Rockwell Automation 5370-CVIM2 Module - Manual (Page 318)

Chapter 7
Inspection Tools
7–80
Image Subtraction: S1 – S2, S1 – S1’, S1 – T
In an image subtraction operation, the image tool subtracts an AOI in the
secondary image designated by
S2 or S1’ (or a “template” designated as T)
from an AOI in the primary image designated by S1 , and places the resulting
image in the designated destination buffer. More specifically, the value of
each pixel in the
S2 , S1’ , or T image is subtracted from the value of each
corresponding pixel in the S1 image. This results in a signed image; that is,
an image whose pixel values lie between –128 and 127.
If S1 and S2 (or S
Ȁ or T) are binary images, a logic XOR (“exclusive OR”)
function will be performed on the two images, as shown by the following
table:
S1 S2, S
Ȁ, or T
Pn or Bn
Black Black Black
Black White White
White Black White
White White Black
NOTE: Since image subtraction creates a signed image, the “ Sign ,”
“ Absolute ,” “ S.Threshold ” or “ S.Clip ” LUT should be used in order to
remove negative values from the image. The Sign , S.Threshold , and S.Clip
LUTs add 128 to each pixel value, thereby placing all values in the 0 to 255
range that the inspection tools require to identify and evaluate features
properly. The
Absolute LUT changes all negative values to positive values.
Typically, image subtraction is used to cancel all features that are common to
two images and leave only those features that are different between the two
images.
The similarities and differences between the image subtraction operations are
detailed in the following sections.
S1 – S2
The S1 – S2 image arithmetic operation subtracts an AOI within the S2
image from an AOI within the S1 image.
Figure 7.65 (page 7–81) is an example of how an S1 – S2 operation could
be set up in the toolset edit panel.