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.