How Facial Feature Extraction Works

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Facial feature extraction is one of the most important challenges in the area of facial image processing. This step is the first in applications like Face Recognition [1], Facial Expression Recognition [2], Face Detection [3], Gender Classification [5], Age Classification [4], Animation [6] etc. Facial feature extraction, in general, refers to the detection of eyes, mouth, nose and other important facial components. Various techniques have been proposed in the literatures for this purpose and can be mainly classified in four groups: geometric feature-based, template-based, color segmentation-based and appearance-based approaches. In geometric feature-based approaches, the features are extracted using anthropometric relation [7], [8] of the face components [9]. Valley detection filters [10] and analysis of horizontal and vertical edge projections are such examples [11]. Functions which are used in this approach are not one-to-one transforms; furthermore, these methods cannot distinct slight changes in the image very well. Template-based approaches match facial components using appropriate energy functional. The best match of a template in the facial image will yield the minimum energy. Template matching [12], Deformable templates [13], ASM [14] and genetic algorithms [15] fall into this category. These methods are time consuming and lost their efficiency due to variation in the illumination and out of axis rotation. In addition, these algorithms fail when the size of pattern is constant and the size of feature changes. Color segmentation techniques [16], [23], on the other hand, use skin color to isolate the face. Any non-skin color region within the face can be represented as a candidate for “eyes” and/or “mouth”. This method... ... middle of paper ... ...n orthogonal projection operator on a vector space is any operator that maps each vector into its orthogonal projection on a hyper-plane (line in ℝ 2 and plane in ℝ 3) through the origin. (Refer to Figure 1a) Figure 1b shows that the orthogonal projection is not one-to-one. Therefore, all distinct points on the same vertical line are mapped into the same point in the xy-plane. This defect causes loss of structure in the data and the feature extraction process may be faced with problems. Figure 2 shows the result of applying horizontal projection function on image X. As it is obvious, the main horizontal change in the picture is occurred in the line 3 (X3), but the projection cannot highlight that. We proposed a linear transformation to cover this weakness. A linear transformation T maps space V to W. Therefore, we need defining a space for representing an image.

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