Feature Detection Filter Using Orientation Fields
A target object is found in a target image, by using a computer for determining an orientation field of at least plural pixels of the target image, where the orientation field describes pixels at discrete positions of the target image being analyzed according to an orientation, and location. The orientation field is mapped against an orientation field in model images in a database to compute match values between the orientation field of the target image and orientation field of model images in the database. The match values are thresholded, and those match values that exceed the threshold to are counted to determine a match between the target and the model.
1 . A method of recognizing a target object in a target image, comprising:
using a computer for determining an orientation field of at least plural pixels of the target image, where the orientation field describes pixels at discrete positions of the target image being analyzed according to an orientation and location;
mapping the orientation field against an orientation field in model images in a database to compute match values between the orientation field or the target image and orientation field of model images in the database;
thresholding the match values against a threshold; and
counting those match values that exceed the threshold to determine a match between the target and the model.
2 . The method as in claim 1 , further comprising determining a common scale for both the model and the target, and changing all images to represent said common scale.
3 . The method as in claim 2 , wherein said common scale is set to 2.5 pixels.
4 . The method as in claim 2 , wherein said threshold is above 3% of a minimum weight.
5 . The method as in claim 1 , wherein said orientation field is one that points parallel to an object when inside the object and points perpendicular to the object when located outside the object.
6 . The method as in claim 1 , wherein said mapping comprises measuring an alignment between orientations of vectors.
7 . The method as in claim 1 , wherein the orientation field has a periodicity of 180 degrees when rotated around a point.
8 . The method as in claim 1 , wherein the orientation field is a tuple {w,θ}, where w is a real valued scalar, and θ is an angle in an interval [0, π), formed by generating an integral field S of an image I by
S
(
x
,
θ
)
=
∫
-
L
2
L
2
I
(
x
+
r
θ
(
s
)
)
s
and determining the tuple as:
w
(
x
)
=
max
φ
∈
[
0
,
π
)
∫
0
π
K
(
φ
,
μ
(
x
)
)
S
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x
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μ
and
θ
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x
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=
arg
max
φ
∈
[
0
,
π
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∫
0
π
K
(
φ
,
μ
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x
)
)
S
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x
,
μ
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μ
where K(μ,λ) is a real-valued function with properties K(μ,λ)=K(λ, μ) and K(λ,λ)=1.
wherein
K (μ,λ)≡cos(2(μ−λ)).
9 . The method as in claim 1 , wherein said computing match values computes an autocorrelation of the orientation field.
10 . The method as in claim 9 , wherein said autocorrelation comprises establishing a match at pixel (i,j) by
M ( i,j )=(1 −a ) C ( i,j )+ a∥A−B (i,j) ∥
where C(i,j) is a match map computed as
C
(
i
,
j
)
=
∑
k
=
1
M
∑
l
=
1
N
w
target
(
i
-
M
2
+
k
,
j
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N
2
+
l
)
w
model
(
k
,
l
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cos
(
2
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θ
target
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i
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M
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+
k
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j
-
M
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+
l
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model
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k
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,
A is an auto-correlation for the model computed as
A (i,j) ( k,l )= w ( i−k,j−l ) w ( k,l )cos(2(θ( i−k,j−l )−θ( i,j ))), (3)
and B(i,j) is a sub-matrix extracted around a neighborhood of C(i,j) a is a parameter which determines how much weight to give alignment C(i,j) for a final measure, versus an amount of auto-correlation. where, if a=0, only an alignment measure is used, and if a=1, only an auto-correlation measure is used, and if a=0.5, alignment and auto-correlation are equally weighted.
11 . A method for generating stable image descriptors for object recognition, which are stable with respect to variations in color, contrast, and lighting, and various image artifacts, the method comprising:
using a computer for generating orientation angles between 0 and 180 degrees at each of a plurality of pixel locations based on line integration;
using the computer for generating orientation weights, which indicates a reliability of the computed orientation angle using line integration; and
assigning all orientations with weights above a certain threshold weight as being active, and all orientations with weight below a certain threshold as being inactive.
12 . A method for generating a scale invariant representation of an orientation field, the method comprising:
for multiple scales, compute average orientation weights for all pixels of at least part of an image;
selecting a scale that maximizes the average orientation weights; and
re-sizing the orientation field based on a relation between an optimal scale and a pre-determined reference scale.
13 . A method for measuring a correlation of two orientation fields 1 and 2, the method comprising:
Compute an alignment between an in orientation field 1 and an orientation in orientation field 2, and assign the alignment a value between −1 and 1, where −1 indicates that an orientation angle 1 of orientation field 1 is perpendicular to an orientation angle 2 of orientation field 2 where, 1 indicates that orientation angle 1 and 2 are parallel, and
0 indicates that the orientations differ by 45 degrees or that at least one of the two orientations have weight 0;
Computing an average alignment of all orientations of orientation field 1 and a corresponding orientations in orientation field 2.