Rockwell Automation 5370-CVIM2 Module - Manual (Page 197)

Chapter 6
Reference Tools
6–38
Stop When Selection
The “ Stop When ” selection field enables you to select either “ First
Feature(s) ” or “ Best Feature(s) ” as the basis for deciding when a template
match has occurred and the feature window can stop searching. Since the
feature window scans the search window in a left–to–right, top–to–bottom
pattern, “
First ” causes the search to stop when the feature window finds the
first template match that satisfies the parameter selections in the First Pass
panel. “ Best ” causes the feature window to scan the entire search window
and to select the best template match (which may also be the first when there
is only one match).
Figure 6.33 (page 6–39) illustrates the difference between First and Best . In
Figure 6.33 (A), the feature window is shown in position over the original
feature image (note that the X and Y coordinates are 293 and 198). In (B),
the search window is shown in position over the search area.
In (C), with the reference window tool in operation and First Features(s)
selected, the feature window finds the first feature in the upper–left corner of
the search window (it qualifies as a match according to the current parameter
settings in the
First Pass panel). This, however, is not the desired feature
(the X and Y coordinates are 141 and 193), and the RMS error is 18.915.
In (D), with Best Feature(s) selected, the feature window finds the best
feature at almost exactly the location of the original feature window. In this
case, the X and Y coordinates are 293 and 197, and the RMS error is 4.013,
which is a better match.
“ Best ” will require more time, since the feature window must always search
the entire search window, but it will usually give you better positional
accuracy, and it will also provide a true measure of the “worst case”
inspection cycle time.
NOTE: During configuration, “ Best ” can be useful to find the “ RMS
Error ” value for the best match, and to use that value as the basis for
determining the appropriate “ Max. RMS Pixel Error ” setting. This is
discussed later in the Pixels Error Parameter Selections section.