JP4158153B2 - Face image detection method - Google Patents

Description

  The present invention relates to a face image detection method for detecting a human face image from a color image obtained from an imaging device such as a video camera or a still camera.

Conventionally, when a human face image is detected from a digital image, there is a method of detecting whether there is a skin-colored region or hair above it by color (for example, see Patent Document 1 below).
JP 2000-187721 A (FIG. 1)

  In the conventional face image detection method described above, a skin color region is extracted from a digital image and, for example, whether there is black hair or a facial feature in the skin color region is detected. It was.

In other methods as well, in order to extract facial features, many processes are required to prepare templates of various sizes and repeat pattern matching.
However, in the natural world, there are many colors that are similar to skin color, and when brightness is included, the problem is how far the skin color is in the color space.

  Therefore, the present invention has been made in view of the above points, and the number of templates for extracting facial features can be extremely reduced, and face image detection that is robust to changes in brightness, face orientation, and size is performed. It is an object of the present invention to provide a face image detection method that can be used.

In order to achieve the above object, a face image detection method according to the present invention includes a pixel density conversion processing step for performing pixel density conversion processing so as to unify the pixel density of a color image captured by an imaging device to a predetermined pixel density. A binarization processing step for binarizing the image subjected to the pixel density conversion processing based on a threshold value set based on a standard deviation of skin color information, and one eye that is a candidate from the binarized image. detected and is compared with a plurality of templates corresponding to the size of a given eye, detects the eyes as a other candidate from the search range for the appropriate the template, detected both eyes distance and the A feature point extraction process step of identifying a face area candidate based on an angle between a direction passing through both eyes and a reference direction, and extracting a feature point from the identified face area candidate; and the feature point extraction process The face region candidates from information contained in the face region candidates is that a recognition processing step of judgment is made as to whether the face.

  By providing this configuration, a template for extracting facial features by unifying facial images with a predetermined size, binarizing based on the standard deviation of skin color information, and making facial features stand out. Even if the brightness, color temperature, face size, and face orientation during imaging change, the face can be detected quickly and stably.

  In the recognition processing step, the left / right symmetry of the face area is used to determine whether the image is a face.

  By providing this configuration, face recognition accuracy can be increased.

  According to the face image detection method of the present invention, an image including a face is unified to a predetermined size, and binarization is performed based on the standard deviation of skin color information to make the facial features stand out, and the facial features are The number of templates to be extracted can be extremely reduced, and a face image robust to changes in brightness, face orientation, and size can be detected.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a flowchart showing a schematic processing content of the face image detection method according to the embodiment of the present invention. As shown in FIG. 1, the face image detection method according to the present embodiment performs pixel density conversion processing that performs pixel density conversion processing so as to unify the pixel density of a color image captured by an imaging device (not shown) to a predetermined pixel density. Processing step S1, binarization processing step S2 for binarizing the image subjected to pixel density conversion processing based on a threshold value set based on the standard deviation of skin color information, and detecting both eyes from the binarized image Then, a face area candidate is identified based on the detected information of both eyes, and feature point extraction processing step S3 for extracting a feature point from the identified face area candidate, and included in the face area candidate subjected to the feature point extraction process Recognition processing step S4 for determining whether or not the face is detected from the information.

  Hereinafter, each processing step will be described in detail. First, pixel density conversion processing is performed on a color image obtained from an imaging apparatus such as a digital still camera (step S1). An image obtained from an imaging device such as a digital still camera has a pixel density that greatly changes depending on the performance of the camera and shooting conditions, and a large number of templates for template matching must be prepared. It is necessary to match. In the pixel density conversion process in step S1, a horizontally long image and a vertically long image are handled in the same manner, but the number of pixels in the longitudinal direction is unified to a predetermined number of pixels, for example, 380 in consideration of the processing time and accuracy of the image.

  Next, a binarization process is performed on the image on which the pixel density conversion process has been performed (step S2). When detecting a human face image, color information is not necessarily required, and detection is possible even from a monochrome binary image. Therefore, the binarization process is performed so as to extract features at the time of detecting a face image, reduce the memory cost, reduce the processing amount, and contribute to the speedup. However, in the binarization process, a large amount of information is lost due to the setting of the threshold value, and therefore, the important point is how to leave only necessary information. That is, it is important to set a threshold value for binarization.

  Therefore, in order to highlight the features of the human face, it is considered to extract the skin color and use the gradation as a threshold value in order to leave the skin color part and leave the eyes, nose and mouth. Hereinafter, specific processing contents of the binarization processing will be described according to the flowchart shown in FIG.

Step S21: Extraction of skin color Since the white balance may be greatly lost depending on the photographing conditions, it is necessary to take a considerably wide skin color range in the color space in order to reliably extract the skin color of the face. However, if the skin color range is expanded, other object colors included in the image will be included, so by extracting the skin color only from the center of the image where the probability that a face exists is high, the effect will be reduced as much as possible. I will decide. For the skin color range to be extracted, hue H, saturation S, and brightness V are used to simplify the calculation.

Here, how to obtain the hue H, saturation S, and brightness V is shown below.
When each pixel RGB value is represented by 8 bits (255 levels),
vRed: Red value,
vGreen: Green value,
vBlue: If the value of Blue is the maximum value and the minimum value is Mx and Mn, the hue H, the saturation S, and the brightness V can be obtained as follows.

Saturation S:
When S = (Mx−Mn) / Mx (Mx ≠ 0) When S = 0 (Mx = 0)

Hue H:
rc = (Mx−VRed) / (Mx−Mn)
gc = (Mx−VGreen) / (Mx−Mn)
bc = (Mx−VBlue) / (Mx−Mn)
When Mx = vRed H = bc-gc
When Mx = vGreen, H = 2 + rc-bc
When Mx = vBlue H = 4 + gc-rc
H = H × 60;
H <0 H = H + 360
However, when S ≠ 0 and S = 0, H = O

Brightness V:
V = Mx
The skin color range is determined in consideration of variations due to lighting conditions.

Step S22: Calculation of skin color standard deviation The average value Vm of the brightness V of the skin color extracted in the skin color extraction (step S21) is obtained, and the number of the extracted colors is N, and the standard deviation σ shown in the equation (1) is obtained. .

Step S23: Determination of the threshold value The threshold value Vs is adjusted so that the feature is slightly emphasized by the constant K as shown in the equation (2) with the skin color standard deviation σ obtained by the equation (1) as a guide.

Step S24: Binarization A binarization process is performed on the image on which the pixel density conversion process has been performed based on the threshold value determined as described above.

  Next, after performing the binarization processing of the image according to the flow shown in FIG. 2, returning to FIG. 1, the feature point extraction processing is performed (step S3). This feature point extraction processing is performed according to the flowchart shown in FIG. In the present embodiment, the feature points are extracted by considering the eyes, nose, mouth, and hair that are the features of the face. I tried to find the angle.

  Step S31: First, eye template matching is performed. The problem when extracting eyes is that images taken with a digital still camera, etc., are different from ID photos such as licenses and passports, and the size of the face in the image is not unified In other words, the size of the eyes is not uniform. For this reason, in this embodiment, as shown in FIGS. 4A to 4C, three templates having different sizes of large, medium, and small are prepared. Thus, the matching degree is obtained by comparing with a part of the extraction target image. Then, locations exceeding a predetermined threshold are stored in the memory. Then, a range to be examined is determined for each size of the template, and only the one having a high matching degree is left to obtain the eye location.

  Step S32: Next, both eyes are detected. The eye location left by the above process is searched. First, the left eye is searched from the size of the template, assuming that the found eye location is the right eye. At this time, in order to allow face orientation, inclination, and individual differences, a search range is determined for each template size as shown in FIG. As a result, the orientation and inclination of the face can be allowed to some extent. The shortest distance is selected from the search range, and the right eye candidate as the detection start point and the coordinates of the left eye candidate detected here are obtained. Then, face area candidates are determined based on the detection result.

  Step S33: Next, face area candidates are specified. In the face area shown in FIG. 6, the size, position, and angle of the face that will exist based on the detected distance W between the eyes and the angle θ are estimated, and the face area is obtained by affine transformation. There are many feature extractions in a real image, even if they are not faces, and the processing time becomes longer if the features are examined in detail for all of them, so the candidates for the face region are determined roughly. Specifically, (a) skin color content rate, (b) hair presence rate, and (c) eyebrow presence rate are examined as described later, and face region candidates are examined by checking whether these exceed a certain threshold. Will be identified.

  However, since there is a case where the hair presence rate is low due to wearing a hat or the like, as many candidates as possible are selected by setting the threshold of the total points for specifying a low value.

(A) Skin color content rate In the face area as shown by the rectangle in FIG. 6, the skin color pixels Ds included in the skin color area set in advance in the HSV color space are counted, and the number Nf of pixels in the entire face area The skin color content Fsr is obtained according to the equation (3).

(B) Hair Presence Rate Next, in FIG. 7 showing a face region cut out based on the distance W between both eyes shown in FIG. 6, a rectangular region (width) slightly above the line connecting the eyes in the face region The hair presence rate Fhr indicating the degree of black pixels (hair) Dh is obtained according to the equation (4) with respect to the total number Nba of head region pixels of (W + 2 · W / 2) and vertical width: W).

(C) Eyebrow Existence Rate Also, an eyebrow presence rate Fbr indicating how many white pixels Db are in the total pixel count Nba in the eyebrow area between the eyes is examined according to Equation (5).

  Step S34: Next, feature extraction is performed. Even if a candidate for a face area is determined, there are many portions in the real image that meet these conditions. Therefore, features are further extracted and converted into points to improve the likelihood of the face as follows.

(D) Degree of matching with face pattern Since the sizes of faces to be detected are various, it is not necessary to prepare all these face patterns, and the enlargement ratio and angle are obtained from the distance between both eyes of the face area and the template, While obtaining coordinates by affine transformation shown in (6) and performing high-speed matching so as to obtain exclusive OR with a binarized real image, the face pattern matching degree Fm shown in equation (7) Ask for.

(E) Symmetry The degree of symmetry of the face area when the face area is folded back from the center in the horizontal direction is defined as the left-right symmetry of the face area, and the symmetry Fsym is obtained according to equation (8).

(F) Overall Point As shown in Equation (9), the result of Equation (3) is added to the values of Equations (7) and (8) above to obtain an overall point Ft. This is used to select overlapping areas.

  Next, returning to FIG. 1, recognition processing is performed (step S4). In recognition, feature points of each face area are obtained for face area candidates, and these are compared with a threshold value and sieved. That is, when the results of the expressions (3) and (7) to (9) satisfy the expression (10), the face area is determined. In the formula (10), I is a symbol meaning a constant proposition. As shown in this equation (10), the recognition processing step determines whether the skin color content rate Fsr, symmetry degree Fsym, face pattern matching degree Fm, and total point Ft are faces when each satisfies the comparison condition with the threshold value. .

  Since many face areas remain around the face in some cases, the face area with the highest total score is selected as the correct face area.

  According to the present invention, an image including a face is unified to a predetermined size, and binarization is performed based on the standard deviation of skin color information to make the facial features stand out. The present invention is capable of detecting a face at high speed and stably even if the brightness, color temperature, face size, and face orientation during imaging can be changed. Applicable to.

It is a flowchart which shows the schematic processing content of the face image detection method which concerns on embodiment of this invention. FIG. 3 is a flowchart according to the embodiment of the present invention and showing the contents of the binarization process of FIG. 1. FIG. 3 is a flowchart according to the embodiment of the present invention and showing the contents of feature point extraction processing of FIG. 1. It is a figure which shows the template for an eye detection used in embodiment of this invention. It is a figure which shows the binocular detection search range in embodiment of this invention. It is a figure which shows the face area | region in embodiment of this invention. It is a figure which shows extraction of the face area | region in embodiment of this invention.

Explanation of symbols

S1 pixel density conversion processing step S2 binarization processing step S3 feature point extraction processing step S4 recognition processing step

Claims (2)

A pixel density conversion processing step for performing pixel density conversion processing to unify the pixel density of the color image captured by the imaging device to a predetermined pixel density;
A binarization processing step for binarizing the image subjected to the pixel density conversion processing based on a threshold value set based on a standard deviation of skin color information;
One candidate eye is detected from the binarized image , compared with a plurality of templates corresponding to a predetermined eye size, and the other candidate eye from the search range corresponding to the template. detects, identifies the candidate face region based on the angle between the direction of the direction and the reference through spacing and the eyes of both eyes detected, extracts a feature point from the identified face region candidate feature A point extraction processing step;
A recognition process step of determining whether the face area candidate is a face from information included in the face area candidate subjected to the feature point extraction process;
A face image detection method provided.   The face image detection method according to claim 1, wherein the recognition processing step uses the left-right symmetry of the face region to determine whether the image is a face.

Images