KR102405635B1 - focus control apparatus and focus measure method - Google Patents

Abstract

The present invention relates to a focus control apparatus and a focus measurement method, comprising: a luminance detector configured to detect luminance values of a plurality of patches constituting an image; a threshold selector for selecting patches having a luminance value less than a reference value from among the luminance values for the patches detected by the luminance detector and outputting a luminance value within the selected patches; a filter unit that receives the luminance value output from the threshold value selection unit as an input, performs filtering, and outputs a filter value; and a control unit configured to obtain a normalized value by dividing the filter value by a representative value of the luminance value output from the threshold selector, and to determine a final focus value based on the normalized value to control the position of the camera lens, focus measurement The method includes: receiving a luminance value of an input image; dividing the input image into patches having a preset size; outputting a luminance value in patches having a luminance value less than a reference value among the patches; outputting a filter value by filtering the luminance value of the received input image; obtaining a normalized value by dividing the filter value by a representative value of the luminance values in patches having a luminance value less than a reference value; and calculating a position of a peak among the normalized values as a focus value.

Description

A focus control apparatus and a focus measure method

The present invention relates to a focus control apparatus and a focus measurement method, and more particularly, to calculate an accurate focus value even in an environment where a bright light source exists in a dark place, and to perform autofocus using the calculated focus value The present invention relates to a focus control device and a focus measurement method.

In general, imaging devices, such as cameras and camcorders, photoelectrically convert an optical signal incident through a lens into an electrical signal, and then perform a predetermined image processing on the photoelectrically converted image signal to perform an imaging operation, Focus is performed by any one of an auto focus and a manual focus method.

Among these focusing methods, the autofocus method is a method of automatically focusing by controlling the position of the lens to a point where autofocus data, which is a value obtained by converting contrast based on luminance for a predetermined distance section from the lens, becomes maximum. , This is a method that automatically focuses on the clearest position without any user intervention. In such autofocus, quick focus setting and accurate focus setting are two main factors.

At this time, in order to accurately execute autofocus, it is necessary to generate a focus value, which is an input of autofocus, and to generate a focus value, FIR (Finite Impulse Response) or IIR (Infinite Impulse Response) Impulse response) characteristic digital filter is used, but in a bright light source that exists in a dark place, that is, in a spot light source environment, the filters required for auto focus do not work properly, so auto focus cannot be performed correctly. is known not to.

The reason is that, in the case of a point light source that shines brightly in a dark area, the bright light and the dark boundary area around the point light source are analyzed with high frequency on a spatial filter.

In other words, since the area of the point light source appears wider and the high-pass filter (HPF) or band-pass filter (BPF) high-frequency component increases at the boundary of the large area, the high-frequency component appears higher when the focus is out of focus. It goes against the basic assumption that the peak is the best-focused location.

Accordingly, there is a need for a focus control apparatus and a focus measurement method having robustness to a point light source environment while satisfying the basic assumption that the focus is best at such a high-frequency peak.

Japanese Patent Laid-Open No. 2006-243101 Korean Patent Publication No. 2000-0021538

The present invention has been devised in view of the various problems described above, and the technical problem to be achieved is to calculate an accurate focus value in an environment where a bright light source exists in a dark place, that is, a point light source environment, and calculate the calculated focus value. An object of the present invention is to provide a focus control apparatus and a focus measurement method capable of performing autofocus using the present invention.

The tasks of the present invention are not limited to the tasks mentioned above, and other tasks not mentioned will be clearly understood by those skilled in the art from the following description.

According to an embodiment of the present invention, there is provided a focus control apparatus comprising: a luminance detector configured to detect luminance values of a plurality of patches constituting an image; a threshold selector for selecting patches having a luminance value less than a reference value from among the luminance values for the patches detected by the luminance detector and outputting a luminance value within the selected patches; a filter unit that receives the luminance value output from the threshold value selection unit as an input, performs filtering, and outputs a filter value; and a controller configured to obtain a normalized value by dividing the filter value by a representative value of the luminance value output from the threshold selector, and to determine a final focus value based on the normalized value to control the position of the camera lens.

Here, the filter unit may be an IIR filter (Infinite Impulse Response Filter), the representative value may be a sum or average value, and the reference value may be determined as a predetermined position on a histogram of the luminance values detected by the luminance detection unit. .

Also, the controller may determine a focus value that maximizes the normalization value among various focus values as the final focus value.

Meanwhile, a focus measurement method according to an embodiment of the present invention includes: receiving a luminance value of an input image; dividing the input image into patches having a preset size; outputting a luminance value in patches having a luminance value less than a reference value among the patches; outputting a filter value by filtering the luminance value of the received input image; obtaining a normalized value by dividing the filter value by a representative value of the luminance values in patches having a luminance value less than a reference value; and calculating a position of a peak among the normalized values as a focus value.

Other specific details of the invention are included in the detailed description and drawings.

According to the focus measurement method according to an embodiment of the present invention, an accurate focus value can be calculated even in an environment in which a bright light source exists in a dark place, and autofocus can be performed using the calculated focus value, thereby improving product reliability. effect can be provided.

The effect according to the present invention is not limited by the contents exemplified above, and more various effects are included in the present specification.

1 is a photograph showing an example of a state in which a bright light source exists in a dark place, that is, in a point light source environment, out of focus.
FIG. 2 is a table showing conventional focus measurement data as opposed to a focus measurement method according to an embodiment of the present invention; FIG.
3 is a diagram illustrating an assumption that, when the contrast of image brightness is large, the focus may be optimal at a point where the brightness of the image is the darkest.
4 is a block diagram schematically illustrating a configuration of a focus control apparatus according to an embodiment of the present invention;
5 is a diagram illustrating an example of a filter unit constituting a configuration of a focus control apparatus according to an embodiment of the present invention.
6 is a flowchart illustrating a focus measurement method according to an embodiment of the present invention.
7 is a diagram illustrating focus measurement data measured by a focus measurement method according to an exemplary embodiment of the present invention.
8 is a schematic configuration diagram showing a state in which a point light source region is detected in a state in which an image is divided into patches;
9 is a photograph showing an example of a state in which a focus is achieved in an environment where a bright light source exists in a dark place, that is, a point light source environment according to an embodiment of the present invention.

Advantages and features of the present invention, and a method for achieving them will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, and only these embodiments allow the disclosure of the present invention to be complete, and common knowledge in the technical field to which the present invention belongs It is provided to fully inform the possessor of the scope of the invention, and the present invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout.

In addition, the embodiments described in this specification will be described with reference to cross-sectional and/or schematic diagrams that are ideal illustrations of the present invention. Accordingly, the shape of the illustrative drawing may be modified due to manufacturing technology and/or tolerance. In addition, in each of the drawings shown in the present invention, each component may be enlarged or reduced to some extent in consideration of convenience of description. Like reference numerals refer to like elements throughout.

Hereinafter, a preferred embodiment of the focus measurement method according to the present invention will be described in detail based on the accompanying illustrative drawings.

First, before describing the focus control apparatus and the focus measuring method according to the exemplary embodiment of the present invention, a conventional autofocus method will be briefly described.

1 is a photograph showing an example of a state where focus is not achieved in an environment where a bright light source exists in a dark place, that is, a point light source environment, and FIG. It is a figure which shows the measurement data in a table.

Also, FIG. 3 is a diagram illustrating an assumption that, when the contrast of image brightness is large, the focus may be optimal at the point where the brightness of the image is the darkest.

Autofocus technology can be largely divided into an active (active) or a passive (passive) method. The active method is a technology that focuses using reflected and returned light or a specific wavelength. Because a separate proximity sensor is required, manufacturing costs increase and objects located opposite to transparent glass cannot focus well. As a result, it is not currently used well.

On the other hand, in the case of the passive method, the frequency characteristic of the image formed on the camera and the image sensor is analyzed, and the point where the high-frequency component where the boundaries of the image is well expressed is the highest as the focus position is determined as the focus position. This is the method mainly used today.

Here, the passive method is divided into a phase difference method and a contrast method. The phase difference method is a method for optically focusing by attaching a separate autofocus sensor that detects a phase difference in the camera, and is mainly used in DSLRs, etc., The contrast method is mainly used in general digital cameras because it does not require additional parts.

In both the phase difference method and the contrast method described above, it was difficult to set the focus when a small and bright light source (point light source) exists in a dark place. While visible, the HPF or BPF high frequency component also increases at the boundary of a large area, so that when focus is not achieved, the high frequency component appears higher.

On the other hand, digital filters with various FIR or IIR characteristics are applied to generate a focus value. Rather than FIR filters, IIR filters that accumulate past results (Infinite Impulse Response Filter) are point light sources. better performance in the environment.

However, even in the case of the IIR filter, it was not able to properly perform under any adverse conditions, and as shown in FIG. 2 , it was confirmed that the focus value exhibited the lowest characteristic at the position where the focus should be the best, as shown in FIG. 2 .

That is, as shown in Fig. 2, when the lens is positioned at about 61 on the X-axis, it is actually a case of focus, but rather, the lens has the highest Y value (the luminance value of the image) at about 1 on the X-axis. ) appears to appear to be in focus, so correct autofocus cannot be performed with these focus values.

On the other hand, in the region where the contrast of the brightness of the image including the light source is greatest, the focus may be rather optimal at the point where the brightness of the image is the darkest (“AN ADVANCED VIDEO CAMERA SYSTEM WITH ROBUST AF, AE, AND AWB”) CONTROL, June-Sok Lee, You-Young Jung, Byung-So0 Kim and Sung-Jea KO, Senior Member, IEEE Department of Electronics Engineering, Korea University, Seoul 136-701, Korea"). Referring to FIG. 3 , it can be seen that the brightest point in a point light source having a large brightness contrast is a point slightly deviated from the center rather than a central position, and the brightness may be lowered at the central position. Therefore, in such a special case, it may be appropriate to set the lowest point between the two peaks as the focus value as in the above paper.

However, not all images include a point light source, and even if a point light source is included, the contrast may not be large enough, and the iris may not be constant when a low-priced lens is adopted, so it is a more general situation. It needs to be considered for application. That is, the assumption that the focus is best at the darkest point within the two peaks in the image is not always true.

Accordingly, the present invention provides a focus control apparatus and a focus measurement method capable of calculating a more accurate focus value in an environment where a bright light source exists in a dark place, that is, a point light source environment, and performing autofocus using the calculated focus value. would like to provide

4 is a block diagram schematically illustrating a configuration of a focus control apparatus according to an embodiment of the present invention.

As shown in FIG. 4 , the focus control apparatus 100 according to an embodiment of the present invention includes a luminance detection unit 110 , a threshold value selection unit 120 , a filter unit 130 , and a control unit 140 . can be made with

The luminance detector 110 may detect a luminance value for each patch or block in an image acquired by a general imaging device.

Here, the patch is a partition of a captured image in a predetermined arrangement, and may be partitioned and formed to include one or more pixels. Although described as an example, the present invention is not limited thereto, and may be divided into various numbers of course.

The threshold selector 120 selects, among the luminance values for each patch in the image, a patch having a luminance value less than a set reference value, for example, a patch other than a patch in which the point light source 150 as shown in FIG. 8 is located. can As described above, the patch having a luminance value less than the reference value can be regarded as a portion in the image excluding the point light source.

The reference value may be set as a fixed value, but as a dependent value of the image, it may be set based on the average value or the median value among the histograms of all luminance values of the image (eg, a value located at 1.5 times the average value, the average value and between the maximum values, etc.).

Also, the threshold selector 120 may select patches having a luminance value less than the reference value, and output only luminance values of pixels within the selected patches.

The filter unit 130 receives the luminance value provided from the threshold value selection unit 120, filters it, and outputs the filter value. The filter unit 130 may be used as an FIR filter, but it is preferable to use an IIR filter.

5 is a configuration diagram illustrating an example configuration of an IIR filter applicable to a focus control apparatus according to an embodiment of the present invention. The IIR filter includes a value of an input signal X[n] and an output signal Y[n]. ]) is applied recursively and as filtering is performed, it becomes an iterative expression in the form of an implementation expression, and the impulse response, which is a characteristic function, has an infinite length.

For reference, the IIR filter has an internal block in the form of a feedback, and the internal block may include at least one buffer (D) and at least one operator (G).

Therefore, it is economical because the order is reduced compared to the FIR filter having the same characteristics, and since it has non-linearity in terms of phase characteristics (that is, the difference in phase for each frequency component is non-linearly different), the input and output waveforms are It has a characteristic that does not have a similar waveform. Therefore, it can be seen that the IIR filter, which cumulatively feeds back the output and uses it as an input, has greater robustness to random noise than the FIR filter.

However, even if the IIR filter 130 is used, an undesired error may occur in an image including a large number of point light sources.

To this end, the control unit 140 obtains the sum or average value of the output of the filter unit 130, and uses this as a representative value of the luminance values output from the threshold value selection unit 120, for example, the sum (Y_sum) or the average value ( Y_mean) to perform normalization. A normalized value at an arbitrary lens position x obtained through this process, that is, N _x may be expressed by Equation 1 or 2 below.

Here, Sum(V _filter ) is the sum of filter values output from the filter unit 130 , and Y_sum is the sum of luminance values in the patches selected by the threshold selection unit 120 .

Here, Mean(V _filter ) is an average value of filter values output from the filter unit 130 , and Y_mean is an average value of luminance values in the patches selected by the threshold value selection unit 120 .

As such, when a normalized value at an arbitrary lens position x is obtained, the focus value FV can be determined by the following equation (3).

That is, the focus value is determined as the position of x that maximizes N _x .

In this way, by performing normalization only with luminance values belonging to patches having a luminance value below a certain level, noise that may occur in luminance values in the point light source region when the point light source is out of focus can be removed and various It is possible to obtain a focus value for the environment of the image. For example, in the present invention, in the image in which the point light source exists as shown in FIG. 8, since the patch region to which the point light source belongs is excluded from the luminance values (eg, Y_sum) used for normalization, the point light source included in the image It is possible to prevent the case where the sum of the output values becomes rather large due to the blurring phenomenon caused by not being focused.

7 is a table showing focus measurement data measured by the focus measurement method according to an embodiment of the present invention, wherein the illustrated dots represent the IIR filter values of the luminance values of the patches selected by the threshold selection unit 120 . Total (Y sum) or the average value (Y_mean).

As described above, it was confirmed as a result of the experiment that the output value obtained by normalizing the IIR filter value according to the embodiment of the present invention fits best when the position of the lens is approximately 60 on the x-axis. Through this, it can be seen that there is a significant difference from the case in which a significant error occurs compared to an accurate focus value (x=61) by simply applying only the IIR filter value as shown in FIG. 2 .

Thereafter, the controller 140 controls the lens of the camera to move to the target position based on the focus value obtained as described above. As described above, when the focus value is adjusted by the focus measurement method according to the embodiment of the present invention, a more accurate focus value can be obtained even in an environment where bright point light sources exist in a dark place as shown in FIG. As a result, a clearer image can be captured.

Those of ordinary skill in the art to which the present invention pertains will understand that the present invention may be embodied in other specific forms without changing the technical spirit or essential features thereof. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. The scope of the present invention is indicated by the following claims rather than the above detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention. do.

100: focuser control device 110: luminance detection unit
120: threshold selection unit 130: filter unit
140: control unit 150: point light source

Claims (6)

a luminance detector configured to detect luminance values of a plurality of patches constituting an image;
a threshold selector for selecting patches having a luminance value less than a reference value from among the luminance values of the patches detected by the luminance detector and outputting a luminance value within the selected patches;
a filter unit that receives the luminance value output from the threshold value selection unit as an input, performs filtering, and outputs a filter value; and
A control unit for controlling the position of the camera lens by dividing the filter value by a representative value of the luminance value output from the threshold value selection unit to obtain a normalized value, and determining a final focus value based on the normalized value,
The plurality of patches are formed by partitioning the image in a predetermined arrangement,
The filter unit outputs the filter value by using the luminance values of the selected patches among all the patches constituting the image. The method of claim 1,
The filter unit is an IIR filter (Infinite Impulse Response Filter). The method of claim 1,
The representative value is a sum or average value. The method of claim 1,
The reference value is determined as a predetermined position on a histogram of the luminance value detected by the luminance detector. The method of claim 1,
the control unit
A focus control apparatus for determining a focus value that maximizes the normalized value among various focus values as a final focus value. receiving a luminance value of an input image;
dividing the input image into patches having a preset size;
selecting patches having a luminance value less than a reference value from among the patches, and outputting a luminance value within the selected patches;
outputting a filter value by performing filtering on the output luminance value;
obtaining a normalized value by dividing the filter value by a representative value of the luminance values in patches having a luminance value less than a reference value;
Comprising the step of calculating the position of the peak in the normalized value as a focus value,
The patches are formed by partitioning the input image in a predetermined arrangement,
The outputting of the filter value includes outputting the filter value using luminance values of the selected patches among all patches constituting the input image.

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