Converting a NURBS dog model to a Polygon in Maya 5

I had an old NURBS model that was made up of 21 separate objects. This creates unsightly seams at the joints. The front legs were made up of 4 separate sections each. The back legs were 3 separate sections each. The tail was separate from the body, as well as both eyes, nose and ears. See a picture of the NURBS model below. I used different colors on each body segment to illustrate how the objects are separate.

I used techniques I read about in "Maya: Secrets of the Pros: Modeling a Subdivision Surface Horse." Only, I did not convert the model to a Subdivision Surface at the end. Many modelers, as I've read in this book and another called "3D Modeling and Texture Mapping" by Tom Capizzi, start with NURBS models and later convert to a Polygon model. NURBS help give an organic shape quicker than Polygons, and when you convert them, you usually get a smooth polygon surface.

In order to get a seamless polygon model out of these segments, I did the following:

  1. I duplicated these segments and then converted the duplicated nurbs to polygons through Modify > Conver t > Nurbs to Polygons. I set the options to Quads, General, Per Span # of Isoparms, Number U and V set to 1. View Screen Capture of menu selection. View screen capture of options.
  2. After converting the Nurbs to Polygons, I then had to assign a general brown shader to it by creating one out of a lambert within the Hypershade Editor. Window > Rendering Editors > Hypershade. Create > Materials > Lambert. Right Click on the new lambert in the graph and select the "attribute editor." Click on the gray box next to "color" and select a brown color. Close the attribute editor. Click on the object you want to be shaded with this color in the viewing window. Then right click on the new shader and select "assign material to selection." View example of the Hypershade in Maya.
  3. I had to move the pivot points of all the separate poly objects because they were located outside the object at the World Space Origin. Select the move tool in Maya, then hit the “insert” key. Now you can move the pivot to the center of the object and hit “insert” key again to get it to stay. Use all orthographic views to center the pivot point. View example of centering a pivot point.
  4. Begin the laborious task of splitting polygons to create edges that can be merged together between objects. If you delete edges, be sure to delete their vertices. To add an edge on a polygon model go to Edit Polygons > Split Polygon Tool > Options. The subdivision number tells Maya how many vertices to set down on the new edge. Leave it at 1. The number of magnets is a way of telling Maya how far into a face to snap a vertice for a new edge. Changing this to 2, e.g. will set a vertice 1/3 of the way into an edge. To delete edges, right click on the object and select "edge." Select one or more edges (shift click for more than one). Then hit delete. Note: you can tell Maya to select contiguous edges by going to Edit Polygons > Selection > Select Contiguous Edges > Options. In the options, tell Maya how many edges to either side of the currently selected edge that you want selected. Hit "Select" and you should see all the selected edges highlighted in orange. View example of multiple selected edges.
  5. I moved some segments down in space and removed overlapping faces in preparation for merging edges to combine them into one contiguous object. To remove faces, I right clicked on the polygon object and selected “faces.” Then you lasso the faces and hit “delete” key. Note: it is important to delete the faces that will fall inside of the combined objects once merged. If left, they wil create rigging and texturing problems later. View example.
  6. Next, check your normals on each object before combining them. Go to Display > Polygon Components > Normals. If any are reversed, you go to Edit Polygons > Normals > Reverse. And select the Reverse and Extract option. They should all be sticking outward at this point. View menu selection example. View example of a selected object (in green) with its normals reversed. Note that the normals are pointing inward on the selected green object.
  7. Then, select all four leg segments and go under Polygons > Combine.
  8. Now you can merge edges together by going to Edit Polygons > Merge Edge Tool. I have the option set to “middle.” Simply click on one edge and shift click on the edge on the other object you want to join it to. If the edges aren’t creating normal looking faces, then probably some of your normals are facing inward. Select the offending faces and go to Edit > Polygons > Normals > Reverse to reverse and extract them again. If you click the "Y" key on your keyboard after merging each edge, you can continue with the next edge and so on. Hit enter when done. View example of edges being merged together on two combined polygon objects. Note the two edges to be joined are orange in color. Also note that all the normals are showing and pointing outward - which is what you want.
  9. The model will need tweaking at this point. This involves adding edges with the "Split Polygon" tool and selecting vertices and moving them around. You just keep doing this until you are happy with what you have. If the model is to be rigged, you should have multiple rings of edges around each joint - knees, elbows, wrists, etc. Usually, five rings of edges work well. View example of edges added around knee joint to form 5 rings - highlighted in orange.

Joining the legs to the torso:

This is a process of deduction. Somehow, you have to have an equal number of edges to merge together on each object. I started deleting faces on the inner surfaces of the leg and torso, where they meet. Then I moved vertices around slightly to see which ones would match up. Some faces were split in order to make edges and vertices line up between the two objects. This process is better taken slow and performed in stages on opposite legs (the two front legs, e.g.) in order not to delete a face you need. Ideally, you want both sides of the animal to be symmetrical.

I joined the tail and forelegs to the torso before joining the hind legs. I chose a lower position on the dog’s body than the tail would normally sit just so I would have a good face to face match up. I figure I can move the vertices around later to get it where I want it. When I converted the nurbs tail to a polygon, I had to set the per span # of isoparms for U to 6 and the per span # of isoparms for V to 3 in order to get 32 faces around the base of the tail that would match the 32 faces coming together at the end of the torso. Type was Quads, Tesselation method was General. Chord Height was set to .01.

Adjusting the model to fit my captured dogs

After I was happy with the general shape of the model, I scaled it to match an australian shephard and a greyhound. I used photographs of the dogs, or their breed, taken from the side. Below, you can view the pictures of how I lined up the model with the photographs. I used X-ray mode in Maya in order to see through the model to the photograph behind it.


Click here to see a movie of the model above.


See a movie of the greyhound model.

Back to Introduction Page.

On to Modeling the Dog Skeleton