A DIFFERENTIAL GEOMETRIC APPROACH FOR VISUAL

NAVIGATION IN INDOOR SCENES

L. Fuentes and M. Gonzalo-Tasis

MOBIVA Group

University of Valladolid

G. Bermudez and J. Finat

MOBIVA Group

University of Valladolid

Keywords:

Visually Navigation, Motion Analysis, Matching Correspondence and Flow, Kalman Filtering.

Abstract:

Visual perception of the environment provides a detailed scene representation which contributes to improve

motion planning and obstacle avoidance navigation for wheelchairs in non-structured indoor scenes. In this

work we develop a mobile representation of the scene based on perspective maps for the automatic naviga-

tion in absence of previous information about the scene. Images are captured with a passive low-cost video

camera. The main feature for visual navigation in this work is a map of quadrilaterals with apparent motion.

From this mobile map, perspective maps are updated following hierarchical grouping in quadrilaterals maps

given by pencils of perspective lines through vanishing points. Egomotion is interpreted in terms of maps of

mobile quadrilaterals. The main contributions of this paper are the introduction of Lie expansion/contraction

operators for quadrilateral/cuboid and the adaptation of Kalman ﬁltering for moving quadrilaterals to estimate

and predict the egomotion of a mobile platform. Our approach is enough modular and ﬂexible for adapting to

indoor and outdoor scenes provided at least four homologue cuboids be present in the scene between each pair

of sampled views of a video sequence.

1 INTRODUCTION

Visually based automatic navigation for platforms is a

common subject in Motion Planning and Navigation

along the nineties, with a lot of applications, includ-

ing interactive wheelchairs navigation. This prob-

lem has been also developed in several related works

(P. Trahanias and Orphanoudakis, 1997), including

landmarks (M. Rous and Kraiss, 2001), stereo vision

(Mazo and et al, 2002) for navigation robotics. Local-

ization of beacons, simultaneous correspondence be-

tween homologue points increase the computational

cost and troubles for decision making.

The design of smart wheelchairs with sensor fusion

and hybrid control is an active research subject(Mazo

and et al, 2002) (Levine et al., 1999) (R. Simpson

and Nourbakhsh, 2004) (Yanco, 1998). Two main

problems concern to safety tasks based in reactive be-

havior (Cort

´

es et al., 2003) and navigation-oriented

tasks focused towards the generation of environment

maps and motion planning (Yuki, 2000). Changing

or non-structured environments, and complex inter-

actions with the environment are beneﬁted from vi-

sual information. Active and passive sensors can be

used for semi-automatic or automatic navigation with

wheelchairs (A. Pruski and Morre, 2002). Active sen-

sors are crucial for obstacle avoidance, but a reactive

behavior is not enough for complex tasks in unknown

environments. An accurate and robust motion plan-

ning, requires detailed information about the scene for

mapping and localizations tasks (P. Trahanias and Or-

phanoudakis, 1997). Modeling intelligent behaviors

for wheelchairs have received an important attention

along last years and it is the main motivation for this

work. Nevertheless, the present approach is focused

towards the exploitation of information provided by

a low-cost video camera. Range sensors require in-

formation fusion and often provide incomplete infor-

mation about changing environment, with some lim-

itations linked to reactive behavior. A low-cost non-

calibrated camera mounted in a mobile platform pro-

vides global information about the scene, which can

be corrected, updated and completed with range sen-

sors for metric information (A. Pruski and Morre,

2002)

Furthermore, to increase the usability in non-

structured environments, it is convenient to avoid bea-

cons or previous information about the scene. To de-

crease the computational effort and to achieve a bet-

ter adaptation to these applications for disabled per-

468

Fuentes L., Gonzalo-Tasis M., Bermudez G. and Finat J. (2006).

A DIFFERENTIAL GEOMETRIC APPROACH FOR VISUAL NAVIGATION IN INDOOR SCENES.

In Proceedings of the First International Conference on Computer Vision Theory and Applications, pages 468-476

DOI: 10.5220/0001378104680476

Copyright

c

SciTePress

sons, we shall suppose that environments are given

by indoor or outdoor architectural scenes. In ordi-

nary architectural scenes it is easy to identify ob-

jects whose boundaries support elements for gener-

ating ”perspective maps” given by vanishing points,

perspective lines and perspective planes. A method-

ology for generating perspective maps is developed in

the §3. Perspective maps provide a 2

1

2

reconstruction.

Most of cameras can be considered as perspective de-

vices which project some part of the visible scene on

the camera plane.

Visual navigation is planned depending on the se-

lection of visual targets. Visual targets must be real-

time located and updated in a faithful representation

of the scene. To simplify, in this work we suppose

that the identiﬁcation of visual targets or tasks to be

achieved is externally performed by the user; other-

wise, a recognition and a motion planning module

must be added for decision making. A difﬁcult prob-

lem concerns to the representation updating for the

workspace. In this work, the chosen representation

is given by the lifting of quadrilaterals maps Q.A

quadrilateral map is a 2d quadrangular mesh adapted

to a perspective representation of the scene (M. Gon-

zalo and Aguilar, 2002) generated by the intersec-

tion of pencils of perspective lines through vanishing

points.

Edges of Q lie on visible support whose boundaries

provide a support for perspective lines. The computer

management of the image information contained in

each view is performed in terms visible elements. The

information management is performed on a symbolic

representation given by a small subgraph of a quad-

tree corresponding to visible elements in each view.

A characterization of simple events linked to quadri-

laterals of Q simpliﬁes the search of homologue ele-

ments. From the motion viewpoint, evolving homo-

logue quadrilaterals give information about the mag-

nitude and direction of motion changes, involving the

mobile platform itself, and other external agents.

Usual accurate volumetric representations have a

high computational cost, and it is difﬁcult to obtain

a faithful representation of the scene which are up-

dated on-line (twice per second). Corridor scenes are

used in the experimental set-up. In this case, by tak-

ing a mobile reference centered in the mobile object,

at most two vanishing points (v

z

, v

x

, e.g.) are at ﬁ-

nite distance, and at least a third one v

y

is at inﬁnite

distance (vertical lines must be parallel). The simple

nature of analyzed scenes allows to generate maps of

cuboids by intersecting pencils of planes through the

three vanishing lines connecting each pair of vanish-

ing points.

The information management in terms of octrees

has in general a complexity O(N

2

log N) in the num-

ber N of planes of the scene. However, the simple na-

ture of the corridor scene, allows to generate a mobile

perspective representation of the volumetric scene.

Each 3d perspective representation of the whole scene

is obtained by lifting the quadrilateral map Q to a 3D

model. The ordered lifting of the quadrilateral map

has a complexity at most O(N log N) linked to or-

dering planes contained in perspective maps which

are meaningful for visibility issues. The incorporation

of a graphics card would allow to avoid this increasing

thanks to the use of a typical z-buffer algorithm. How-

ever, for lowering costs, a simpliﬁed perspective rep-

resentation is introduced, which reduces the computa-

tion to visible cuboids. The resulting maps of cuboids

C play a very similar role to the maps of quadrilater-

als. To give a representation of contraction/expansion

of cuboid/quadrilateral maps we introduce Lie con-

traction/expansion operators along motion directions.

The construction of contraction/expansion operators

between maps of cuboids C and maps of quadrilater-

als Q requires a robust estimation of vanishing points

and the ego-motion description in terms of maps of

quadrilaterals, which is the main contribution of this

work. To achieve it, we use a variant of Kalman Fil-

tering (Marion, 2002)

Kalman ﬁltering is a tool for control of mobile sys-

tems, including motion estimation, tracking, and pre-

diction from estimation. They have been used in Mo-

tion Analysis by Computer Vision, in particular to

provide an assistance for visually guided automatic

navigation. A Kalman ﬁlter (Faugeras, 1993) is a

recurrent technique for solving a dynamic problem

by the least squares method. Measures can be cor-

rupted by white noise, and must be corrected. In this

work, an adaptation of Kalman ﬁltering is developed

for maps of quadrilaterals, including an implementa-

tion in C++ for motion estimation and tracking in ar-

chitectural indoor scenes (M. Gonzalo and Aguilar,

2002) by developing some aspects appearing in (Mar-

ion, 2002)

The paper is organized as follows: We start with

a very short review of related approaches. Next, we

develop some elements for modeling the scene. The

fourth section is devoted to the motion analysis and to

sketch the adaptation of Kalman ﬁltering to maps of

quadrilaterals.

2 RELATED APPROACHES

The design of smart wheelchairs with sensor fusion

and hybrid control is an active research subject along

last ten years ((Levine et al., 1999), (Mazo and et al,

2002), (R. Simpson and Nourbakhsh, 2004), (Yanco,

1998)). Two main problems concern to safety tasks

based in reactive behavior following agent-based

technologies (Cort

´

es et al., 2003) and navigation-

oriented tasks focused towards the generation of en-

A DIFFERENTIAL GEOMETRIC APPROACH FOR VISUAL NAVIGATION IN INDOOR SCENES

469

vironment maps and motion planning ((A. Pruski and

Morre, 2002), (Yuki, 2000)). The integration of em-

bedded systems for real-time sensor-based navigation

is a challenge specially in presence of human interac-

tion (J. Minguez and Montano, 2004).

Changing or non-structured environments, and

complex interactions with the environment are ben-

eﬁted from visual information. Visually based auto-

matic navigation for platforms is a common subject

in Robotics Navigation along the nineties. Its appli-

cation to wheelchairs has been also developed in sev-

eral related works (P. Trahanias and Orphanoudakis,

1997), including landmarks (M. Rous and Kraiss,

2001), stereo vision (Mazo and et al, 2002), between

others. Localization of beacons, simultaneous cor-

respondence between homologue points increase the

computational cost and decision taking. The nearest

approach to ours is (Toedem

´

e and Gool, 2004). In our

work the accent is put in explaining the generation

and updating of the geometric model, by sketching

algorithms for grouping around geometric primitives

of positive dimension. Often, beacons or ”salient”

corners can be partially occluded for relative localiza-

tions of the mobile platform. Thus, it is advisable to

work with geometric primitives supported on lines or

quadrilaterals, instead of points. Robustness of per-

spective models suggests the choice of 2d perspective

maps as the initial support for relative localization and

tracking of apparent motion of the platform.

3 MODELLING THE SCENE

On-line generation and updating of a perspective map

provide us to get a model of the scene. The experi-

mental set-up is as follows: To imitate arbitrary mo-

tions of a disabled person mounted in a wheelchair,

a mobile platform is manually displaced along a cor-

ridor with diffuse light and non-conditioned material

in an arbitrary (sometimes erratic) way . Kinematic

characteristics of the trajectories are unknown. The

mobile platform has a low-cost uncalibrated video

camera providing low-quality images, which are sam-

pled twice each second.

3.1 Perspective Models

Lines of the ordinary space provide the main features

for perspective models. The set of space lines through

an eventually mobile point (focus of the camera) is

a homogeneous compact manifold called a projec-

tive plane P

2

. Often, in architectural scenes, some

”meaningful lines” can be grouped in pencils through

each ”vanishing point”. Structural elements for a

perspective map of the scene are given by vanish-

ing points V

i

, perspective lines PL

j

and perspective

planes PP

k

. Vanishing points v

i

are the intersection

locus of at least four perspective lines PL

j

parallel in

the scene (three lines can converge in a corner which

is not a vanishing point in general). In complex archi-

tectural scenes there can be a lot of vanishing points,

due to the misalignment of walls or faades, in indoor

or outdoor scenes, respectively. For clarity purposes,

we restrict ourselves to simpler cases of indoor scenes

with at least three independent vanishing points.

In frontal views of a large number of architectural

simple scenes, there are three main vanishing points.

Typical frontal views have two ﬁxed vanishing points

at inﬁnite distance, labelled as v

x

and v

y

, correspond-

ing to the intersection of pencils of horizontal and ver-

tical lines, respectively. Usually, the vanishing point

v

y

is ﬁxed. However, rotations of the camera give

displacements of v

x

, providing the main input for the

motion estimation. Furthermore, even for rectiﬁed

views, turning a corridor generates abrupt changes

in the identiﬁcation of perspective elements involving

the localization of v

x

.

Figure 1: Perspective Model and Quadrilateral Map.

Collinear segments are grouped to obtain ”large”

lines

j

with a length (number of collinear mini-

segments) larger than a threshold selected by the user.

So, a collection of allowed lines

j

i

is obtained as

candidates for perspective lines through the vanishing

point v

i

. Candidate perspective lines are obtained by

an automatic grouping of segments, and a selection

of pencils Λ of lines which converge in a vanishing

point v

i

. Validation of v

i

is performed by minimiz-

ing

(

T

j

i

v

i

)

2

. After estimating each vanishing point

v

i

, candidate perspective lines are replaced by new

perspective lines to provide a ”coarse rectiﬁcation”

linked to a globally coherent perspective representa-

tion of the scene; this step is necessary for correcting

views provided by the low cost video camera.

Each pair of vanishing points V

i

, V

j

∈ P

3

deter-

mines an horizon line L

∞,ij

= V

i

V

j

⊂ P

3

which is

computed by the cross-product V

i

∧V

j

; any point on

L

∞,ij

is also a vanishing point. Often, visual target

VISAPP 2006 - MOTION, TRACKING AND STEREO VISION

470

Figure 2: Perspective Model and Quadrilateral Map.

is located in L

∞,ij

, and the visual perception of ego-

motion in the image is represented by an apparent dis-

placement along the horizon line L

∞,ij

; in this case, it

sufﬁces to track this displacement for egomotion es-

timation. To achieve a 2

1

2

reconstruction as general

framework for visual navigation, a projective repre-

sentation of the plane at inﬁnity is required.

Three non-collinear vanishing points V

i

, V

j

, V

k

generate a plane at inﬁnity in P

3

which is denoted

as Π

∞

without specifying the choice of vanishing

points. Thus, π

∞

can be taken as an invariant refer-

ence plane for motion estimation and tracking. Simi-

larly, pencils L

i

, L

j

, L

k

or perspective lines through

the vanishing points V

i

, V

j

, V

k

give a dual refer-

ence.

Let us denote by L

i

a pencil of perspective lines

through the vanishing point v

i

. Two pencils , L

i

, L

j

of coplanar perspective lines determine a perspective

plane which is denoted by L

i

+ L

j

. The perspec-

tive representation of an ordinary architectural scene

is visualized by means of three pencils L

h

, L

v

, L

d

of

horizontal, vertical and depth perspective lines. This

representation is a local version of the projective rep-

resentation, and it can be interpreted as a dual refer-

ence system for the projective plane which extends

the plane of each view. A perspective representation

of the scene in terms of perspective lines is more ro-

bust than representations based on points.

There are three basic perspective models which are

labeled as frontal, angular and skew depending on the

availability of one, two or three vanishing points at ﬁ-

nite distance. If there is no vanishing point at ﬁnite

distance we have a parallel projection, which is not

a perspective model, properly said. A corridor scene

(resp. room scene) is represented by a frontal (resp.

angular) perspective model, whereas in turning a cor-

ner of a corridor an angular perspective model pro-

vides the transition between two frontal perspective

model. In outdoor architectural scenes, the localiza-

tion of the camera gives skew perspective models for

the transition between angular perspective models.

3.2 Quadrilaterals Map

The low quality of conventional video cameras and

the bad illumination conditions, impose serious re-

strictions for achieving robust results in real-time, in-

cluding an on-line localization for a mobile platform.

Visual perception of rectangular regions is more ro-

bust than corners surrounding them. In frontal per-

spective models one has at least a vanishing point v

z

at ﬁnite distance. In this model, rectangular regions

are ideally perceived as trapezoids, i.e., as quadrilat-

erals with two parallel vertical edges. To simplify the

tracking of quadrilaterals, we restrict ourselves to hor-

izontal and vertical segments. Hence, the ﬁrst task is

the construction and tracking of two trapezoidal maps

linked to v

z

. There are two extremal perspective

lines PL

M,z

, PL

m,z

through v

z

with maximal M

and minimal m slope and afﬁne equations

M,z

=0,

m,z

=0, respectively. Both perspective lines decom-

pose the plane in four signed regions, depending on

the sign evaluation for afﬁne equations at each point.

Vertical (respectively, horizontal) segments contained

in regions (+, −) and (−, +) (respectively, in regions

(+, +) and (−, −)) are extended till to arrive to ex-

tremal perspective lines. So, we generate two trape-

zoidal maps T

z,ver

and T

z,hor

linked to the vanish-

ing point v

z

. Both trapezoidal maps are bounded

by extremal perspective lines with vertical and hor-

izontal parallel segments, respectively. After retrac-

ing extremal perspective lines, each trapezoid of a

trapezoidal map linked to a vanishing point is char-

acterized by three points. For each oriented trapezoid

T

i,j

∈T

z,j

with j = ver or j = hor, we associate

in a canonical way a pair of bivectors representing the

trapezoid uniquely; a bivector is the cross-product of

two ordinary vectors.

The same argument is applied for angular and skew

perspective models, but by replacing the trapezoid by

a general quadrilateral, i.e is the image of a rectan-

gle by an afﬁne transformation, whose opposite edges

are supported on perspective lines through the same

vanishing point. Two coplanar pencils Λ

i

and Λ

j

of

projective lines through the vanishing points V

i

, V

j

give a quadrilateral map. The quadrilateral map Q

is supported by a plane through the vanishing line

L

∞,ij

:= V

i

V

j

. Data structures for a quadrilateral

map Q are supported on visible segments lying in two

coplanar pencils L

i

, L

j

of perspective lines. Thus,

for a typical architectural scene, we have three fam-

ilies of maps of quadrilaterals Q

12

, Q

13

, Q

23

linked

to pencils of perspective planes through the vanish-

ing lines L

∞,12

, L

∞,13

, L

∞,23

. A sheaf depending

on two (resp. three) parameters is called a net (resp.

web). Hence, by packing quadrilateral maps a web of

quadrilaterals is obtained.

The basic cell of a a quadrilateral map is

given by an ordered quadrilateral Q

ij

with edges

A DIFFERENTIAL GEOMETRIC APPROACH FOR VISUAL NAVIGATION IN INDOOR SCENES

471

a

i1

, b

j1

, a

i2

, b

j2

. Edges are supported on two pairs

of lines lying on pencils of perspective lines; so we

have a

i1

⊂

i1

∈L

i

, b

j1

⊂

j1

∈L

j

and so on. The

oriented area for the quadrilateral Q

ij

is given by the

bivector

A(Q

ij

)=

1

2

[a

i1

× b

j1

+ a

i2

× b

j2

] (1)

In particular, if Q is a map of quadrilaterals for a

frontal perspective model, then each quadrilateral is a

trapezoid. Quadrilateral maps are referred in projec-

tive representation with respect to the common van-

ishing point (corresponding to the preservation of ver-

tical direction), and a rotation of the horizon line op-

posite to the vanishing point. In the general case,

rotation is computed following the methodology de-

scribed in (Jelinek and Taylor, 2001).

Figure 3: Perspective Model and Quadrilateral Map.

Turning a corridor implies severe alterations in

maps of quadrilaterals, and it is necessary to have a

3d model for integrating turning quadrilateral maps in

a common framework. The proposed common frame-

work for the on-line management of 2

1

2

d information

is given by a map of cuboids. A cuboid is given by

the cross product of three linearly independent vec-

tors; a parallelepiped is a cuboid given by the triple

product of three orthogonal vectors. In architectural

scenes, edges (respectively, faces) of cuboids are sup-

ported on perspective lines (respectively, planes). The

intersection of two adjacent cuboids is a face of one of

them. Modeling of cuboid maps C is similar to mod-

eling of trapezoidal maps, but increasing the dimen-

sion in a unity. So, instead of considering pencils Λ

i

of lines through vanishing points V

i

, it is necessary to

consider pencils L

k

of planes through vanishing lines

V

k

V

. Computer management is also similar, but by

replacing quadtrees by octrees. To avoid excessive

subdivisions, only non-empty cellules are subdivided

depending on the identiﬁcation of visible perspective

elements in the 3d scene. Furthermore, only local in-

formation is preserved in short memory whereas some

part of the scene is visible. In this way, a veriﬁcation

of global coherence is avoided in updating 3d models.

4 LIE OPERATORS FOR

EXPANDING/CONTRACTING

MAPS

Motion estimation from optical motion is not well-

determined. It is necessary to add constraints, predic-

tions and measurements about the scene or the mo-

tion, which must be estimated, validated and updated.

In this section, a differential model is provided for a

simultaneous evaluation of the motion and the scene

structure from the updating of perspective represen-

tation of the scene. The key tool is the introduction

of simple transformations between perspective maps

which can be read in terms of maps of quadrilaterals

and cuboids. Some additional concepts are needed for

showing the main steps for both operations

4.1 Vector Fields for Motion

Estimation

A smooth vector ﬁeld X is a section of the tangent

bundle T R

3

R

3

× R

3

, i.e., a representation of the

velocities ﬁeld at each point. Hence, it is given by

a

∂

∂x

+ b

∂

∂y

+ c

∂

∂z

, where a, b, c are smooth functions

or more synthetically as

a

i

∂

∂x

i

.

The optical ﬂow is a 2d vector ﬁeld θ which is the

projection of the true 3d motion ﬁeld. The egomotion

is a piecewise smooth vector ﬁeld ξ providing a esti-

mation of the optical ﬂow along a temporal sequence

of views with integral curve parametrized by λ.

Our differential geometric approach to the estima-

tion of the 3d motion vector ﬁeld requires 1) the es-

timation of the 2d egomotion vector ﬁeld ξ, the ex-

pression of 2d optical ﬂow θ from the 2d egomotion

vector ﬁeld ξ and the lifting of θ to the 3d motion ﬁeld

by using the updating of the perspective model of the

indoor scene.

4.2 Differential Forms for the

Perspective Representation of

the Scene

A differential form ω is a section of the cotangent

bundle Ω

1

R

3

:= HOM(T R

3

,ε

1

R

3

) R

3

×R

3

where

ε

1

R

3

:= R

3

×R is the trivial vector bundle of rank 1 on

R

3

. A differential form ω ∈ Ω

1

is given by

b

i

dx

i

.

The evaluation ω(X) of a differential form on a ﬁeld

VISAPP 2006 - MOTION, TRACKING AND STEREO VISION

472

gives a real number. Similarly, Ω

1

c

denotes the ﬁrst

degree differential forms with compact support.

The canonical bases dx

1

,dx

2

,dx

3

and

∂

∂x

1

,

∂

∂x

2

,

∂

∂x

3

are dual between them, i.e.,

dx

i

(

∂

∂x

j

)=δ

ij

(delta de Kronecker) for 1 ≤ i, j ≤ 3.

In particular, dz vanishes

∂

∂x

, and

∂

∂y

. Thus, in an

ideal euclidian model where z represents the depth,

it sufﬁces to make adjustments with respect to the

component c of

∂

∂z

. Unfortunately, the perception

model of any video camera is not euclidian. Thus,

some additional considerations are required.

A map of Segments (resp. Quadrilaterals, resp.

Cuboids), is given as the support of a pencil of 1-

degree (resp. a net of 2-degree, resp. a web of 3-

degree) differential form with compact support ω ∈

Ω

k

c

(R

3

) of degree k for k =1(resp k =2, resp.

k =3. The choice of compact support for differ-

ential forms is justiﬁed by practical and theoretical

reasons: the objects to be tracked (quadrilaterals) are

compact and the need of representing volumetric el-

ements by ”non-trivial” models from the cohomolog-

ical viewpoint. To ﬁx ideas, a static simple corridor

scene is modeled in a frontal perspective view with

vertical and horizontal trapezoidal maps correspond-

ing to lateral walls and ﬂoor-roof, respectively. Pen-

cils of lines belonging to each trapezoidal map verify

incidence conditions with respect to vanishing points.

For example, the vertical left trapezoidal map is deter-

mined by two pencils of lines through the vanishing

points v

y

for parallel vertical lines and v

z

for depth

map. Similarly, the horizontal lower trapezoidal map

is determined by two pencils of lines through the van-

ishing points v

x

for parallel horizontal lines and v

z

for depth map.

The real motion of the mobile platform generates

an apparent displacement of the vanishing points v

x

and v

z

(vertical direction is preserved). The apparent

displacement of vanishing points generates displace-

ments of vertical and horizontal quadrilateral maps

in each view. The displacement is controlled by 1)

a transversal direction to the vertical trapezoid given

a perspective horizontal line passing through v

x

for

vertical trapezoidal map; and 2)a transversal direc-

tion to the horizontal trapezoid given a perspective

depth line passing through v

z

for horizontal trape-

zoidal map.

The exterior differential of a k-degree differential

form gives a (k +1)-degree differential form. In par-

ticular, if we would have the simplest representation

for the left vertical trapezoidal map given by the 2nd

degree differential form ω

vert

= a

vert

(x, y, z)dz∧dx

, then its differential dω

vert

=

∂a

vert

∂y

dx ∧ dy ∧ dz

would give a cuboid map relative to the lacking or-

thogonal direction. Similarly, if we would have the

simplest representation for the lower horizontal trape-

zoidal map given by the 2nd degree differential form

α

hor

= b

hor

(x, y, z)dy ∧ dz , then its differential

dα

hor

=

∂b

hor

∂x

dx ∧ dy ∧ dz would give a cuboid

map relative to the lacking orthogonal direction. As

both cuboid maps must be the same, one has a struc-

tural constraint dω

vert

= dα

hor

. Hence, it sufﬁces to

consider only a trapezoidal map. By visibility reasons

it is more practical to restrict to a left vertical trape-

zoidal map T , e.g., generated by the intersection of

pencils of projective lines through v

z

and v

y

.

4.3 Lie Contraction/Expansion

Operators

Even when prior information about the scene is avail-

able, optical ﬂow is not easy to compute due to noise,

partial occlusions, etc. Quadrilateral are easier than

points for tracking, and they provide a support for dif-

ferential forms with compact support. Each mobile

vector, bivector or three-vector (with support contain-

ing a segment, a quadrilateral or a cuboid) is the dual

of a 1, 2 or 3-degree differential form in a point. Ho-

mologue elements are linked along the motion by the

directional or Lie derivative.

The Lie (directional) derivative of a k-th degree dif-

ferential form ω is the k-th degree differential form

deﬁned by L

ξ

ω(x)=

d

dλ

(f

∗

λ

ω)(x) |

λ=0

; let us re-

mark that

d

dλ

is a linear operator.

The motion ﬁeld θ corresponding to the apparent

motion of homologue elements along the sequence of

views is topologically modeled by a local diffeomor-

phism f

λ

(ξ) arising from the integration of the vec-

tor ﬁeld ξ. The linearization of the diffeomorphism

f

λ

(ξ) (differential evaluated at origin) gives an ele-

ment of the general linear group GL(3; R). Its semi-

direct product with the translation gives the afﬁne lin-

ear map between homologue elements along the se-

quence of temporal views. The estimation of θ is

performed from the comparison between homologue

trapezoids or, more generally, quadrilaterals in terms

of Kalman ﬁlters (see below).

The ﬂow conservation’ equation f

∗

λ

(dω)=d(f

∗

λ

ω)

implies the conservation of homologue 2d rigid el-

ements (trapezoidal or quadrilateral maps) along the

temporal sequence of views. The lifting of the ﬂow

conservation to the 3d ambient space implies the

preservation of homologue 3d rigid elements along

the motion. The homologue rigid elements are visu-

ally perceived as cuboids corresponding to different

views of the same rectangular parallelepiped. Hence,

they are related between them through afﬁne transfor-

mations which must be estimated on line.

The exterior differentiation commutes with the Lie

derivative which is a linear operator

d

dλ

. Thus,

d(L

X

ω)=L

X

(dω) for every ω ∈ Ω

k

M

, which can be

expressed as dL

X

= L

X

d in a more synthetic way.

A DIFFERENTIAL GEOMETRIC APPROACH FOR VISUAL NAVIGATION IN INDOOR SCENES

473

The piecewise-linear expansion of the 2d quadri-

lateral map Q (given in particular by a the trapezoidal

map T ) to a cuboid map C is locally given by the sup-

port of the exterior product ω ∧ η where η is a dif-

ferential form supported on lines through the lacking

vanishing point v

x

.

The apparent displacement of rigid elements iden-

tiﬁed on the quadrilateral map Q (respectively, cuboid

map C) is modeled by the Lie derivative of the 2-

degree differential form ω (respectively, the 3-degree

differential form dω) along the motion ﬁeld ξ.

Given a differentiable manifold M , for any vec-

tor ﬁeld X ∈X(M) on M and for any k +1-

degree differential form ω ∈ Ω

k+1

M

, the contrac-

tion of ω along the vector ﬁeld X, denoted as i

X

ω

is deﬁned as the k-th differential form given by

i

X

ω(X

1

,...,X

k

):=ω(X, X

1

,...,X

k

) for any

vector ﬁelds X

1

,...,X

k

∈X(M)

The piecewise contraction of the cuboid map C to

the three quadrilateral maps Q

ij

, for 1 ≤ i<j≤ 3

is locally given by the Lie contraction of Ω along the

Lie vector ﬁeld ξ of the egomotion.

The simplest local representation of mobile maps

of cuboids (respectively, quadrilaterals) is given

by orthographic three-dimensional reconstructions,

where basic pieces are rectangular parallelepipeds,

which are distorted by perspective effects. By means

a projective transformation, it is possible to send the

three vanishing points to the inﬁnity, to construct a

parallepiped model for the scene with euclidian infor-

mation (for scaled orthographic resulting representa-

tion), solve the contraction/expansion, and to perform

the inverse transformations. However, this would

have a high computational cost, and it would be some

difﬁcult to obtain a real-time implementation. Thus,

it is important to develop a forward estimation of

egomotion directly supported on maps of homologue

quadrilaterals. To ﬁx ideas, let us restrict to the trape-

zoidal map T corresponding to v

y

and v

z

5 MOTION ANALYSIS

The general problem is which kind of information

about the motion vector ﬁeld Θ can be extracted from

the egomotion vector ﬁeld ξ in an indoor scene. Ho-

mologue quadrilaterals allow to evaluate magnitude

and direction of motion ﬁeld. Usual differential mod-

els for ego-motion estimation are based on optical

ﬂow. Some troubles are related with aperture prob-

lem, noise for geometric features, indeterminacy or

ambiguity about homologue elements, between oth-

ers. In our case, the differential formalism allows to

reduce the estimation/tracking of a trapezoid to the es-

timation/tracking of two segments supported on per-

spective lines through v

z

. An important issue con-

cerns to the estimation of 1D (perspective lines) or

2D data (trapezoids, quadrilaterals), and the adapt-

ability to changing conditions. Thus, we develop an

approach able of supporting estimation and tracking

of positive dimensional elements.

After modeling, main problems concern to 1) the

prediction from the current localization, 2) the iden-

tiﬁcation of homologue quadrilaterals along sampled

views of a video sequence, 3) model’s validation and

4) prediction updating.

5.1 Dynamic Modeling

A discrete dynamical system can be represented by

a state equation s

i

= ψ

i,i−1

s

i−1

w

i

where s

i

is the

state vector at instant i, ψ

i,i−1

is a linear matrix, and

w

i

is the random noise representing a perturbation on

the system. To simplify, one supposes a white noise,

i.e. E[w

i

]=0,E[w

i

w

T

i

]=Q

i

where the covari-

ance matrix Q

i

is usually determined in a heuristic

way. If we suppose that data are not correlated be-

tween them, then a diagonal matrix can be chosen for

Q

i

. To avoid heuristics, a choice based on triangu-

lar matrices allows to maintain dependence relations

between nested elements.

5.2 Dynamical Estimation of the

Trapezoidal Map

In this subsection, wand adaptation of (Faugeras,

1993) is developed for estimating and measuring the

trapezoidal map as a discrete dynamical system. We

have followed an incremental estimation approach:

• Static states for each trapezoid of the trapezoidal

map T are represented by a 2d vector t

i

corre-

sponding to a coordinate of a corner, and the width

of trapezoid.

• Kinematic states s

i

are given by a 4d vector (t

i

,

˙

t

i

)

where

˙

t

i

is the temporal variation of t

i

.

• The plant’s or state’s equation is s

i

= ψ

i|i−1

s

i−1

+

w

i

where ψ

i|i−1

s

i−1

is a triangular matrix respon-

sible for the kinematic coupling and w

i

is a white

noise with E[w

i

.w

T

i

]=Q

i

. To begin with, ψ

1|0

=

I

2

I

2

0 I

2

where I

2

is the 2 × 2 identity matrix.

• The measurement equation is given by m

i

=

H

i

s

i

+ ν

i

with noise ν

i

. It gives as output the 4× 4

linear transformation H

i

for each trapezoid.

• Covariance matrix: Due to the lack of information

about estimated values, we take an initial covari-

ance matrix P

i

given by a 4 × 4 diagonal matrix

with high positive eigenvalues.

VISAPP 2006 - MOTION, TRACKING AND STEREO VISION

474

• The prediction of the covariance matrix before

comparing the 0-th (before motion) and the ﬁrst im-

age is equal to P

1|0

= ψ

0

P

1

ψ

T

0

+ Q

0

where ψ

0

has

been estimated before, and Q

0

= E[w.w

T

] for the

i-th trapezoid.

5.3 Kalman Filtering

A Kalman ﬁlter (Faugeras, 1993) is a recurrent linear

technique for solving a dynamic problem by the least

squares method, allowing to integrate noised mea-

sures. Measures can be corrupted by white noise, and

must be corrected without having all the measures. In

our case, we have adapted Kalman ﬁltering to maps of

quadrilaterals, (Marion, 2002). The estimation based

on KF is performed without waiting to have all the

measures.

Typical steps of standard KF are given by initial-

ization, measurement, validation and updating of pre-

diction. The initialization has been already described

below.

5.3.1 Mahalanobis Distance for Updating

Measurements

Under uncertainty conditions, the covariance matrix

Λ

i

provides information about the dispersion of data,

and allows to compute the Mahalanobis distance

d

iM

(x,µ):=(x − µ)Λ

i

(x − µ)

T

of a vector x with

respect to the current mean value µ

. In our case, the

comparison between 0-th and ﬁrst images gives an

initial measurement matrix H. From H, the covari-

ance matrix Λ

0

= HR

0

H

T

+ R

1

.

Along the iteration,d

iM

=(m

i+1

−

Hs

i

)Λ

−1

i

(m

i+1

− Hs

i

)

T

where m

i+1

is the

measure at time i +1with covariance matrix R

i

, and

Λ

i

= HP

i

H

T

+ R

i+1

.

5.3.2 Validation

If the computed Mahalanobis distance for the segment

representing the trapezoid, is lesser than a selected

threshold, then the estimation is accepted. Otherwise,

another trapezoid must be selected, and the process

re-starts.

5.3.3 Updating of the Kalman Filter

The information is updated in the standard form as

follows: 1) Gain matrix: K

0

= R

0

H

T

(HR

0

H

T

+

R

1

)

−1

; 2) Updating of the state’s estimation: ˆs

1

0s

0

+

K

0

(m

1

− Hs

0

); 3) Updating of P

0

in

ˆ

P

0

=(R

0

+

H

T

R

−1

1

H)

−1

The process is iterated for all trapezoids contained

in the central part of the image with homologues eas-

ily identiﬁable in the precedent and consecutive view

of the sequence.

5.3.4 Extended Kalman Filter EKF

Most processes in real world are not linear. Hence, we

must adapt or extend KF (EKF) to the non-linear case:

In each step a linearization of the precedent estimated

state is performed. If the initialization is not enough

good or if the noise is high, then the convergence is

not guaranteed. In this case, it is necessary to modify

initial constraints or to perform a new processing of

the acquired information.

5.4 Estimation and Tracking of

Quadrilaterals

The pipeline for estimating and tracking quadrilater-

als is as follows

1. Grouping of mini-segments in long segments

2. Computation of intersections of lines supported by

long segments

3. Identiﬁcation of putative perspective lines

4. Estimation of vanishing points

5. Retracing of ”true” perspective lines

6. Grouping of perspective lines in three pencils L

i

through vanishing points V

i

7. Maps of Quadrilaterals Q

ij

linked to each pair L

i

,

L

j

of perspective lines.

8. Identiﬁcation of allowed simple events involving

changes in quadrilaterals (grouping or decomposi-

tions).

9. Representation of transformations involving inde-

composable quadrilaterals by triangular matrices.

10. Solving systems by successive reduction in trian-

gular matrices.

6 CONCLUSION

A geometric framework based on mobile perspective

models is superimposed to structured scenes. An on-

line updating of changing perspective models is per-

formed from quadrilateral maps by sampling two im-

ages each second. Expansions of maps of quadri-

laterals are constructed as maps of cuboids by using

differential constraints. A new representation of the

egomotion is provided in terms of Lie derivative of

the quadrilateral maps and the Lie contraction of the

cuboid map along the egomotion ﬁeld ξ. The esti-

mation of the egomotion is directly computed on the

quadrilateral map by using an adaptation of Kalman

ﬁltering. Some challenges for the next future are re-

lated with a) the implementation of a real-time mod-

ule for generating and tracking mobile perspective

A DIFFERENTIAL GEOMETRIC APPROACH FOR VISUAL NAVIGATION IN INDOOR SCENES

475

models will be developed, b) improve metric localiza-

tion from uncalibrated video sequences, c) identiﬁca-

tion of mobile human beings in the scene compatible

with the motion mounted on the platform, d) evalua-

tion of behaviors to improve the interaction.

ACKNOWLEDGEMENTS

The authors acknowledge to the MAPA Project

and specially thanks to J.J. Fernandez-Martin and

J.I.SanJose Alonso for their partial ﬁnantial support.

REFERENCES

A. Pruski, M. E. and Morre, Y. (2002). Vahm: A user

adapted intelligent wheelchair. In IEEE Intl Conf on

Control Application (Glasgow, Scotland, UK). IEEE.

Cort

´

es, U., Annicchiarico, R., V

´

azquez-Salceda, J., Ur-

diales, C., namero, L. C., L

´

opez-S

´

anchez, M.,

S

`

anchez-Marr

`

e, M., and Caltagirone, C. (2003). e-

tools: The use of assistive technologies to enhance

disabled and senior citizens’ autonomy. In EU-LAT

Workshop on e-Health, pages 117–133.

Faugeras, O. (1993). Three-dimensional Computer Vision.

A Geometric Viewpoint. The MIT Press, Cambridge,

Mass.

J. Minguez, L. M. and Montano, L. (2004). An architec-

ture for snsor-based navigation in realistic dynamic

and troublesome scenarios. In Proc. IROS). IEEE.

Jelinek, D. and Taylor, C. (2001). Reconstruction of linearly

parametrized models from single images with cam-

era of unknown focal length. IEEE Trans. on Pattern

Analysis and Machine Intelligence, 23(7):767–773.

Levine, S. P., Bell, D. A., Jaros, L. A., Simpson, R. C., and

Koren, Y. (1999). The navchair assistive wheelchair

navigation system. IEEE Trans on Rehabilitation En-

gineering, 7 (4):443–451.

M. Gonzalo, J. Finat, M. A. and Aguilar, S. (2002). Dynam-

ical trapezoidal maps for coarse perspective model in

indoor scenes. In ISPRS’02, 1st International Confer-

ence on Enterprise Information Systems. ISPRS.

M. Rous, H. L. and Kraiss, K. F. (2001). Vision-based

landmark extraction using indoor scene analysis for

mobile robot navigation. In IEEE/RSJ International

Conference. IEEE.

Marion, G. (2002). Estimacin y Prediccin del Movimiento

Propio en Escenas de Interior mediante Filtros

Kalman. Master’s thesis, Valladolid.

Mazo, M. and et al (2002). Experiences in assisted mo-

bility: The siamo project. In Proc. IEEE Intl Conf

on Control Applications (Anchorage, Alaska), pages

766–771. IEEE.

P. Trahanias, M. Lourakis, A. A. and Orphanoudakis, S. C.

(1997). Navigational support for robotic wheelchair

platforms. In Proc. ICRA-IEEE. IEEE.

R. Simpson, E. LoPresti, S. H. and Nourbakhsh, I. (2004).

The smart wheelchair component system. J. of Reha-

bilitation Research and Development, 41:429–442.

Toedem

´

e, M. Nuttin, N. T. and Gool, K. V. (2004). Vision

based intelligent wheelchair control: the role of vision

and inertial sensing in topological navigation. J. of

Robotic Systems, 21(2):85–94.

Yanco, H. A. (1998). Integrating robotic research: a sur-

vey of robotic wheelchair development. In AAAI

Spring Symp. on Integrating Robotic Research (Stan-

ford, CA). IEEE.

Yuki, O. (2000). Corridor Navigation of a Mobile Robot us-

ing a camera and sensors-multiagent approach. The-

sis, UCLA University, Los Angeles, CA.

VISAPP 2006 - MOTION, TRACKING AND STEREO VISION

476