CN102314043A - Auxiliary focusing method for human face block - Google Patents

Abstract

The invention discloses a method for auxiliary focusing of a human face block, which is suitable for an image capturing device with an automatic focusing program, wherein the automatic focusing program has a preset sampling interval. The method for auxiliary focusing of the face block comprises the following steps: capturing an image to be focused, wherein the image to be focused is provided with a human face block; calculating the face parameters of the face blocks in the image to be focused; looking up a parameter focal length comparison table according to the human face parameters to obtain a focusing section; and executing an automatic focusing program according to the focusing section and the face sampling interval to obtain a target focal length, wherein the face sampling interval is smaller than a preset sampling interval.

Description

The method of face block auxiliary focus

technical field

The present invention relates to a method for assisting focusing, in particular to a method for assisting focusing with human face blocks.

Background technique

When shooting an image, it is necessary to correctly focus on the subject to obtain a clear picture, and today's general digital cameras and other image capture devices have built-in auto-focus functions. Autofocus can be roughly divided into two categories, one is active autofocus (active autofocus), and the other is passive autofocus (passive autofocus).

Active autofocus technology uses a group of infrared emitters or laser emitters and relative receivers to project light patterns on the subject, and then calculates the distance between the camera and the subject by methods such as triangulation. distance, and this distance is the focal length. However, the precision of the active autofocus method is poor, that is to say, it is impossible to obtain a very accurate focal length, and it cannot be used in the situation where there is glass or other things between the image capture device and the object to be photographed. In addition, the use of active autofocus requires an additional light transmitter and receiver in the camera, which is costly.

Passive autofocus captures many images from the closest macro distance to the infinity area before the official shooting, and analyzes the clarity of the obtained images to determine the focal length. The passive auto-focus method only needs to use the image sensor and computing unit of the camera itself, so the manufacturing cost can be reduced. However, passive autofocus often sacrifices precision to avoid spending too much time on autofocus due to the large amount of computation required.

In recent years, the number of users who use image capture devices such as digital cameras to capture images has increased significantly. In addition to photography professionals, there are also more and more general users who use digital cameras to shoot. Usually, the images captured by general users often have problems such as focal length inconsistencies because the shooting skills of ordinary users are not as good as those of professionals. However, when taking a photo, the image of a person is often the most important part of the picture. The traditional auto-focus method may judge the background or other things other than the characters as the subject of photography, but there is a problem that people who should be the focus of the shooting are out of focus instead. Furthermore, once the precision of the auto focus is insufficient and a focus error occurs, it is likely to cause serious blurring of the captured image. Especially when the captured image is a portrait, photographic imperfections appearing on people's faces are even worse.

Contents of the invention

In order to solve the above problems, the present invention provides a method for assisting focusing of face blocks, which is suitable for an image capture device with an auto-focus program. The auto-focus program has a preset sampling interval, and the face block-assisted focusing method includes: capturing an image to be focused, wherein the image to be focused has a face block; calculating a person of the face block in the image to be focused face parameters; refer to a parameter focal length comparison table according to the face parameters to obtain a focus section; and perform an auto-focus procedure according to the focus section and a face sampling interval to obtain a target focal length, wherein the face sampling interval is smaller than the preset sampling interval .

The face parameter can be a length ratio or an area ratio between the face block and the image to be focused. The parameter focal length comparison table may correspond to a face detection program, and the image capture device detects a face block in the image to be focused by using the face detection program.

According to an implementation example of the present invention, the step of consulting the parameter focal length comparison table according to the face parameters to obtain the focus segment may include: consulting the parameter focal length comparison table according to the face parameters, and obtaining a predicted focus position corresponding to the face parameters; And according to a zoom ratio of the image capture device, the focus segment is obtained with the predicted focus position as a reference point.

In addition, the focus segment may have a front segment and a rear segment. The front segment is located between the predicted focus position and a short focus distance of the image capture device, and the rear segment is located between the predicted focus position and a far focus distance of the image capture device. And according to an embodiment of the present invention, the length of the front section may be the same as that of the rear section. According to another embodiment of the present invention, the length of the front section may be greater than or shorter than the length of the rear section.

After the focus segment is obtained, the auto-focus program can calculate the target focal length by a quadratic polynomial approximation method.

To sum up, the face block assisted focusing method consults the parameter focal length comparison table according to the face parameters of the face block, and executes the auto-focus procedure with high precision in the obtained focus area. Therefore, the accuracy of autofocus can be improved to obtain a better target focal length, and a clear human face image can be captured.

Description of drawings

FIG. 1 is a schematic block diagram of an image capture device according to an embodiment of the present invention;

FIG. 2 is a flow chart of a method for assisting focusing of a face block according to an embodiment of the present invention;

FIG. 3A is a schematic diagram of a face block according to an embodiment of the present invention;

3B is a schematic diagram of a face block according to another embodiment of the present invention;

FIG. 4A is a schematic diagram of a focus section according to an embodiment of the present invention;

FIG. 4B is a schematic diagram of a focus section according to another embodiment of the present invention;

FIG. 4C is a schematic diagram of a focus region according to yet another embodiment of the present invention; and

FIG. 5 is a schematic diagram of a sharpness curve according to an embodiment of the present invention.

Among them, reference signs

20 image capture device

22 lens unit

24 photosensitive units

26 microprocessors

28 storage units

30 To focus on the image

32 face blocks

40 predict focus position

42 focus section

44 front section

46 rear section

50 sharpness curve

52 Quadratic polynomial fitting curve

Detailed ways

The detailed features and advantages of the present invention are described in detail below in the implementation manner, and its content is enough to make any person skilled in the art understand the technical content of the present invention and implement it accordingly, and according to the content disclosed in this specification, the patent scope of the application and the accompanying drawings , any person skilled in the art can easily understand the related objects and advantages of the present invention.

The present invention provides a method for assisting focusing of a face block, which is suitable for an image capture device with an Auto Focusing (AF) program, wherein the Auto Focusing program has a preset sampling interval.

Please refer to FIG. 1 , which is a schematic block diagram of an image capture device according to an embodiment of the present invention. However, the image capture device applicable to the method for assisting focusing of face blocks according to the present invention is not limited to FIG. 1 .

The image capture device 20 can be, for example, a digital camera, a camera with a camera function, a video camera or a webcam. The image capture device 20 includes a lens unit (lens unit) 22 , a photosensitive unit (photosensitive unit) 24 , a microprocessor (micro processor unit) 26 and a storage unit (storage unit) 28 . When capturing an image, the image capturing device 20 focuses the lens unit 22 on a focusing distance (also referred to as focus). Then when the user shoots an image, the microprocessor 26 controls the lens unit 22 to a focus position corresponding to the focus distance, and stores the image obtained by the lens unit 22 and the photosensitive unit 24 in the storage unit 28 .

The focus distance refers to the distance from the optical center point of the lens unit 22 to the subject or person, and the unit of length may be meters or centimeters. The focus position can also be referred to as focus length, which refers to the distance from the optical center of the lens unit 22 to the photosensitive unit 24 . The image capture device 20 can control the lens device 22 to different focusing positions through a step motor (not shown). The focusing position is generally taken as a unit of a step (step, also referred to as a step distance).

It should be noted that the relationship between the focus distance and the focus position is not necessarily proportional. For example, the number of steps corresponding to a focusing distance of 1 meter is not necessarily twice the number of steps corresponding to a focusing distance of half a meter; the number of steps corresponding to a focusing distance of 2 meters is not necessarily the same as that corresponding to a focusing distance of 1 meter. twice the number of steps.

Next, please refer to FIG. 2 , FIG. 3A and FIG. 3B , which are respectively a flow chart of a method for assisting focusing of a face block according to an embodiment of the present invention, and schematic diagrams of face blocks according to different embodiments of the present invention.

The image capturing device 20 first captures an image to be focused 30 (step S110 ), wherein the image to be focused 30 has a face block 32 . The face block 32 refers to an image of the face portion in the to-be-focused image 30 after the face of the subject is captured as the to-be-focused image 30 . And the microprocessor 26 of the image capture device 20 can execute a face detection program to detect the face block 32 in the image 30 to be focused.

The image capture device 20 can usually use an auto-focus program to move the lens unit 22 to different focus positions one by one according to a preset sampling interval to perform auto-focus. However, when the face block 32 is detected in the image to be focused 30 , it means that the user intends to shoot at least one person. Then, the method for assisting focusing by the face block can improve the accuracy of the auto-focus procedure for the subject and obtain a target focal length by the following steps, which is better than using the target focal length to capture the subject to obtain an excellent portrait image.

或是 After obtaining the face block 32, the microprocessor 26 calculates a face parameter of the face block 32 in the image to be focused on (step S120). According to the present invention, the face parameter may be a length ratio or an area ratio between the face block 32 and the entire image to be focused 30 . For example, assume that the resolution of the image 30 to be focused is 640×480, and the resolution of the face block 32 detected by the face detection program is 120×80. Then the face parameters can be

or

Since the face parameter adopts the proportional value between the face block 32 and the image to be focused on 30 instead of an absolute value, the face block-assisted focusing method is applicable to different image formats and various image capture devices 20 .

Then the microprocessor 26 consults a parameter focal length comparison table according to the face parameters to obtain a focus segment (step S130 ). The focus area is the range of possible focus positions when capturing the image of the subject, so the auto-focus procedure should be executed in the focus area with high focus accuracy.

Preferably, the parameter focal length comparison table can correspond to the face detection program. Because even if the same image to be in focus 30 is processed, different face detection programs may be used to find out the face blocks 32 of different sizes or different positions. Therefore, the parameter focal length comparison table can be designed in conjunction with the face detection program.

As shown in FIG. 3A and FIG. 3B , generally speaking, the subject who is closer to the image capture device 20 will form a larger face block 32 in the image 30 to be focused; A distant subject will form a smaller face block 32 in the image to be in focus 30 .

According to an embodiment of the present invention, the step of consulting the parameter focal length comparison table according to the face parameters to obtain the focus segment may include: consulting the parameter focal length comparison table according to the face parameters, and obtaining a predicted focus position corresponding to the face parameters; And according to a zoom ratio of the image capture device 20 , the focus segment is obtained with the predicted focus position as a reference point.

The zoom magnification is the zoom magnification set for the image capture device 20 when capturing the image 30 to be focused, which also corresponds to the focal length. For example, when the lens unit 22 of the image capture device 20 can zoom with a focal length of 37 mm (millimeter) to 260 mm, the zoom ratio is 37 mm to 260 mm. The lens unit 22 with zoom function can adjust the position of the lenses in the lens unit 22 before shooting, so as to change the focal length. The end of the shortest focal length (37mm) is called the wide-angle end (wide), and the image capture device 20 can capture images in a larger range; otherwise, the end of the longest focal length (260mm) is called the telephoto end (tele), and the visual It seems that the image of the scene in the distance is brought closer to the image capture device 20, so that the image is enlarged.

The zoom ratio will affect the preset sampling interval. For example, when the zoom ratio is set close to the wide-angle end, the preset sampling interval may be 20 steps; and when the zoom ratio is set close to the telephoto end, the preset sampling interval may be 30 steps. The image capture device 20 can obtain the focus segment according to the zoom ratio when capturing the image to be focused 30 and use the predicted focus position as a reference point, that is, obtain the range of the most likely focus position when capturing the image of the subject.

Please refer to FIG. 4A , FIG. 4B and FIG. 4C , which are schematic diagrams of focusing regions according to different implementation examples of the present invention.

In step S130 , the focus segment 42 is obtained from the predicted focus position 40 , and the focus segment 42 may have a front segment 44 and a rear segment 46 . The front segment 44 is located between the predicted focus position 40 and a near focal distance (Near) of the image capture device 20, and the rear segment 46 is located between the predicted focus position 40 and a far focus distance (Far) of the image capture device 20. between. The focal length of the near focal length is longer (for example, 200 steps), and the focal length of the far focal length is shorter (for example, 100 steps). And different magnifications also correspond to different near focus distances and far focus distances.

And according to the present invention, the length of the front section 44 can be the same as the length of the rear section 46 ; or the length of the front section 44 can be greater or smaller than the length of the rear section 46 .

The actual data is taken as an example for illustration below, but the method for assisting focusing of the face block according to the present invention is not limited to these example data.

When the image 30 to be focused is captured, the zoom ratio of the image capture device 20 is closer to the wide-angle end, which means that it is possible to focus on a subject far away. Assume that the preset sampling interval at this time is 20 steps, and the predicted focus position 40 obtained by looking up the table is the 150th step. The range from 10 steps from the predicted focus position 40 to the far focus direction of the current magnification to 20 steps from the predicted focus position 40 to the near focus direction can be designated as the focus section 42 . Therefore, the focus section 42 is the range between the 140th step and the 170th step.

For another example, when the image capture device 20 captures the image 30 to be focused, the zoom ratio is closer to the telephoto end, and the default sampling interval is 30 steps. Then, the range from the 150th step distance of the predicted focus position 40 to the far focal length direction of the current zoom ratio can be designated as the focus area. Paragraph 42. Therefore, the focusing section 42 is a range between the 120th step and the 165th step.

However, according to another implementation example of the present invention, instead of recording the predicted focus position 40 corresponding to the face parameter, the parameter focal length comparison table records a predicted focal length corresponding to the face parameter. Moreover, the image capture device 20 can also obtain the in-focus segment 42 according to the magnification ratio and the predicted focal length according to the above-mentioned steps.

After the focus segment 42 is obtained, the microprocessor 26 executes an auto-focus procedure according to the focus segment 42 and a face sampling interval to obtain the target focal length (step S140 ). The face sampling interval is smaller than the preset sampling interval, that is, the microprocessor 26 executes the auto-focus procedure with an auto-focus precision higher than the preset value in the focus section 42 . The face sampling focal length is also related to the zoom ratio. For example, when the zoom ratio is close to the wide-angle end, the face sampling interval can be 4 steps; and when the zoom ratio is close to the telephoto end, the face sampling interval can be 6 steps.

Based on the above example of actual data, in the focus section 42 between the 140th step and the 170th step, the auto-focus procedure can be executed at a sampling interval of 4 steps. Alternatively, in the focus section 42 between the 120th step and the 165th step, the auto-focus procedure can be executed at a sampling interval of 6 steps.

After the focus segment 42 is obtained, the auto-focus program can calculate the target focal length by a quadratic polynomial approximation method.

Please refer to FIG. 5 , which is a schematic diagram of a sharpness curve according to an embodiment of the present invention.

The quadratic polynomial approximation method can also be called the quadratic polynomial curve fitting method. The auto-focus program captures an image at multiple focus positions within the focus area 42 and calculates a resolution of these images. Then, each focus in the focus section 42 and the corresponding sharpness can be plotted as X-axis and Y-axis, and a sharpness curve 50 can be obtained. Then, a quadratic polynomial curve fitting is performed according to the sharpness curve 50 to obtain a quadratic polynomial fitting curve 52 , and the focus position corresponding to the peak of the quadratic polynomial fitting curve 52 is calculated as the target focal length.

More specifically, the auto-focus program captures an image every other face sampling interval within the focus section 42 to calculate the sharpness. The face sampling interval can be calculated by dividing the focus segment 42 by a sampling point. In the same focusing section 42 , the more sampling points, the smaller the face sampling interval, and the higher the auto-focusing accuracy.

For example, when the face block 32 is not detected in the image to be focused 30 , the auto-focus program takes 10 sampling points in the range from macro to infinity to perform auto-focus. And when the face block 32 is detected in the image to be focused on 30 , 10 sampling points are also taken in the focused area 42 for auto-focusing. In this way, although it takes the same time to perform the quadratic polynomial approximation method with 10 images, a more accurate target focal length can be obtained and focus errors can be avoided.

The aforementioned parameter focal length comparison table can be established by experiment. The establishment of the experimental method refers to shooting subjects corresponding to various facial parameters with different focal lengths, and calculating the predicted focus position 40 corresponding to each facial parameter, so as to establish a parameter focal length comparison table.

In addition, although the aforementioned implementation example takes the image 30 to be in focus with a single face block 32 as an example, the method for assisting focusing by the face block is also applicable to the image 30 to be in focus with multiple face blocks 32 . For example, the average value, median or mode of all face parameters can be taken to look up the table; or a plurality of face parameters can be used to look up the table to obtain a plurality of predicted focus positions 40 and then take the average, median The digit or mode is used as the final predicted focus position 40 .

To sum up, according to the face block assisted focusing method of the present invention, the parameter focal length comparison table is consulted according to the face parameters of the face block, and the auto-focus procedure is executed with high precision in the obtained focus area. Since there is no need to set additional hardware such as a light transmitter and a receiver in the image capture device, the manufacturing cost is relatively low. Furthermore, by improving the precision of the auto-focus, a more accurate target focal length can be obtained compared with the known technology, so that the user can capture clear images of human faces.

Certainly, the present invention also can have other multiple embodiments, without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and deformations according to the present invention, but these corresponding changes All changes and modifications should belong to the scope of protection of the appended claims of the present invention.

Claims (9)

1. A method for assisting focusing of a face block, suitable for an image capture device with an auto-focus program, characterized in that the auto-focus program has a preset sampling interval, and the face block assists focus Methods include: capturing an image to be focused, wherein the image to be focused has a face block; calculating a face parameter of the face block in the image to be focused; Consulting a parameter focal length comparison table according to the face parameters to obtain a focus segment; and The auto-focus procedure is executed according to the focus segment and a face sampling interval to obtain a target focal length, wherein the face sampling interval is smaller than the preset sampling interval. 2 . The method for assisting focus by a face block according to claim 1 , wherein the face parameter is a length ratio between the face block and the image to be focused. 3 . 3. The method for assisting focusing of a face block according to claim 1, wherein the face parameter is an area ratio of the face block to the image to be focused. 4. The method according to claim 1, characterized in that, the parameter focal length comparison table corresponds to a face detection program, and the image capture device uses the face detection program in the Detect the face block in the focused image. 5. The method for assisted focusing of face blocks according to claim 1, wherein the step of consulting a parameter focal length comparison table according to the face parameters to obtain a focus section comprises: Consult the parameter focal length comparison table according to the face parameter, and obtain a predicted focus position corresponding to the face parameter; and According to a zoom ratio of the image capture device, the focus segment is obtained with the predicted focus position as a reference point. 6. The method according to claim 5, wherein the focus section has a front section and a back section, and the front section is located between the predicted focus position and the image capture The rear section is located between the predicted focus position and a far focal distance of the image capture device, and the length of the front section is the same as the length of the rear section. 7. The method according to claim 5, wherein the focus section has a front section and a back section, and the front section is located between the predicted focus position and the image capture The rear section is located between the predicted focus position and a far focal distance of the image capture device, and the length of the front section is greater than the length of the rear section. 8. The method according to claim 5, wherein the focus section has a front section and a back section, and the front section is located between the predicted focus position and the image capture The rear section is located between the predicted focus position and a far focal distance of the image capture device, and the length of the front section is smaller than the length of the rear section. 9 . The method for assisting focusing of a face block according to claim 5 , wherein the auto-focus program calculates the target focal length by a quadratic polynomial approximation method. 10 .

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