Poor Data In Equals Poor Data Out - 3D Tracing

By Kevin Paddy
What is 3D tracing? Are you doing it? To answer these questions, we must understand what 3D really means. 3D is the integration of 2D point data with third dimensional height points to create a three-dimensional (3D) circumference value.

Figure 1 is an example of standard 2D data: Figure 1 shows an oval shape with just 32 points. A single tracing point is represented by a radius value as well as an angle of degree. In most cases, tracers send anywhere from 400 to 18,000 2-dimensional points. While 18,000 points would seem to be more accurate, in all cases the interface compresses or expands the number of tracing points to 400 to 800.

Understanding 3D Data

The example shown in Figure 2 (3D data) shows the use of Z data. The number of Z points will vary from tracer to tracer, and of course, interface manufacturer to interface manufacturer. Tracers typically send 200 to 400 Z points and interface manufacturers typically send 200 points. Depending on your tracer manufacturer and/or interface system, the way this data is sent varies. There are three typical ways this data is passed on:

1 The Z data from the tracer is passed to the host without changing OMA/VCA label (ZFMT)

2 The Z data is converted to a 3D circumference value OMA/VCA label (CIRC)*

3 The Z data is converted to a frame curve usually in a diopters value. (FCRV)

*Note—OMA/VCA label CIRC is not specified as 2D data or 3D data, it can be either.

Any of these three formats are acceptable, provided the data is sent correctly. The most important part is that the host interface understands the data received. It is equally important that each edger understands the data being sent to it. As an example, sending FCRV as the third dimensional data value to an edger that does not understand FCRV renders this information useless and does not allow you to edge three dimensionally. This occurs far more often then you may expect. All new tracers have the ability to measure the third dimension of 3D, however sending this information correctly can be an issue. Figure 3 is an example of improperly passing on the third dimension of 3D data.

Figures 2 and 3 are identical shapes, however because they were loaded differently in the tracer or based on how the tracer calculates the third dimension, the data sent to the host can vary greatly.

Example of improper use of data If the host calculates a frame curve (FCRV) based on Figure 3, the curve may be calculated at 20 diopters. If the frame curve is calculated from proper data (Figure 2), the curve is calculated at closer to 6 diopters. Obviously if the edging size is calculated from a 20 diopter curve versus a 6 diopter curve, final edging sizes vary greatly.

While it is likely you will not be able to view this data, working closely with your tracer, interface, and edger manufacturers is your best way to understand what data these systems are operating with. From this you can optimize your system to make full use of the data provided. Tracing is a critical first step—and providing quality trace data improves more than just your finishing results.

Remember: poor data in means poor data out.


May/June LabTalk 2017