.
- Since the curve is projecting through the sphere, it will also create another projection of the circle on the opposite side of the surface. You may want to delete that depending on your intentions with the projection.
Intersecting Surfaces
1. Overlap two surfaces.
2. Select both surfaces and hit Edit NURBS > Intersect Surfaces. This will create a curve based on the two surface’s penetration locations. This tool can come in handy for creating the slight bulge of the eye.
Trim Tool 
The Trim Tool can be a useful tool for creating holes in geometry but be warned that creating trim edges increases the geometry quickly and causes texturing issues later on.
Trimming a hole on a plane:
1. Create a sphere.
2. Create a curve that fits within the boundaries of the surface plane.
3. Select both objects and go to Edit NURBS > Project Curve on Surface.
4. Hit the Trim Tool (Edit NURBS > Trim Tool) and click on the plane. Which side the yellow dot appears on of the surface will not be removed. So, if you select the geometry outside the projection, then the surface within the projection will be deleted. Hit Enter to complete the function.
Booleans (are evil)
- Boolean tools are good tools but the surface they modify turn into trim edges, which cannot be attached. This can cause enveloping and texturing issues later on.
- Union Tool
- Subtract Tool
- Intersect Tool
Attach Surfaces
As the name suggests, you can connect two separate NURBS surfaces to make one.
A bit of warning: if one surface has more divisions that the one you are attaching to, you division count will go up drastically and this will reshape both surfaces slightly. Maya will try its best to average the next isoparms locations but it may not be to your liking. The solution is to make sure both surfaces have an equal number of isoparms. Use the Rebuild Surfaces function to achieve this.
How to attach NURBS surfaces:
1. Select both surfaces.
2. Go to Edit NURBS > Attach Surfaces option box.
3. Check the option for Attach Method to Connect, Multiple Knots to Keep and uncheck Keep Originals. If you leave Attach Method to Blend, your two surfaces will “melt” together rather than rigidly connect (can be handy if you want a more organic surface). If you leave Keep Originals turned on, it will create another surface so that you will have two NURBS surfaces in one location (usefully in testing out what the new surface will look like).
Detach Surfaces
1. Select a trim edge or isoparm(s).
2. Go to Edit NURBS > Detach Surfaces. Wherever you selected the trim edge or isoparms, it will break the surface apart into two separate NURBS surfaces.
Insert Isoparms 
One way to add more divisions to a NURBS surface and to have more CVs to model with is to insert isoparms.
1. Select the surface and right-click on the object and go to isoparms for your component selection type.
2. Select one of the current divisions and pull a new isoparms from it. This is not moving the original isoparms but drawing a new one from it. You cannot just pull one from anywhere because Maya needs to know which direction you want to create the new isoparms.
3. Go to Edit NURBS > Insert Isoparm. Now, wherever you placed the dotted yellow line, Maya will now insert a new division there.
4. To insert multiple isoparms at one time, after pulling the initial isoparms out, hold down shift and pull out as many as you like the hit Insert Isoparm.
5. You can also insert isoparms in between selections. Go to Edit NURBS > Insert Isoparm option box. Change Insert Location to Between Selections and hit Insert. Using this option is great if you’re trying to place an isoparms that is equal-distant from one isoparms to the next.
Reverse Surface Direction
Necessary when attaching NURBS surfaces or preparing to texturing.
Select the NURBS surface and hit Edit NURBS > Reverse Surface Direction..
Rebuild Surfaces
At some point in modeling, it may become necessary to rebuild a surface because you either have too many isoparms or the surface is getting out of control or you just want to make the model a little lighter.
1. Select the geometry and go to Edit NURBS > Rebuild Surfaces option box.
2. With this window open, you’ll have to do a little testing to make your old surface like new again. Play with the different options under Rebuild Type, Number of Spans U, and Number of Spans V. Then hit Apply to keep the option window open so you can go back and make any changes to your choices under this window.
Sculpt Surfaces Tool 
This tool comes in very handy when you want to sculpt surfaces to have a more natural and organic feel to it.
This tool works by pushing and pulling CVs on the U and V.
1. Select your surface and go to Edit NURBS > Sculpt Surfaces Tool option box.
2. The Tool Settings window pops up and gives a wide range of option for sculpting purposes. Under the Sculpt tab we have the following options:
- Under Stamp Profile, you have Radius (U) and Radius (L). These two options control the diameter of the brush.
- Opacity is a filter for the strength of the sculpt tool. Lets say you’re pushing a surface with 10 units of force. With the Opacity set to 25%, the brush would push with a force of 2.5 units.
- Operation
- Push: Pushes vertices away from the viewer.
- Pull: Pulls vertices toward the viewer.
- Smooth: Averages the height and distance between the surrounding vertices.
- Erase: Tries to flatten out the surface by averaging the vertices around it.
- Under the Stroke tab we have the following options:
- Reflect Paint
- Reflect On/Off
- This option can cut your work time in half if you’re trying to model something that is symmetrical on both sides (like a face for example).
- Depending on the surface creation history,
you may have to play with the values in U/V
Dir.
- U Dir
- V Dir
- Invert Reference Vector On/Off
- Stylus Pressure
- None, Opacity, Radius, Both.
- Maya, as most other 3D packages do, can use a stylus pen-tablet. This input device can be very powerful for sculpting (and many other functions) because of the pressure sensitivity of the stylus. When using this type of input device, think of using like the Airbrush tool in Photoshop. The lighter the touch, the lighter the feedback and vice-versa.
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