IEEE Robotics & Automation Magazine - December 2013 - 86

y
Line Length

z
z

y x

y
Robot Base
Reference Frame

Line Orientation

x
Camera
Reference Frame

x
Figure 2. A home cleaning robot with our embedded vision
board, including a camera, an ARM11 processor, and 64 MB
RAM. (Photo courtesy of the Future IT Laboratory of LG
Electronics, Inc.)

(a)
y
1st

iw-avg.
2nd

iw-avg.

ivc1
Orthogonal
Lines

ivc2

x

(b)
Figure 4. The visual compass concept: (a) line length and angle
in the camera reference frame and (b) two rotational angles
2nd
obtained from two dominant orientations i 1st
w -avg. and i w -avg. of
the orthogonal lines.

(a)

Vertical
Lines

(b)
Figure 3. Extracted lines in 320 # 240 resolution images
captured from a single camera pointing in the upward direction.
The red and green lines represent vertical lines and nonvertical
lines, respectively. The vertical lines are highlighted with yellow
dotted circles. (a) Image 1. (b) Image 2. (Images courtesy of the
Future IT Laboratory of LG Electronics, Inc.)

robot orientation with respect to a pose stored in a visual map
after building the visual map in advance. In this section, we
present an algorithmic visual compass, simply referred to as
the visual compass, as a virtual sensor to correct an absolute
orientation of the mobile robot to within !45° under an
orthogonal structure assumption in an indoor environment.
86

*

IEEE ROBOTICS & AUTOMATION MAGAZINE

*

DECEMBER 2013

In [12], a visual compass algorithm is proposed that estimates a camera orientation in an EKF-based SLAM framework based on distant points, ideally at infinity, as features.
However, the visual compass algorithm described in [12] is
inappropriate for visual SLAM in indoor environments,
since it is effective for a large scale of outdoor scenes where
a number of distant points are available as features for a
visual compass.
Figure 3 shows two images captured from the upwardlooking camera (shown in Figure 2) to detect the orthogonal
lines. The lines are extracted from an image based on the
edge-following approach [25] to extract all lines needed to
detect the orthogonal lines. All vertical lines to the ground
are easily detected and removed since vertical lines projected in the image plane vanish to the optical center. The red
vertical lines are highlighted with yellow dotted circles in
Figure 3. The remaining lines outlined in green are considered for detecting the orthogonal lines.
Figure 4(a) shows the information used to detect the
orthogonal lines, while Figure 4(b) provides an example of
computing the visual compass orientation using the two
2nd
dominant orientations i 1st
w -avg. and i w -avg. of the orthogonal
2nd
lines. The two dominant orientations i 1st
w -avg. and i w -avg. are



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