JP2011013978A - Method and apparatus for detecting object based on estimation of background image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and an apparatus for detecting objects based on estimation of a background image, capable of detecting the shape of a moving object with ease and at low cost, and thereby recognizing the movement or behavior of the moving object.SOLUTION: The method and apparatus for detecting objects use one image input means capable of moving slowly, and detect an image 11 of a moving object from the background image 10 of a plurality of image frames F, Ftaken at preset time intervals. For each pixel 12 forming the image frame F, a background image estimation means determines using the image frame Fthe normal distribution of density values of the pixel 12 forming the image frame F, estimates the background image 10 based on a comparison between the density value of each pixel 12 of the image frame Fand its normal distribution, and obtains the image 11 of the moving object.

Description

The present invention relates to an object detection method and an object detection apparatus based on background image estimation for detecting a moving object image from a background image of an image frame photographed by one image input means (for example, a camera) that can move slowly.

A technique for detecting a moving object from an image captured by a moving camera is important for safe driving of a car and environment recognition of a mobile robot.
As a conventional method, a detection method (stereo method) using a stereo camera system is mainly used (see, for example, Patent Documents 1 to 3).

JP 2001-243456 A JP 2001-283203 A JP 2004-280194 A

However, when a stereo camera system is used, an area where a moving object exists can be detected, but since the area is indicated by a frame, detection of the shape of the object itself is not easy. For example, when the moving object is a person, even if the area can be detected, the movement and behavior of the person are not known.
Moreover, although the correlation between the images acquired by the two cameras on the left and right is necessary, the correlation cannot be performed as accurately as humans do.
Furthermore, two cameras are required, and the cost for installation is high.
In addition, the conventional detection method (stereo method) for moving objects (obstacles) is mainly the detection method when driving on general roads in in-vehicle vision, and detection when the moving speed is slow, such as near an intersection. There are few ways.

The present invention has been made in view of such circumstances, and an object detection method and object detection based on background image estimation that can detect the shape of a moving object easily and at a low cost, and can also recognize the movement and behavior of the moving object. An object is to provide an apparatus.

The object detection method based on the background image estimation according to the first invention in accordance with the above object uses a single image input means capable of slow movement, and from a background image of a plurality of image frames taken at a preset time interval, An object detection method for detecting a moving object image,
Obtaining a normal distribution of density values for each pixel constituting each image frame, estimating the background image from a comparison of the normal distribution of density values of each pixel of each image frame and the density value of each pixel, A moving object image is obtained.

In the object detection method based on background image estimation according to the first invention, the background image uses the normal distribution A of density values for each pixel constituting the previous image frame A out of the image frames. From the correspondence between the position of each pixel in the frame A and the position of each pixel in the next image frame B of the image frame A, a normal distribution B of density values for each pixel constituting the image frame B is obtained, and the image frame It is preferable to estimate based on whether the density value for each pixel constituting B is within a preset value range of the normal distribution B of density values.

In the object detection method based on background image estimation according to the first aspect of the invention, the reference image frame of the previous image frame A gives a normal distribution of density values to each pixel constituting the reference image frame. Thus, the normal distribution A of the density values is preferable.

In the object detection method based on background image estimation according to the first invention, it is preferable that the normal distribution of density values for each pixel constituting the background image and the moving object image is updated with newly obtained density values. .

In the object detection method based on background image estimation according to the first invention, the image input means is preferably a vehicle-mounted camera or a mobile robot camera.

The object detection device based on the background image estimation according to the second invention according to the above object uses one image input means capable of slow movement, and from the background images of a plurality of image frames taken at a preset time interval, An object detection device for detecting a moving object image,
A normal distribution of density values is obtained for each pixel constituting each image frame, the background image is estimated from a comparison between the normal distribution of density values of each pixel of each image frame and the density value of each pixel, and A background image estimating means for obtaining the moving object image from the estimated background image;

In the object detection device based on the background image estimation according to the second invention, the background image estimation means uses a normal distribution A of density values for each pixel constituting the previous image frame A among the image frames. From the correspondence between the position of each pixel of the image frame A and the position of each pixel of the next image frame B of the image frame A, a normal distribution B of density values for each pixel constituting the image frame B is obtained, The background image may be estimated based on whether or not the density value of each pixel constituting the image frame B is within a preset value range of the normal distribution B of the density value.

In the object detection device based on background image estimation according to the second invention, the background image estimation means uses the normal distribution A of density values for each pixel of the reference image frame of the previous image frame A as the reference. It is preferable to obtain a normal distribution of density values for each pixel constituting the image frame.

In the object detection device based on background image estimation according to the second invention, the normal distribution of density values for each of the pixels constituting the background image and the moving object image is further updated with newly obtained density values. It is preferable to provide a distribution update means.

In the object detection device based on background image estimation according to the second invention, the image input means is preferably an in-vehicle camera or a mobile robot camera.

An object detection method and an object detection device based on background image estimation according to the present invention obtain a normal distribution of density values for each pixel constituting each image frame, and obtain a normal distribution of density values of each pixel of each image frame and each image. Since the density value of each pixel on the frame is compared, the background image can be estimated and an image of the moving object can be obtained. Thus, since the normal distribution of the density values obtained for each pixel constituting the image frame is used for estimating the background image, the shape of the moving object can be directly obtained. Accordingly, when the moving object is a person, for example, motion recognition (walking, running, stopping, squatting, falling, etc.) can be performed by using the motion recognition method described in Japanese Patent Application No. 2008-14124.
Furthermore, since the image of the moving object is obtained by a single image input means capable of slow movement, unlike the conventional stereo method, it is not necessary to perform the association processing between the left and right images having instability. Moreover, the cost can be kept lower than the stereo method using two cameras, which is economical.

In particular, when the normal distribution of density values for each pixel constituting the background image and the moving object image is updated with newly obtained density values, the normal distribution of the background image and the moving object image is updated to the latest information. Therefore, the detection accuracy of the moving object is further improved.
In addition, when the image input means is an in-vehicle camera, particularly by detecting a person (obstacle), for example, it is possible to determine a dangerous person on a pedestrian crossing and give a warning signal to the driver, It can contribute to the improvement of driving safety technology. Further, when the image input means is a mobile robot camera, for example, if the intelligent robot has this function, it becomes possible to support or protect various activities of a person.

It is explanatory drawing of the preparation process of the object detection method based on the background image estimation which concerns on one embodiment of this invention. It is explanatory drawing of the feature point detection process of the same object detection method. It is explanatory drawing of the feature point tracking process of the same object detection method. It is explanatory drawing of the camera motion model estimation process of the same object detection method. It is explanatory drawing of the normal distribution calculation process of the same object detection method. It is explanatory drawing of the moving object image detection process of the same object detection method. It is explanatory drawing of the normal distribution update process of the same object detection method. (A) is a photograph of an original image taken from a car stopped in front of a pedestrian crossing, (B) is a photograph of a background image of (A), and (C) is a photograph of a person detected from (A). (A) is a photograph of an original image taken from a car that is turned left at an intersection, (B) is a photograph of a background image of (A), and (C) is a photograph of a person detected from (A).

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
As shown in FIGS. 1 to 7, the object detection method based on background image estimation according to an embodiment of the present invention includes a single camera (an example of an image input unit) that is attached and fixed to an automobile and that can move slowly. This is a method for detecting a human image (an example of a moving object image) 11 from background images 10 of a plurality of image frames taken at preset time intervals. The plurality of photographed image frames are photographed in time series, the previous image frame A is an image frame F _i−1, and the next image frame B that is continuous thereto is an image frame F _i. When the number of all image frames is I, i is a positive integer not less than 1 and not more than I. Details will be described below.

The camera is an in-vehicle camera attached and fixed to the front of the automobile (front of the driver's seat). The camera is not limited to this, and may be, for example, a camera for a mobile robot that is attached and fixed to the mobile robot.
This camera is, for example, a digital video camera or a CCD camera. Note that the size (number of pixels) of the captured image frame is, for example, about (100 to 500) × (100 to 500) pixels. Also, the image frame shooting speed is, for example, about 10 to 100 frames per second.
The photographing by this camera is performed when the automobile moves slowly or stops. Here, slow movement means, for example, a speed having an upper limit of 10 km / hour, further 5 km / hour, a lower limit exceeding 0, or 1 km / hour.

When the automobile (camera) is stopped, the conventional method cannot detect the shape of the human image itself with one camera. In addition, when the automobile (camera) moves slowly, the position of the background image also moves in the plurality of captured image frames, so that a human image in each image frame cannot be detected. Here, slow travel means, for example, the speed at which a traveling automobile tries to stop, or the speed at which a once stopped automobile starts to move again.
Therefore, the following operations constituting the object detection method according to the present embodiment are performed by a computer (illustrated) of an object detection apparatus (hereinafter also simply referred to as an object detection apparatus) based on background image estimation according to an embodiment of the present invention. No), and the image is processed by a program in the computer to detect a human image. This computer is a conventionally known computer including a RAM, a CPU, a ROM, an I / O, and a bus for connecting these elements, and has a background image estimating means. Note that the object detection apparatus also includes the above-described camera.
Hereinafter, a case where the camera moves slowly will be described.

First, i = 1, as shown in FIG. 1, the image frame F _i-1 as a reference a previous image frame of the image frames F _{i (where} is the first initial image frame, a plurality of images A normal distribution A of density values is obtained for each pixel 12 constituting an image frame in the middle of the frame). The normal distribution has two parameters, an average value μ and a variance σ ² . Here, the average value μ is a parameter representing the estimated pixel value of the background image, and has the same meaning as the moving average model. Further, the variance σ ² represents the variation (stability) of pixel values in the time direction in the pixel. That is, the variance is small for pixels with little change in pixel value, and is large for pixels with much change.
The processing for obtaining the normal distribution A of density values is performed by using the calculation means in the computer after transmitting the data of the image frame F _i-1 captured by the camera to the computer and storing it in the storage means. The data of the image frame F _i next to the image frame F _i-1 is also transmitted to the computer and stored in the storage means.

In the case of a color image, each pixel has a color expressed in RGB. This RGB is a kind of color expression method, and is a kind of additive color mixture that reproduces a wide range of colors by mixing three primary colors of red (Red), green (Green), and blue (Blue). Note that the color representation of each pixel is not limited to RGB, and may be represented by other representation methods (for example, an alpha channel (Alpha) representing transparency (transparency) used for image composition) RGBA and so on). Each pixel may be expressed in black and white.

As a result, a three-dimensional normal distribution N _i−1 (f; μ, Σ) _{(x, y)} of RGB density values is given to the pixel p (x, y) on the initial image frame F _i−1 . This is performed for all the pixels constituting the image frame F _i-1 and stored in the storage means.
However, the normal distribution N _i-1 (f; μ, Σ) _{(x, y)} of the above-described concentration value is expressed by the equations (1) and (2).

Here, Σ represents, as an example, a covariance matrix when it is assumed that each component of RGB is independent and has the same variance. According to this assumption, complicated calculation such as calculation of an inverse matrix can be avoided, and high-speed processing is possible (the preparation process).

Next, a feature point detection method (for example, Harris operator) is applied to the image frame F _i-1 stored in the storage means of the computer. As a result, as shown in FIG. 2, K feature points P _{k, i-1} (k = 1, 2,..., K) surrounded by thick circles can be obtained. Is stored in the storage means. The number of feature points depends on the calculation processing speed, but is, for example, about 50 or more and 500 or less (here, 100 or more and 200 or less).
As described above, by using the feature point detection method, it is possible to detect feature points that are easily taken in the image frame F _i-1 (easy to calculate a motion vector), and any point in the image frame F _i-1 can be detected. The error that occurs when trying to obtain the motion vector can be reduced (the feature point detection step).

Then, as shown in FIG. 3, on an image frame F _i stored in the storage means of the computer, the feature points and the P _k, P _k point corresponding to _i-1, a _i, feature point tracking method (e.g. , LK method: Lucas-Kanade method, local gradient method). This LK method is a method of calculating a motion vector using a linear solution of a constraint equation in the vicinity of the target pixel, assuming that the motion vector of the small region pixel in the vicinity of the target point is the same as the motion vector of the target point. .
As a result, as shown in FIG. 3, the position of the feature point P _{k, i-1} in the image frame F _{i- 1} (the position surrounded by the circle of the thin line in FIG. 3) in the image frame F _i It is detected whether it corresponds to ₍ a position surrounded by a thick circle in FIG. 3, that is, the corresponding point P _{a (k), i} ), and this is stored in the storage means.

Note that the LK method has a drawback that it is vulnerable to a large movement, and therefore an improved method using the resolution hierarchical structure of an image can be used.
In this method, two images are sequentially compressed to create a pyramid-shaped hierarchical structure. The movement on the image frame becomes smaller as the resolution is lower. Using this fact, a motion vector is estimated from an image with low resolution by the LK method, and the result is used as an initial value for motion vector estimation of the next layer. By repeating this process until the size of the original image is reached, the LK method can cope with a relatively large movement (the feature point tracking step).

Next, a set of feature points and corresponding points stored in the storage means of the computer (P _{k, i−1} , P _{a (k), i} ) (k = 1, 2,..., K; a ( k) by using a function) that gives the number of the feature points corresponding to k, as shown in FIG. 4, two-dimensional projective transformation between image frames F _i-1 and the image frame F _i T _{i-1 → i} Ask for. Thereby, since the regularity (formula) at the time of the movement of the feature point can be obtained from the pair of the image frame F _i-1 and the image frame F _i , this is stored in the storage means.
At this time, RANSAC (RANdom Sample Consensus) is used to remove a set of corresponding points that cause local motion (outlier), and this is stored in the storage means.

By using RANSAC, based on random sampling, a motion in an image frame can be discriminated as either a global motion (inlier) or a local motion. A camera motion model parameter estimation algorithm using this RANSAC is shown below.
1) R vectors are randomly selected from all motion vectors.
2) The model parameter is calculated by the least square method using the selected vector.
3) The error E = {(u−mu) ² + (v−mv) with respect to the model vector (mu, mv) calculated with the obtained model parameter in all calculated motion vectors (u, v). ² } ^1/2 is measured, and the number of vectors (inliers) whose error E falls within the threshold is measured.
4) Repeat the above 1) to 3) sufficiently many times.
5) The final model parameters are calculated using all the inliers in the model parameters from which the most inliers are obtained (the camera motion model estimation step).

Subsequently, the normal distribution N _i (f; μ, Σ) _{(x, y)} of the RGB density values of the pixel p (x, y) is obtained on the image frame F _i .
Specifically, as shown in FIG. 5, a point (x, y) _i-1 on the image frame F _i-1 corresponding to the position (x, y) _i of each pixel p on the image frame F _i is represented by , Using the inverse transformation of the two-dimensional projective transformation T _{i-1 → i} described above (dotted line arrow shown in FIG. 5). Then, the RGB average values μ _R , μ _G , μ _B and variance σ ² are obtained by bilinear interpolation from the normal distribution of four pixels (texture pixels) nearest to the point (x, y) _i−1. This is obtained, and this is set as a normal distribution N _i (f; μ, Σ) _{(x, y)} of the density value at the position (x, y) _i of the pixel p.

The above procedure is performed for all pixels in the image frame F _i, and stores it in the storage means.
In this way, the normal distribution N _i (f; μ, Σ) _{(x, y)} of the density value for each pixel constituting the image frame F _i is represented by each pixel p of the image frame F _i−1 as the camera moves. point '(x, _y) position of each pixel p of _i-1 positions the image frame _{F i} (x, _y) from the correspondence between the _i, normal distribution of the density values of each pixel in the image frame _{F i-1} N _i-1 (f; μ, Σ) _{(x, y)} can be used for the calculation (the normal distribution calculating step).

Then, if the RGB density values f _R , f _G , and f _{B at} the position (x, y) _i of the pixel p on the image frame F _i obtained by the computing means of the computer satisfy Expression (3), the pixel p Is a pixel (B) constituting a background image, and if not satisfied, a pixel (F) constituting a human image as a foreground. Note that Tσ in Expression (3) is a threshold constant. Further, the RGB density values f _R , f _G , and f _B (pixel values) at the position (x, y) _i of the pixel p are values for each pixel constituting the image frame F _i. Is different from the normal distribution in that the former is a specific numerical value, whereas the latter represents a spread (distribution) of the numerical value due to image noise, a change in brightness between image frames, or the like.
The same process by performing for all pixels, since the foreground pixels of the image frame F _i (F) is obtained, this is stored in storage means.

Specifically, as shown in FIG. 6, the RGB density values f _R , f _G , and f _B for each position (x, y) _{i of} the pixel p constituting the image frame F _i are the values of the image frame F _i . The background or the foreground is estimated based on whether or not the normal distribution N _i (f; μ, Σ) _{(x, y)} of the density value is within a preset value (here, σ).
The background means a set of pixels whose movement is caused only by the movement of the camera when the position of the camera moves (in the case of a still camera, it means a set of pixels that are stationary between consecutive image frames). ), Foreground means a set of pixels other than the background.
As a result, the normal distributions N _i−1 (f; μ, Σ) _{(x, y)} and N _i (f; μ, Σ of density values obtained for the pixels constituting the image frames F _i−1 and F _i are obtained. ) From _{(x, y)} , the background image that moves with the movement of the camera can be estimated (the moving object image detection step).
The background image estimation described above is performed by processing by background image estimation means in the computer.

It should be noted that the normal distribution of density values for each pixel 12 constituting the background image 10 and the person image 11 is preferably updated with newly obtained density values as shown in FIG.
Specifically, the normal distribution of the position (x, y) _i of the pixel p is updated using Expressions (4) to (7). The update is performed using different update formulas depending on whether the pixel is a background pixel or a foreground pixel.

Here, f _i (x, y) is the density value at the position (x, y) of the pixel p at the sample time i, α is the variation learning rate defined by the equation (5), and β is the equation (6). As defined, variation learning based on the number of consecutive foreground extracted frames C _i (x, y) (the number of times that the position (x, y) of the pixel p is determined to be the foreground consecutively by i at the sample time i) Rate. Also, c in equation (5) is a normalization constant, and k in equation (6) is a constant determined by experiment.
A new normal distribution N _i ^# (f; μ ^# , Σ ^# ) _{(x, y)} is obtained from the updating formulas (4) and (7). In this way, the normal distribution is updated using the newly obtained (obtained) density value, and this is updated with the new normal distribution N _i (f; μ, Σ) _(at the position (x, y) _i of the pixel p. _{x, y)} .
This procedure is performed for all the pixels and stored in the storage means (the normal distribution updating step).
As described above, when the normal distribution is updated, a normal distribution update means is further provided in the computer.

Next, assuming i = 2, the above-described feature point detection step, feature point tracking step, camera motion model estimation step, normal distribution calculation step, moving object image detection step, and normal distribution update step are sequentially performed. At this time, the data of the image frame F _i next to the image frame F _i-1 is transmitted to the computer and stored in the storage means in the feature point detection step.
Further, assuming that i: = i + 1, the process of sequentially performing the above steps is repeated for i ≦ I (the number of all image frames), and the process for all image frames is completed.
Thus, the normal distribution of each pixel on the background image 10 that has been estimated by comparing the density value of each pixel on the image frame F _i, human image 11 is obtained.
The background image estimation and human image detection described above are performed by background image estimation means in the computer.
Therefore, even if the camera moves slowly and the position of the background image 10 moves on the image frames F _i-1 and F _i , the human image 11 can be detected and its shape can be recognized. Note that the shape of the human image can be recognized even when the camera is stopped.

Next, examples carried out for confirming the effects of the present invention will be described.
In this example, a camera is mounted on the camera, and human images are detected from the image frames for two cases: shooting from a car stopped in front of a pedestrian crossing and shooting from a car stopped by turning left at an intersection. The results will be described.
Note that the size of the captured image frame is 320 × 220 pixels, and the imaging speed of the image frame is 30 frames per second.

First, a result of detecting a human image using an original image taken from a car stopped in front of a pedestrian crossing will be described with reference to FIGS.
By applying the object detection method based on the background image estimation of the present invention described above to the original image shown in FIG. 8A, the background image of the original image shown in FIG. 8B can be estimated. It was confirmed that a human image can be detected from the original image shown in FIG.

Next, the result of detecting a human image using an original image taken from a car that has turned left at an intersection will be described with reference to FIGS.
By applying the object detection method based on the background image estimation of the present invention described above to the original image shown in FIG. 9 (A), the background image of the original image shown in FIG. 9 (B) can be estimated. It was confirmed that a human image can be detected from the original image shown in FIG.

From the above, it was confirmed that the shape of the moving object can be detected easily and at low cost by using the object detection method and the object detection device based on the background image estimation of the present invention. As a result, for example, by using the motion recognition method described in Japanese Patent Application No. 2008-14124, the motion and behavior of the moving object can be recognized.

As described above, the present invention has been described with reference to the embodiment. However, the present invention is not limited to the configuration described in the above embodiment, and the matters described in the scope of claims. Other embodiments and modifications conceivable within the scope are also included. For example, a case where an object detection method and an object detection apparatus based on background image estimation according to the present invention are configured by combining some or all of the above-described embodiments and modifications is also included in the scope of the present invention.

The object detection method and object detection apparatus based on background image estimation of the present invention can be used for, for example, a safety system provided in various vehicles (for example, automobiles, buses, trains) and a visual information processing system of a mobile robot.

10: background image, 11: human image (moving object image), 12: pixel

Claims (10)

An object detection method for detecting a moving object image from background images of a plurality of image frames taken at a preset time interval using one image input means capable of slow movement,
Obtaining a normal distribution of density values for each pixel constituting each image frame, estimating the background image from a comparison of the normal distribution of density values of each pixel of each image frame and the density value of each pixel, An object detection method based on background image estimation, characterized by obtaining a moving object image. 2. The object detection method based on background image estimation according to claim 1, wherein the background image uses the normal distribution A of density values for each pixel constituting the previous image frame A out of the image frames. From the correspondence between the position of each pixel of A and the position of each pixel of the image frame B next to the image frame A, a normal distribution B of density values for each pixel constituting the image frame B is obtained, and the image frame B An object detection method based on background image estimation, wherein a density value for each pixel constituting the pixel is estimated based on whether or not the density value is within a preset value range of the normal distribution B of density values. 3. The object detection method based on background image estimation according to claim 2, wherein a reference image frame of the previous image frame A is obtained by giving a normal distribution of density values to each pixel constituting the reference image frame. An object detection method based on background image estimation, wherein the density distribution is a normal distribution A. 4. The object detection method based on background image estimation according to claim 1, wherein a normal distribution of density values for each pixel constituting each of the background image and the moving object image is newly obtained. An object detection method based on background image estimation, characterized by being updated with a value. 5. The object detection method according to claim 1, wherein the image input means is an in-vehicle camera or a mobile robot camera. Detection method. An object detection device that detects a moving object image from background images of a plurality of image frames taken at a preset time interval using one image input means capable of slow movement,
A normal distribution of density values is obtained for each pixel constituting each image frame, the background image is estimated from a comparison between the normal distribution of density values of each pixel of each image frame and the density value of each pixel, and An object detection apparatus based on background image estimation, comprising background image estimation means for obtaining the moving object image from the estimated background image. 7. The object detection device based on background image estimation according to claim 6, wherein the background image estimation means uses the normal distribution A of density values for each pixel constituting the previous image frame A out of the image frames. From the correspondence between the position of each pixel in the image frame A and the position of each pixel in the next image frame B of the image frame A, a normal distribution B of density values for each pixel constituting the image frame B is obtained, and the image Based on background image estimation, wherein the background image is estimated based on whether or not the density value for each pixel constituting the frame B is within a preset value range of the normal distribution B of the density value Object detection device. 8. The object detection apparatus based on background image estimation according to claim 7, wherein the background image estimation means uses a normal distribution A of density values for each pixel of a reference image frame of the previous image frame A as the reference. An object detection apparatus based on background image estimation, wherein a normal distribution of density values is given to each pixel constituting an image frame. The object detection device based on background image estimation according to any one of claims 6 to 8, further newly obtaining a normal distribution of density values for each pixel constituting each of the background image and the moving object image. An object detection apparatus based on background image estimation, comprising normal distribution updating means for updating with a density value. 10. An object detection apparatus based on background image estimation according to claim 6, wherein the image input means is an in-vehicle camera or a mobile robot camera. Detection device.