A method and apparatus are provided for identifying differences between a
stored pattern and a matching image subset, where variations in pattern
position, orientation, and size do not give rise to false differences. The
invention is also a system for analyzing an object image with respect to a
model pattern so as to detect flaws in the object image. The system
includes extracting pattern features from the model pattern; generating a
vector-valued function using the pattern features to provide a pattern
field; extracting image features from the object image; evaluating each
image feature, using the pattern field and an n-dimensional transformation
that associates image features with pattern features, so as to determine
at least one associated feature characteristic; and using at least one
feature characteristic to identify at least one flaw in the object image.
The invention can find at least two distinct kinds of flaws: missing
features, and extra features. The invention provides pattern inspection
that is faster and more accurate than any known prior art method by using
a stored pattern that represents an ideal example of the object to be
found and inspected, and that can be translated, rotated, and scaled to
arbitrary precision much faster than digital image re-sampling, and
without pixel grid quantization errors. Furthermore, since the invention
does not use digital image re-sampling, there are no pixel quantization
errors to cause false differences between the pattern and image that can
limit inspection performance.