OPTIMAL SPANNING TREES MIXTURE BASED PROBABILITY

APPROXIMATION FOR SKIN DETECTION

Sanaa El Fkihi

1,2

, Mohamed Daoudi

GET/TELECOM Lille1, LIFL (UMR USTL-CNRS 8022), Cit´e Scientiﬁque - Rue Guglielmo Marconi , Villeneuve d’Ascq, France

Driss Aboutajdine

GSCM

LRIT Faculty of Sciences, University Mohammed V, Rabat, Morocco

Keywords:

Optimal spanning tree, dependency tree, mixture of trees, skin detection, classiﬁcation, probability graphical

models, tree distributions.

Abstract:

In this paper we develop a new skin detection algorithm for learning in color images. Our contribution is based

on the Optimal Spanning Tree distributions that are widely used in many optimization areas. Thus, by making

some assumptions we propose the mixture of the Optimal Spanning Trees to approximate the true Skin (or

Non-Skin) class probability in a supervised algorithm.

The theoretical proof of the Optimal Spanning Trees’ mixture is drawn. Furthermore, the performance of our

method is assessed on the Compaq database by measuring the Receiver Operating Characteristic curve and its

under area. These measures have proved better results of the proposed model compared with the results of a

random Optimal Spanning Tree model and the baseline one.

1 INTRODUCTION

Tree distributions are well-known machine learning

solutions to deal with probability estimation problem.

(Chow and Liu, 1968) supplied an heuristic to ﬁnd

maximum likelihood Markov Trees called Optimal

Dependency Trees or Optimal Spanning Trees. The

heuristic problem aimed to provide an efﬁcient al-

gorithm to ﬁnd a maximum-weight spanning tree

(MWST) proved to be the optimal one in the sense

of Maximum Likelihood criterion. Since then, many

methods based on that work have been extended: the

polytrees (Pearl, 1988); the mixtures of trees with ob-

served structure variable (Geiger, 1992); the mixture

of Tree-Union (Torsello and Hancock, 2006).

In addition, the authors of (Meila and Jordan, 2000)

proposeda mixture of trees with hidden structurevari-

able. When the variable is a class label, the mixture

model is the bayesian network. Otherwise, the class

variable is considered as the training data in an unsu-

pervised algorithm used to learn the mixed trees.

The MWST has applications in many optimiza-

tion areas. However, this tree is not usually unique.

Indeed, considering a graph with identically weighted

edges, all spanning trees are MWSTs. Consequently,

different tree probability distributions can approxi-

mate on the best way the true probability distribution.

In many domains what is required is not necessarily

the best spanning tree, but rather a ’perfect’ one with

some other properties that may be difﬁcult to quan-

tify. So, what could be the ’perfect’ spanning tree in

the skin detection application?

Research has been performed on the detection of

human skin pixels in color images by the use of var-

ious statistical color models (Jedynak et al., 2005),

such Gaussian mixture and histograms (Jones and

Rehg, 1999). The comparison results of these lat-

ters, estimated with EM algorithm, found that the

histogram model is slightly superior in terms of skin

classiﬁcation for the standard 24-bit RGB color space.

Moreover, in addition to the semi-supervised ap-

proach for learning the structure of Bayesian network

classiﬁers based on an Optimal Spanning Tree (Sebe

et al., 2004), the Best-Tree distribution algorithm ap-

proximating the skin and non skin probability distrib-

utions has been also proposed (ElFkihi et al., 2006).

Since quantifying other ’perfect’ Skin (or Non-

Skin) tree properties is unobvious, our aim is to pro-

vide a learning algorithm for skin/non-skin classi-

ﬁcation, seeking a spanning tree which emphasizes

382

El Fkihi S., Daoudi M. and Aboutajdine D. (2007).

OPTIMAL SPANNING TREES MIXTURE BASED PROBABILITY APPROXIMATION FOR SKIN DETECTION.

In Proceedings of the Second International Conference on Computer Vision Theory and Applications - IFP/IA, pages 382-385

 SciTePress

the data dependencies’ degrees, and approximates the

probability distribution; without adding information.

The next section points out the mathematical for-

mulation of the studied problem. Section 3 details our

approach while section 4 is devoted to experiments

and results. Conclusions are postponed to section 5.

2 PROBLEM FORMULATION

In the RGB color space, we note x

the color of a

pixel s. y

= 1 is a skin pixel label and y

= 0 a

non-skin pixel label. Let r be an integer value, we

deﬁne the following neighborhood system:

= {(i, j)/|i− i

| < r,| j − j

| < r} \ {(i

, j

)} (1)

We consider (x

,... ,x

) an observation vector

standing for an image patch (h × h,h = 2r − 1). This

vector is decomposed until a low-level elements; and

the resultant vector is x = (x

,. .. ,x

), where n = 3h

In practice, the joint probability distribution

P(x,y

) is unknown, instead we have a seg-

mented Database which is a collection of samples

{(x

(1)

),... ,(x

(N)

)} that we suppose inde-

pendent; x

( j)

is a color image, and y

( j)

is its binary

skinness image. Our objective is to construct proba-

bilistic classiﬁers that represent the posterior proba-

bilities P(y

= i|x) (i = 0, 1) at s, given its neighbors.

We consider the observation vector x and G(V,E)

its corresponding undirect graph representation. V is

the vertex set representing observations and E is the

edge set enclosing dependencies between the obser-

vations. In case G is a tree, noted T, the formulation

probability represented by such structure and approxi-

mating the true probability P is given in (Pearl, 1988):

P(x) ⋍

∏

(u∼v)∈T

)

∏

u∈V

) (2)

where P

) and P

) are marginals of P(x),

and u ∼ v means two neighboring vertices u and v.

(Chow and Liu, 1968) calculated the Kullback-

Leibler (KL) divergence of the true probability and

the one approximated by a tree. They proved that

minimizing this distance is equal to maximizing:

∑

(u∼v)∈T

KL(P

),P

)) (3)

which is the weight of the spanning tree T. An arbi-

trary solution is readily obtained in polynomial time

by simple greedy algorithms (Bach and Jordan, 2003),

and it is called an Optimal Spanning Tree (OST).

The OSTs of a graph emphasize the important depen-

dencies between its different components. Hence, we

propose the next mixture of the tree probabilities:

∑

T∈Θ

λ(T)P(x|T) (4)

with Θ is the set of all OSTs, and P(x|T) is the prob-

ability of a tree T while λ(T) is a mixture coefﬁcient

verifying

∑

T∈Θ

λ(T) = 1 and λ(T) ≥ 0.

Hence, the probability classiﬁer we are looking for

is the mixture probability that we have to compute by

determining λ(T) and solving the likelihood function:

mix

= argmax

′

∑

i=1

∑

T∈Θ

log(λ(T)P(x

|T)) (5)

where a tree T

′

represents a mixture probability.

3 THE MIXTURE MODEL

First, we propose the next theorem:

Theorem

1 The true probability distribution (P(x))

is most efﬁciently approximated by the tree proba-

bility distribution (P(x|T

mix

)) obtained by the mix-

ture of Optimal Spanning Trees (T

mix

) than the one

(P(x|T

)) obtained by an Optimal Spanning Tree

). Otherwise:

KL(P(x),P(x|T

mix

)) ≤ KL(P(x),P(x|T

)) (6)

We conclude that our proposal is justiﬁed because

of its improvementcompared with an OST model (our

theorem proof is postponed to the appendix).

In the next, we will discuss two main steps that are

the training and the inference.

3.1 Training Step

In order to select the OSTs we have to list all the graph

spanning trees, which is algorithmically difﬁcult. To

deal with this, we suggest to select K OSTs (K ∈ N

∗

)

using the K-Best-Spanning-Trees algorithm given in

(Katoh et al., 1981) with respect to this constraint:

= argmax

, 1 ≤ i ≤ k (7)

where W

is the weight of the OST numbered i and

is the weight of the spanning tree T.

Let noted the set of the selected trees by Θ

, and

P(x|y

= 1) by p(x), and P(x|y

= 0) by q(x).

The procedure to obtain the Skin mixture of the

OSTs distribution (T

mix

) is:

Procedure

1 Distribution of the skin Optimal Span-

ning Trees’ mixture.

• Input : Dataset

n

(1)



,··· ,



(n)

o

1. Deﬁne the neighborhood system (equation(1)).

2. Construct the vector of observations x.

3. Build a complete non-oriented graph of x.

4. Let two different vertices u and v. Use the empiri-

cal estimators to compute p

) and p

5. Compute the edge cost between u and v:

KL(p

), p

)) (8)

6. Apply the K-Best-Spanning-Trees algorithm.

7. Estimate the mixture coefﬁcients (section 3.2)

• Output

: T

mix

and the mixture coefﬁcients.

3.2 Parameters’ Estimation

To estimate the mixture coefﬁcients of Skin OSTs’

mixture, we propose a graphical model allowing to re-

store each tree of the optimal considered ones. First,

because of the fact that the different trees have the

same vertices, the vertex set of the graph is the same

as the one of these trees. Second, so as not to loose

any information brought by the different trees, we

suggest to keep all the edges of these ones. Further-

more, we propose this edge cost between u and v:

u∼v

= KL(pmix

), pmix

)pmix

))

(9)

where pmix

and pmix

are marginals of the Skin

mixture probability deﬁned like in equation (4), pmix.

Then, we suggest to use the EM algorithm to ﬁnd the

Skin OST performing the mixture model, such: the E

step estimates the λ(T) and the M step re-estimates

the parameters of the model to solve the equation (5).

3.3 Inference

The state of the pixel y

given the observation vector

x, is:

P(y

= i|x) ≈

∑

T∈Θ

λ(T)P(y

= i|x, T);i = 0, 1 (10)

By applying the Bayes’ rule on each T ∈ Θ

, we

obtain:

P(y

= 0|x) ≈

∑

T∈Θ

λ(T)q(y

= 0|T)q(x|T)

p(y

= 1|T)p(x|T) + q(y

= 0|T)q(x|T)

(11)

P(y

= 1|x) ≈

∑

T∈Θ

λ(T)p(y

= 1|T)p(x|T)

p(y

= 1|T)p(x|T) + q(y

= 0|T)q(x|T)

(12)

All the elements of equations (11) and (12) are previ-

ously computed in the procedure 1.

Figure 1: Some inputs (color images) and outputs (grayscal-

ing images) of the three compared models.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

False positive rate

True positive rate

Baseline model

Chow & Liu Optimal−Spanning−Tree model

Mixture of Optimal−Spanning−Trees model

Figure 2: The ROC curves of the three compared models.

4 EXPERIMENTS AND RESULTS

All experiments are made on the Compaq Database

(Jones and Rehg, 1999) which is split into two almost

equal parts randomly (the training and the test parts).

We deﬁne the neighborhood system of a pixel in

which r = 2. In order to evaluate the performances of

the proposed model we compare it to two other mod-

els: the model based on a random OST (Chow and

Liu, 1968) and the baseline one in which pixels are

considered independent (Jones and Rehg, 1999).

In ﬁgure 1, the second column corresponds to the

outputs of the OST model. The outputs of our pro-

posed model are shown in the third column while the

ones of the baseline are given in the fourth column.

We present the Receiver Operating Characteris-

tic (ROC) curves of the considered models (ﬁgure 2);

where false positive rate is the proportion of non-skin

pixels classiﬁed as skin whereas detection rate is the

proportion of skin pixels classiﬁed as skin.

The ROC curves show an improvement of the perfor-

mance for all false positive rate of the mixture model

compared to an OST one. Especially, an increase skin

detection of the mixture model compared to the base-

line is detected from 0.04% to 0.48% false positive.

In addition, using [0;0.07], the area under the ROC

curve (AUC) is equals to 0.0296 for the Baseline,

0.0382 for the OST, and 0.0414 for our approach.

Figure 3 shows some cases where our detector

failed due to over-exposure, or to skin-like color.

Figure 3: Some examples where our mixture model fails.

5 CONCLUSION

In this paper, we have presented a new algorithm de-

voted to the mixture of OSTs to deal with the prob-

lems of either classiﬁcation or probability approxima-

tion of skin/non-skin. It emphasizes and takes account

of the useful information of each existing OST.

A theoretical proof of our mixture model of this spe-

ciﬁc kind of trees was drawn. Furthermore, the ROC

curve and the AUC measures on the Compaq data-

base proved that the performance of the OSTs’ mix-

ture model is better compared to other basic ones.

In further work, we propose to generalize our ap-

proach to take account of the error-tolerant notion in

order to manage the trees’ range to be chosen.

ACKNOWLEDGEMENTS

This work is supported by ”le programme Eiffel Doc-

torat” of French Ministry for Foreign Affairs.

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APPENDIX

We use the notations given in theorem(1) to prove this

latter. We have:

KL(P(x),P(x|T

mix

)) =

∑

x∈V

P(x)log

P(x)

P(x|T

mix

)

(13)

∑

x∈V

P(x)log(P(x)) −

∑

x∈V

P(x)log

∏

u∈V

)

−

∑

x∈V

P(x)log

∑

T∈Θ

λ(T)

∏

(u∼v)∈T

)

(14)

By using the Jensen’s inequality reverse, we obtain:

KL(P(x),P(x|T

mix

)) ≤

∑

x∈V

P(x)log(P(x)) −

∑

x∈V

P(x)log

∏

u∈V

)

−

∑

x∈V

P(x)

∑

T∈Θ

λ(T)log

∏

(u∼v)∈T

)

(15)

However

∑

x∈V

P(x)

∑

T∈Θ

λ(T)log

∏

(u∼v)∈T

)

∑

T∈Θ

λ(T)

∑

(u∼v)∈T

KL(P

),P

))

(16)

Moreover,

∑

T∈Θ

λ(T) = 1 and for an Optimal Span-

ning Tree T

of Θ

we have:

= W

, ∀T ∈ Θ

(17)

Therefore

∑

T∈Θ

λ(T)

∑

(u∼v)∈T

KL(P

),P

)) =

∑

(u∼v)∈T

KL(P

),P

)) (18)

It follows equation (6). Proof concluded.