KSA FAB LAB > Digitizing > Microscribe with Rhino

Digitizing with Rhinoceros

This document describes how to use the Microscribe digitizing arm within the Rhino modeler.

Connect to the Digitizer

To capture geometry from within Rhino, you must first connect Rhino to the digitizing arm. This can be done in several ways:

1. From the Command prompt, type "Digitize" and hit Enter.
2. Open the "Tools" menu at the top of the screen, choose "3-D Digitizer", and then choose "Connect"
3. Open the "3-D Digitizing" menu (right-click in an empty toolbar area to activate the list of menus) and click on the icon.

You will next be prompted to select what type of digitizer you will be using. Make sure the "MicroScribe Digitizer" option is selected, and click "OK".

Orient the Digitizer

You will next be asked to calibrate the digitizing arm. You have two choices:

1. Accept the native digitizer coordinates
2. Calibrate the arm yourself

If the digitizer can comfortably reach all portions of the model you wish to scan, you can accept the native digitizer coordinates. However, if your model is too complex or too large to be digitized without moving the model in some way, you will want to calibrate the arm yourself. This will allow you to re-calibrate the arm after moving your model.

Calibrating the arm involves identifying the origin and X and Y axes in the physical world, and the corresponding origin and axes within Rhino. The real-world points must be attached somehow to your model, as it is critical that your model must not move relative to those points.

Move the tip of the digitizing stylus to the point on your model you wish to use as the origin, and step on the left pedal of the digitizer. Follow the remaining prompts in Rhino to identify the X and Y axes - typically the X axis is a left-right axis, while the Y axis moves towards you and away from you. The Z axis is the vertical axis.

After identifying these points in the physical world, you are then prompted to identify the origin and axes within the Rhino modeling space. Use your mouse to click on the origin and axes in response to the screen prompts.

Capturing Geometry

After connecting to and calibrating the digitizing arm, you will be able to use it as a 3D input device in conjunction with a wide variety of tools. Think of the arm as a 3D mouse - any command you might use in Rhino can be used with the digitizing arm describing the geometry, rather than the standard mouse.

Several common ways of digitizing are as follows:

Points

The simplest way to capture geometry is via a stream of points. Right-click the Point icon , which tells Rhino you will be capturing multiple points instead of just one.

Then, making sure to keep the tip of the arm in contact with your model, hold down the F12 key and drag the tip around your geometry. You will be able to see the points as they are captured in the modeling window. Make sure to adequately cover the surface to be captured, and then release the F12 key before picking the digitizing tip from your model.

After you have a field of points, you will see something like the illustration at right above. These points can then be reconstituted into a surface using a software tool such as FormZ's Reverse Engineering Plugin into the surface shown at right below.

Curves

A slightly more complex method of working is to draw regular curves across the surface to be captured, which are then skinned (via the loft tool, for example). Start the polyline or curve tool, and use the digitizer to draw lines at regular intervals across your model. When you are done, use the loft tool to stretch a surface across those curves.

Using more curves, and defining more points along each curve, will help produce a surface which more accurately describes your surface. Pay particular attention to regions of fine detail in your model.

At right is a series of images showing the same geometry sampled with varying resolutions. The top image shows the surface described by points. The second image shows the surface skinned with three lines of three control points each (9 total inputs). The third image shows the same surface skinned with five lines of five points each (25 inputs). The bottom image shows the surface skinned with 9 lines of 9 points each (81 inputs). The progressively better fit attained by each surface illustrates the need to provide as many input point as possible.

DigSection

In addition to working with the native Rhino commands, there are also commands available which are specific to digitizer input. Perhaps the best of these is the DigSection command, which automates and regularizes some of the "curves" method described above.

To use the DigSection command, type "DigSection" at the command prompt. Rhino will ask you to define a plane in space at one end of your object. Then, you will define the spatial axis along with that plane will be arrayed. Define the start of the axis at one extreme of your model, and the end of the axis at the other extreme of your model. Then, using a technique similar to the point capturing method above, define a stream of points that stretch across all the section planes. While you are constantly registering points, they will only be created in the Rhino file when they intersect one of the section planes.

At the conclusion of the DigSection command, Rhino will create a series of curves - one in each section plane - based on the number of points that were defined on that plane. These regular curves can then be used to loft a surface, similar to the Curves method above.