Biomedical Engineering Reference
In-Depth Information
the central position of the blob, respectively. Here, we use the top left of the image
as the origin of the
x
and
y
axes. Also, each blob is labeled with an element from
a label set
L ¼
{BG, FG{UPLEG, LRLEG, LLLEG, HEAD}}, which stands for
background and foreground, and foreground includes upper legs, lower right leg,
lower left leg, and head. The relations between all blobs are represented by an RAG
in which each graph node is associated with a blob. An edge connects two blobs if
they are adjacent in the image.
3.2
Initial Association
As in [
8
], the dissimilarity between the
i
th blob
B
t
i
at time
t
1 and the
j
th blob
B
j
at time
t
is evaluated by the Mahalanobis distance, as follows:
t
1
;t
i
¼ðf
t
1
i
f
j
Þð
S
t
1
f
t
f
Þ
1
ðf
t
1
i
f
j
Þ
T
þ
S
(2)
D
;
j
t
1
f
t
Here,
f
are the covariance matrices of all the feature vectors at time
t
1
and time
t
, respectively. Then the label of blob
i
*attime
t
1,
LðB
t
1
i
and
S
S
Þ
, is assigned
to
B
j
as its initial label according to
t
1
;
t
t
1
;
t
LðB
j
Þ¼fLðB
t
1
Þj
D
¼
min
i
;
i ¼
1
; ;
ng
(3)
D
i
i
;j
i;j
where
n
is the number of blobs at time
t
1. That is,
B
j
is assigned with the label of
the blob that is most similar to it in the previous frame.
For blobs assigned with body part labels, i.e.,
Li 2
{UPLEG, LRLEG, LLLEG,
HEAD}, the geometrical constraints are further checked to exclude possible
mislabels by computing the distance from those blobs to their corresponding sticks.
Here, the distance between a blob and a stick is defined as the distance from the blob
centroid to the line segment defined between the stick end points. If this distance is
smaller than a threshold
t
s
, this blob is added to the candidate list of its
corresponding body part
C
Li
. In our implementation,
t
s
is the minor axis length of
the corresponding stick.
3.3 Semantic Graph Growing
In order to reduce mislabeling in initial association, we propose a skeleton-based
graph growing method to find the correct leg blobs. For each blob
B
j
in the current
frame, we define its distance to the foreground in the previous frame as
d
FG
ðB
j
Þ¼
min
i
t
1
;t
i
fijLðB
t
1
i
with
Þ2f
FG
gg
, which reflects the possibility of a blob
D
;
j
