Converting Objects to Subdivision Form

The Convert to SDS tool allows you to convert other geometric objects, such as NURBS surfaces and quadrics, to SDS objects.

Curves are converted to sets of SDS vertices. No faces will be created.

Surfaces are converted to SDS surfaces consisting of triangles (faces with three vertices) and quadrangles (faces with four vertices).

The standard Realsoft 3D installation contains a scanned NURBS head. You can convert it to the SDS form as follows:

1. Go to the select window's geometry tab and select 'Paste from file'. Load the file 'models/head'.

2. Select the loaded head. Activate the To SDS tool from the 'SDS' tab of the toolbar. By default, the tool examines the complexity of the selected object and attempts to generate an appropriate number of faces for the SDS object. This automatic resolution detection is usually desired so just select 'Accept' from the View's pop-up menu to finish the conversion. A SDS head is created.

3. Delete the original NURBS head.

[Note] Note
You can also check the 'Del orig.' option in the control bar, which deletes the original object after converting it to SDS. Then step 3 is not needed.

Inverting SDS surfaces

Conversion to SDS form works by evaluating surface points of the selected object to form the control polygon of the SDS object. Therefore, the generated SDS surface is typically slightly smaller than the original surface. A SDS surface usually shrinks inwards from its control polygon. The shrinking can be compensated by appying the 'Invert' tool. The actual surface will then pass through the previously used control points.

Converting polygonal models to the SDS form

The following three tools can be used to achieve a successful conversion from polygonal models to SDS surfaces:

Convert to SDS

The To SDS tool handles polygonal (typically triangle based) models by simply converting the polygons to corresponding SDS faces. Polygonal models often consist of triangles. Unfortunately a non-symmetric triangular structure, when used as a SDS control polygon, usually gives a very poor rendering quality. The triangular nature of the model can be seen from the rendering.

Merge Triangles to Quadrangles

The 'Merge Triangles to Quadrangles' tool scans through the selected triangles (or the whole object if it is not in the editing mode) and attempts to merge them to quadrangles. Whenever two associated triangles form an approximately rectangular face, the tool merges the triangles to one rectangle. This simplifies the model. Also, quadrangles usually produce much nicer SDS surfaces.

Note that often it is not possible to merge all the triangles.

The tool can be found from the main menu bar: 'Methods/Sub Division Surface' submenu includes a full SDS specific toolkit.


Finally, you can invert the generated surface so that the surface passes through the original polygon points.

Create a coffee cup

Any geometric object that defines a true surface can be converted to a subdivision surface. You can use, for example, all NURBS modeling tools to create subdivision objects easily. This is demonstrated in the following tutorial.

Let's first create two simple NURBS surfaces, which represent the body and the handle of a coffee cup.

1. Create a vertical curve representing the rotation axis of a coffee cup. To do this, select the NURBS/Curve tool and click the left mouse button in the view window. Finish the curve by selecting Accept from the view's pop-up menu.

Nurbs Curve tool

Points of the curve

2. Select the created axis curve and activate the NURBS/Curve tool again. When the curve tool is active, the control bar shows you a number of curve specific creation options. Set the Pen option to Rotate.

Activate the Curve tool and set Pen to Rotate

3. Enter a number of points through the view window to define the profile curve for the cup. You can see the rotated surface in real time. Accept when the profile is ready.

Define a profile curve for the cup

Create a NURBS circle representing the cross-section of a handle

4. Switch to the side view and create a circle representing the cross-section of the handle. For example, select Nurbs Circle tool from the NURBS tools menu. Then enter two points through the view window to define the center and the radius for the circle.

5. Select the created circle, activate the NURBS/Curve tool again and set the Pen option to Sweep. Enter a number of points through the view window to create a handle for the cup. Then click Accept.

Two NURBS surfaces
Delete the construction curves

Now the NURBS modeling specific part of this tutorial is done and we can convert the NURBS objects to subdivision objects.

Before we do this, delete the construction curves, which were used to create the surfaces. These curves are located inside the NURBS meshes and allow you to edit the shape of the meshes. However, we only want to convert the surfaces to subdivision objects. So, select the curves and delete them.

6. Select both meshes and select the pull-down menu Tools/SDS/Convert to SDS or Triset. This activates the conversion tool. The control bar allows you to define the number of faces to be created. Default options are OK, so just click the Accept button. Two subdivision objects are created. Delete the original NURBS surfaces.

NURBS meshes converted to SDS objects

[Note] Note
You can also check the Del orig option in the control bar, which deletes the original objects after converting them to SDS.

We now have to weld the handle to the body of the cup. To do this, we need to merge the two separate SDS objects into a single object.

7. Select both SDS objects and click the Merge O (merge objects) button in the control bar. This merges the two selected objects into a single object.

Merge O tool

We could now blend the handle to the cup by creating faces manually between the edges of the handle and the body of the cup. However, this is quite a time consuming job. We use the 'Tunnel' tool instead. This tool allows you create a surface through any two faces. The only problem is that there is no face in the ends of the handle surface. This problem is easy to fix:

8. Switch to the point editing mode and select all the points of the upper end of the handle. Then click the Add button to create a closing cap face. Repeat the operation also for the lower end of the handle.

Add face tool

9. At this point, make sure that the handle is placed so that the end faces of the handle match well two faces of the body. If not, select all points of the handle, move them from the front view and rotate them from the top view. To deselect points, press the Alt key and click on, or drag around, the points you wish to deselect. Pressing Ctrl + t deselects all selected points.

Select the end points of the handle and click the Add tool
Select the faces to be welded...

10. Now switch to the face mode and select the upper end face of the handle. Hold down the Shift key and select a suitable face from the body of the cup. Then click the 'Tunnel' tool to create a surface through the selected faces.

...and click the Tunnel tool

Repeat this for the second end of the handle.

The handle merged to the cup