Heading Direction from Silhouettes



Fig. 1
Some shapes of legs for which it is easy to infer body direction



The first one is the inflections of the knees. When a leg is well separated from the other and the knee is inflected, a coarse body direction can be inferred without ambiguity. Figure 2a illustrates an example of shape legs where feet are cut. Our visual system can easily give an estimate of body direction because the feet have limited possibilities of poses due to the geometry of one leg (high inflexion). Figure 2b illustrates the correct poses and the directions can be inferred using the feet shapes, however Fig. 2c shows impossible situation. The directions of the lines joining inflexion points of the same leg are used to infer the body direction.

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Fig. 2
Shapes of legs with inflected knee

The second one is the direction of shape foot. Indeed, our visual system encounters difficulties by looking at legs shapes without feet and cannot estimate body direction for many configurations even if the body is moving and legs are well separated but without inflexion of knees. For example, seeing to the outlines of Fig 3a, without feet we cannot recognize to what direction body is moving. This ambiguity is clear seeing at the original shapes (see Fig. 3b) and at new shapes obtained drawing feet (see Fig. 3c). The base lines of the feet are good features because they indicate the body direction. Their use is explained in Sect. 2.2.

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Fig. 3
Ambiguity in body direction estimation in case of missed shape feet

The third feature concerns the variation of silhouette’s width along the shape head-shoulders and the length of each shoulder. The ratio of the width of the upper part (head) and the lower part (shoulders) with the varying of the shoulders length are related to the angle of rotation. We noticed that there’s an opposite relationship between the ratio and the orientation angle.



2.2 Inferring Body Direction



Body Direction Estimation Using Feet’s Features:

This task consists to split the lower human shape into separated legs, separated lower legs or grouped legs (The two first cases include the case where the knee of one leg is inflected). We associate to each foot a base line defined by two extremities of the foot located between the heel and the toes. The outline of lower part is processed in order to determine the baseline of the feet located between the heel and the toes. Firstly, high convexities points Cv 1 and Cv 2 characterizing the outline foot are located (see Fig. 4). Secondly, the last point of interest Cc representing a high concavity on this outline is located, such as the distances CcCv 2 is minimal. The convex point that represents toes, will be the closest point to the concave point of the feet outline, the other convex point will obviously correspond to the heel. Thus the base line joins the two convexities of the foot and the orientation of feet corresponds to the vector carried by the feet base line.

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Fig. 4
Steps of body direction estimation based on foot directions

Applying the 2D quasi-invariant, the angle between the two vectors measured in 3D-space varies slowly in the image as viewpoint varies [2]. As in the scene the disposition of foot vectors is restricted by the human physic constraints, it will be the same case in image plane; the body direction is inferred as the average of foot directions. Once the base lines of feet are extracted, body orientation is computed as the resultant vector of the two orientations (see Fig. 5a). When one foot is not put on the ground, which correspond to a high inflection of the knee, the resultant vector will have the direction of the base line of the other foot (see Fig. 5b).

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Fig. 5
Body orientation from feet (In red color the feet orientations of foot and in blue the body orientation)


Body Direction Estimation Using Knee’s Features:

Extraction of inflection points consists to find the best concave or convex pixels of the lower part of the silhouette using the Chetverikov’s algorithm [5]. Among the selected points of inflection p, $p^*$ which is the farthest to the line binding p – and $p^+$ is chosen. The position of $p^-, p^+$ to $p^*$ is a parameter (see Fig. 6).

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Fig. 6
Location of inflection points on outline legs

Many types of knees inflexion may be located (see Fig. 7). The direction of the body follows the direction of the inflected knee considered as the direction of the line joining the concave point to the convex one. Only the direction left towards right and inversely will be considered.

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Fig. 7
Some cases of knee inflexion and the inferred direction of them


Body Direction Estimation Using Head-Shoulders Features:

Applying the algorithm of D. Chetverikov [5], the two concave points (left and right) delineating the head and the two convex points (left and right) extremities of shoulders are located. Head is separated by locating the pixel having the minimum angle among the selected point candidates. The two convex pixels are located based on high curvature. Each pixel is characterized by the fact that it is the farthest from the line (L) connecting the beginning of the shoulder and the end pixel of the head-shoulders outline (see Fig.  8).

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Fig. 8
The pixel p is the farthest from the line L

When human is in the centre of field view of the camera, the average of computed ratios R w (ratio of the widths of head and shoulders) estimated are given by Table 1 and the Fig. 9 illustrates an example corresponding to the rotation of a person towards the left using the ratio R w of head-shoulders.


Table 1
Body direction inferred from head-shoulders features



























Body direction
Ratio R w

$[0^{\circ}, 15^{\circ}]$

$\geq 1.82$

$[15^{\circ}, 30^{\circ}]$

$[1.70, 1.81]$

$[30^{\circ}, 45^{\circ}]$

$[1.61, 1.69]$

$[45^{\circ}, 60^{\circ}]$

$[1.51, 1.60]$

$[60^{\circ}, 75^{\circ}]$

$[1.36, 1.5]$

$[75^{\circ}, 90^{\circ}]$

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Jun 14, 2017 | Posted by in GENERAL SURGERY | Comments Off on Heading Direction from Silhouettes

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