National Instruments IMAQ Vision for LabWindows TM /CVI User Manual
Page 58
Chapter 5
Performing Machine Vision Tasks
IMAQ Vision for LabWindows/CVI User Manual
5-6
ni.com
Using Pattern Matching to Build a Coordinate Transform
You can build a coordinate transform using pattern matching. Use
imaqFindTransformPattern()
to define a coordinate system based on
the location of a reference feature. Use this technique when the object under
inspection does not have straight, distinct edges. Complete the following
steps to build a coordinate reference system using pattern matching.
Note
The object may rotate 360
° in the image using this technique if you use
rotation-invariant pattern matching.
1.
Define a template that represents the part of the object that you want
to use as a reference feature. Refer to the
section of this chapter for more information about defining a template.
2.
Define a rectangular ROI in which you expect to find the template.
3.
Use the options parameter to select your options for finding the pattern
and the results that you want to overlay onto the image. When setting
the
Mode
element, select
IMAQ_MATCH_ROTATION_INVARIANT
when
you expect your template to appear rotated in the inspection images.
Otherwise, select
IMAQ_MATCH_SHIFT_INVARIANT
. Set the options
parameter to
NULL
to use the default options.
4.
Choose the mode for the function. To build a coordinate transform for
the first time, set the mode parameter to
IMAQ_FIND_REFERENCE
.
To update the coordinate system in subsequent images, set the mode
parameter to
IMAQ_UPDATE_TRANSFORM
.