1962 IRE TRANSACTIONS ON INFORMATION THEORY
179
Visual Pattern Recognition by Moment Invariants”
MING-KUEI HUt
SENIOR MEMBER, IRE
Summary-In this paper a theory of two-dimensional moment
invariants for planar geometric figures is presented. A fundamental
theorem is established to relate such moment invariants to the well-
known algebraic invariants. Complete systems of moment invariants
under translation, similitude and orthogonal transformations are
derived. Some moment invariants under general two-dimensional
linear transformations are also included.
Both theoretical formulation and practical models of visual
pattern recognition based upon these moment invariants are
discussed. A simple simulation program together with its perform-
ance are also presented. It is shown that recognition of geometrical
patterns and alphabetical characters independently of position, size
and orientation can be accomplished. It is also indicated that
generalization is possible to include invariance with parallel pro-
jection.
I.
INTRODUCTION
I%
ECOGNITION of visual patterns and characters
independent of position, size, and orientation in
the visual field has been a goal of much recent
research. To achieve maximum utility and flexibility, the
methods used should be insensitive to variations in shape
and should provide for improved performance with re-
peated trials. The method presented in this paper meets
a.11 these conditions to some degree.
Of the many ingeneious and interesting methods so
far devised, only two main categories will be mentioned
here: 1) The property-list approach, and 2) The statistical
approach, including both the decision theory and random
net approaches.’ The property-list method works very
well when the list is designed for a particular set of pat-
terns. In theory, it is truly position, size, and orientation
independent, and may also allow for other variations.
Its severe limitation is that it becomes quite useless, if
a different set of patterns is presented to it. There is no
known method which can generate automatically a new
property-list. On the other hand, the statistical approach
is capable of handling new sets of patterns with little
difficulty, but it is limited in its ability to recognize pat-
terns independently of position, size and orientation.
This paper reports the mathematical foundation of two-
dimensional moment invariants and their applications to
visual information processing.’ The results show that
recognition schemes based on these invariants could be
truly position, size and orientation independent, and also
flexible enough to learn almost any set of patterns.
In classical mechanics and statistical theory, the con-
* Received by the PGIT, August 1, 1961.
t Electrical Engineering Department, Syracuse University,
Syracuse, N. Y.
1 M. Minsky, “Steps toward artificial intelligence,”
PROC.
IRE,
vol. 49, pp. 830; January, 1961. Many references to these methods
can be found in the Bibliography of M. Minsky’s article.
2 M-K. Hu, Pattern recognition by moment invariants,” PROC.
IRE (Correspondence), vol. 49, p. 1428; September, 1961.
cept of moments is used extensively; central moments,
size normaliza,tion, and principal axes are also used. To
the author’s knowledge, the two-dimensional moment
invariants, absolute as well as relative, that are to be
presented have not been studied. In the pattern recogni-
tion
field,
centroid
and size normalizatfion have been
exploited3-5
for “preprocessing.” Orientation normaliza-
tion has also been attempted.5 The method presented
here achieves orientation independence without ambiguity
by using either absolute or relative orthogonal moment
invariants. The method further uses “moment invariants”
(to be described in III) or invariant moments (moments
referred to a pair of uniquely determined principal axes)
to characterize each pattern for recognition.
Section II gives definitions and properties of two-
dimensional moments and algebraic invariants. The mo-
ment invariants under translation, similitude, orthogonal
transformations and also under the general linear trans-
formations are developed in Section III. Two specific
methods of using moment invariants for pattern recogni-
tion are described in IV. A simulation program of a simple
model (programmed for an LGP-SO), the performance
of the program, and some possible generalizations are
described in Section V.
II.
MOMENTSANDALGEBRAIC INVARIANTS
A. A
Uniqueness
Theorem Concerning Moments
In this paper, the two-dimensional (p + n)th order
moments of a density distribution function p(z, y) are
defined in terms of Riemann integrals as
m m
m,, =
ss
xpYaPb, Y) &J dY,
-m -m
p, q = 0,1,2, *-* .
(1)
If it is assumed that p(z, y) is a piecewise continuous
therefore bounded function, and that it can have nonzero
values only in the finite part of the xy plane; then moments
of all orders exist and the following uniqueness theorem
can be proved.
Uniqueness Theorem: The double moment sequence
{m,,] is uniquely determined by p(s, y); and conversely,
p(z, y) is uniquely determined by {m,,) .
It should be noted that the finiteness assumption is
important; otherwise, the above uniqueness theorem might
not hold.
3 W. Pitts and W. S. McCulloch, “How to know universals,”
Bull. Math. Biophys., vol. 9, pp. 127-147; September, 1947.
* L. G. Roberts, “Pattern recognition with an adaptive network,”
1960 IRE
INTERNATIONAL CONVENTION RECORD,
pt. 2, pp. 66-70.
6 Minsky,
op.
cit., pp. 11-12.