The technique used to obtain the three-dimensional data during this study is described below. The following technique is presented as a series of steps in order to make the process as clear as possible. Many of the steps are required for other methods common to the instrument, but it is relevant to the description of the new method to include them here. The software used was Rhinoceros V1.0 for windows. Hardware included an Immersion Microscribe three-dimensional digitizer connected to a Pentium II, 400 MHz PC.
Step 1: Select a suitable specimen for digitizing. For the purposes of this study each element needed to be complete and relatively undistorted. Other studies may not require such rigorous standards. Figure 3 shows a sauropod humerus that meets the necessary requirements of the current study.
Step 2: Position the digitizer, computer, and specimen relative to one another. The computer must be positioned so that commands can be entered while digitizing is in progress. Position the digitizer and specimen relative to one another so the digitizing arm has the maximum possible range of motion around the bone. It is important not to move the specimen or the digitizer during a scan. In the following steps, it is assumed that the digitizer is positioned behind the specimen and in front of the user with the digitizer power switch and ports facing away from the user (Figure 1). Also, the foot pedal is assumed to be positioned such that the cables coming from the pedal face away from the user.
Step 3: Position and stabilize the bone for digitizing. It is best to find the most stable orientation. In this study, the humerus and femur were found to be most stable when positioned with either the anterior or posterior side laying on the floor or table. The lower limb bones are more stable when oriented with the lateral or medial side facing up or down. An exception to this is the sauropod ulna, which is difficult to stabilize so that it can be digitized from both sides easily. Place clay underneath corners of the bone until it is stabilized and will not move with normal hand pressure.
Step 4: Mark registration points on the specimen. Because the specimen will need to be digitized from both sides, it is essential to mark registration points to facilitate integration of scanned halves. Initially, numerous registration marks were made around the bone, but it was soon realized that four registration points are adequate for most elements. White correction fluid was used here, but a pencil mark could have been used as well. Remove marks after digitizing is complete.
Step 5: Initialize the digitizer. Before setting up the digitizer, make an XY coordinate plane on the surface that the digitizer and specimen are resting on. Select TOOLS, DIGITIZE, CONNECT from the main task bar, or press the digitizer icon and select the appropriate digitizer. Follow the prompts and enter the origin, x-axis and y-axis by moving the digitizer stylus to the appropriate location on the coordinate grid and pressing the right foot pedal or mouse button. The XY axes must be entered in the positive direction; otherwise the digitized image will appear upside down. The digitizer is now ready to receive data.
Step 6: Set up the program to receive data. In the Rhinoceros program there are no default units selected. Go to the TOOLS menu and select OPTIONS and then select UNITS. Here the user can select the desired units of measurement for data collection. Millimeters were chosen for the current project. Next select the method by which data will be collected. This can be done either by pressing the SKETCH CURVE button (dotted curved line on the digitizing menu) or by selecting TOOLS, DIGITIZE, SKETCH CURVE from the main menu.
Next, the user must select what kind of data is to be collected. For the first part of the digitizing process, point data must be collected in the form of registration points. To do this, deselect CLOSED CURVES and CURVES; then select POINTS. Data spacing (the interval at which data points will be captured) must also be set at this point. For large limb elements such as sauropod bones, 10 mm seems to work well. If the element is smaller, a shorter interval is necessary. Finally, press the OK button and move to Step 7.
Step 7:
Input registration points. Select the SKETCH CURVE icon and move the digitizer arm to one of the marked registration points and press the right mouse key or foot pedal (remember to immediately release the mouse button or foot pedal or the digitizer will continue collecting data). A point will appear on the appropriate spot on the displayed grid (it may be necessary to zoom out to see the point if a large specimen is being digitized). Next press the left mouse button or left foot pedal, and the previous command line will reappear. Move to the next point and repeat the process. Do this for all the preselected registration points. After entering the last registration point, zoom out and check that all the registration points appear on the screen (Figure 1A) and go to Step 8.
Step 8: Digitize the first half of the element. Select the SKETCH CURVE icon, press the O key, and then press ENTER to display the DIGITIZE OPTIONS menu. Deselect POINTS and select CURVES (be sure that the CLOSED CURVES option is not selected). Then select OK. Move the digitizer stylus to the desired starting point. It is not necessary to begin a scan at one end of the element; however, all but a small number of elements in this study were digitized from end to end. Starting on the side farthest from the user, position the stylus roughly halfway down the side of the element. This may also be done on the near side, but it was found to be very difficult to push the stylus away from the operator in a smooth curve over the surface of the element. Press the right input button and continue to hold it as the stylus is pulled approximately halfway down the other side of the bone and release the input button. The curve described will display as a series of points at the selected interval as the stylus is being moved and will change to a solid curve after the input button is released.
After finishing the first curve, press the left input button and the cursor will reappear. Move the stylus to the next desired starting position and press and hold the right input button. From this point, follow the same procedure as before and continue down the length of the element to the other end (Figure 2A). The interval between each curve is arbitrary, but curves should be drawn more frequently over areas exhibiting key features such as muscle scars. The wider the interval, however, the smoother the lofted surface will be. Make fewer curves in areas where there are minimal morphological data, such as the shaft of limb bones.
Step 9: Loft a surface over the curves. It is essential to create a rendered surface model of each completed scan in order to check the accuracy of the model. Select the LOFT option from the SURFACE menu or select the LOFT icon. Using the computer mouse, select the first curve at one end of the element. The curve will turn yellow indicating it is selected (Figure 3A). When the curve is selected, the point at which the cursor is placed on the curve determines the direction of the curve. That is, if the point selected is to the right or left of the midline of the curve that direction will be assigned as the direction of the curve. It is essential that each curve be selected by moving the cursor to the same side of the curve midline.
Failure to do this will produce a twisted surface (Figure 4A). Also, each curve must be selected in order from one end of the element to the other. Failure to do this will produce a surface that doubles back on itself (Figure 5A). Curves may be skipped but not subsequently selected for a given lofting operation. After all the curves have been selected, press the ENTER key. A wireframe surface will appear over the curves and a dialog box entitled LOFT OPTIONS will appear (Figure 6A).
It is at this point that the surface must be evaluated for accuracy. There are many ways to do this, but I have found the following sequence to produce the quickest and best results. Leave the default settings and select SHADED PREVIEW. The screen will now show a shaded surface. Compare this surface to the original element (Figure 2). Rotate the element by selecting the ROTATE option under the VIEW menu or by selecting the rotate icon from the taskbar. Holding down the left mouse key, rotate the surface in various directions and check for errors in the surface relative to the original. If any errors are apparent, select CANCEL in the LOFT OPTIONS dialog box and delete the erroneous curve or curves. At this point, a new curve can be added following the instructions above, or the surface may be lofted again minus the erroneous curve. Assuming the surface accurately represents the element being digitized, other options in the LOFT OPTIONS dialog box may be changed as the user desires. When the desired surface function is found, select OK.
Step 10: Save the file. The file may now be saved in one of a number of file formats. All files were initially saved as ProE\NT IGES (Initial Graphics Exchange Specification) files. The IGES format is compatible with most three-dimensional modeling programs.
Turn the specimen over and stabilize it. At this point the user can either select NEW from the FILE menu or simply delete the data that was just collected and rename the file. Beginning with Step 7, digitize the other side of the element. With both sides digitized, the two files can be imported into another program (e.g., Surfacer) and registered together to form a three-dimensional solid (Figure 7A).
