KR20080000237A - Back light compensation method of camera - Google Patents

Abstract

A method for compensating for back light in a photographing apparatus is provided to overcome a problem of image distortion caused by a BMB(Back Mask BLC) scheme, which is always performed regardless of environmental conditions. N pixels (refereed to as clipped pixels) above predetermined threshold luminance value in an input image are counted(S100). It is determined if the N is greater than or equal to a predetermined threshold value(S102). The input luminance level of all pixels of the input image is lowered according to a predetermined rule if the N is greater than or equal to the threshold value(S104). An auto exposure is adjusted according to the changed input luminance level if N is smaller than the threshold value or after the input bright level of the pixels is lowered(S106). The predetermined rule is that luminance values of pixels exceeding the threshold luminance value are reduced by a predetermined value.

Description

Back light compensation method of camera

1 is a block diagram showing an example of a simplified internal configuration of a CCD surveillance camera to which the backlight compensation method of the present invention is applied.

2 is a flowchart for describing a backlight compensation method of the photographing apparatus according to an exemplary embodiment of the present invention.

FIG. 3 is a gamma characteristic curve illustrating an example of a method of lowering a luminance level in operation S104 of FIG. 2.

FIG. 4 is a diagram for explaining an example in which an image scaling method is performed to prevent overexposure after a luminance fix is performed at a zero level with respect to a clip pixel.

5 is a flowchart for explaining a backlight compensation method of the photographing apparatus according to another exemplary embodiment of the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photographing apparatus using an imaging device such as a surveillance camera, and more particularly to a photographing apparatus having a BACK LIGHT COMPENSATION function.

The BLC in the image pickup device using the image pickup device is a function to prevent the subject from becoming indistinguishable because the background becomes bright and the subject becomes dark when the subject is photographed in a backlit state. Refers to a function used to improve image quality.

When photographing a subject using a digital imaging device such as a digital still camera or a surveillance camera for a closed circuit television (CCTV), if there is an external illuminator such as natural light or fluorescent light around the subject, the subject is an external illuminator It is put in the backlight state by the image, and it is not clear but it takes a dark image.

This means that the subject is captured in the normal shooting mode without setting the backlight compensation mode, and the low exposure level of the subject is not taken into account, and the automatic exposure adjustment is performed according to the high brightness level of the external illuminant incident through the aperture of the photographing apparatus. Exposure). That is, in the general imaging mode, the amount of light incident on the CCD (Charge Coupled Device) is reduced through AE control such as reducing the aperture ratio of the aperture according to the high luminance level of the external light emitter.

In order to solve this problem, a back light compensation (BLC) function is inherent in the photographing apparatus. The BLC function is to reduce the effects of external illuminators and other interference on the subject in order to secure a clear image quality of the subject, thereby correcting the backlight so that an image having an optimal clarity is photographed.

As an example of such a BLC function implementation, a so-called BMB (Black Mask BLC) is a BLC technique for fixing pixel information above a specific luminance level to a constant luminance value.

The conventional BMB technique is designed to operate at all times without considering low light conditions such as nighttime or the intensity and size of actual surrounding light emitters, so that the ambient light is sufficiently high or the size and intensity of the surrounding light emitters do not require BLC. In this case, since the pixel information above the specific luminance level is fixed at a constant luminance value, unnecessary image distortion may be caused.

An object of the present invention is to provide a backlight compensation method for adjusting an input luminance level by determining an appropriate backlight compensation condition of an image input through an imaging device.

In the backlight compensation method of the photographing apparatus according to the present invention for achieving the above technical problem, (a) the number N of pixels (hereinafter referred to as clip pixels) having luminance Y equal to or greater than a predetermined threshold luminance Yth in the input image. Counting; (b) determining whether the number N of the clip pixels is equal to or greater than a predetermined threshold number Nth; (c) if it is determined that the number N of the clip pixels is equal to or greater than a predetermined threshold number Nth, lowering the luminance value of the clip pixels by a predetermined rule; And (d) determining that the number N of the clip pixels is less than a predetermined threshold number Nth or after step (c), performing automatic exposure adjustment according to the lowered luminance value.

The predetermined rule of step (c) may be to reduce the luminance value of the pixel that is equal to or greater than the threshold luminance by a predetermined value.

In addition, the predetermined rule of step (c) may be performed by changing the maximum level of the luminance value of the pixel that is greater than or equal to the threshold luminance by a predetermined value and scaling the luminance value of the pixel that is greater than or equal to the threshold luminance in accordance with the changed maximum level. have.

In addition, the predetermined rule of step (c) may be to fix the luminance value of the pixel that is equal to or greater than the threshold luminance to a constant value. The predetermined rule of the step (c) is to fix the luminance value of the pixel which is equal to or greater than the threshold luminance to zero level, and to adjust the luminance value of all pixels in the frame with respect to the threshold pixel number fixed to the zero level.

Preferably, the method further comprises the step of scaling by a ratio.

It is preferable to further determine whether the low-light situation before step (a), and to proceed to the step (a) if it is determined that the low light situation.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Like reference numerals in the drawings denote components that perform the same function.

1 is a block diagram showing an example of a simplified internal configuration of a CCD surveillance camera to which the backlight compensation method of the present invention is applied, including a lens unit 100, a CCD 102, a CDS / AGC / AD unit 104, The DSP 106, the lens driver 110, and the monitor 108 are provided.

The lens unit 100 optically processes light from a subject. The lens unit 100 may include a zoom lens, a focus lens, a compensation lens, and a diaphragm, and may include a zoom motor, a focus motor, and an aperture motor for adjusting them.

The CCD (Charge Coupled Device) 102 converts light from the lens unit 100 into an electrical analog signal.

The CDS / AGC / AD (Correlation Double Sampler / Auto Gain Control / Analog-to-Digital Converter) section 104 processes the analog signal from the CCD 102, removes the high frequency noise, and adjusts the gain. After that, it is converted into a digital signal.

The DSP (Digital Signal Processor) 106 processes the digital signals from the CDS / AGC / AD unit 104 to generate digital image signals classified into luminance (Y) and chroma (C) signals. The DSP 106 also controls the operations of the CCD 102 and the CDS / AGC / AD unit 104. In addition, the DSP 106 drives the zoom motor, the focus motor, and the aperture motor of the lens unit 100 through the lens driver 110 to adjust the lens unit 100.

Here, the DSP 106 performs the Back Light Control (BLC) method of the present invention, which will be described in detail below with reference to FIG. 2. The CCD 102, the CDS / AGC / AD unit 104, The lens driving unit 110 is controlled, thereby performing Auto Exposure (AE).

Although not shown in the drawing, a microcontroller for controlling components of a camera device such as a DRAM (Dynamic Random Access Memory), a driving motor, or the like may be provided in the DSP 106 or a peripheral circuit.

The monitor 108 receives the digital video signal generated from the DSP 106 by a predetermined signal format such as Composite Video Banking Sync (CVBS) and displays it on the screen.

2 is a flowchart for describing a backlight compensation method of the photographing apparatus according to an exemplary embodiment of the present invention.

First, the number N of pixels (hereinafter referred to as clip pixels) having luminance Y equal to or greater than the threshold luminance Yth in the input image is counted (S100).

Then, it is determined whether the number N of clip pixels is equal to or larger than the predetermined threshold number Nth (S102). A first feature of the invention is in the step S102. That is, the backlight compensation is performed selectively by determining the condition of the input image. By the step S102, the conventional BMB technique is designed to operate at all times without considering the intensity and size of the actual ambient light emitter, and the pixel information above a specific luminance level even when the size and intensity of the ambient light emitter is small and BLC is not necessary. Since it is fixed at a constant luminance value, it is possible to improve a problem that causes distortion of an unnecessary image.

If it is determined that the number N of clip pixels is less than the predetermined threshold number Nth, the luminance clip process of step S104 is not performed.

However, if it is determined that the number N of clip pixels is equal to or larger than the predetermined threshold number Nth, the luminance value of the pixel that is equal to or greater than the threshold luminance Yth is lowered by a predetermined rule (Y? F (Y) <Y) (S104).

Then, automatic exposure adjustment is performed according to the changed luminance value (S106).

Steps S100 to S106 may be continuously performed until the BLC end command S108 is issued.

Hereinafter, the embodiments of step S104 will be described in detail.

Here, the predetermined rule Y? F (Y) <Y means a change in luminance value by the function value f (Y) for Y smaller than Y.

Here, f (Y) may be to reduce the luminance value of the pixel that is greater than or equal to the threshold luminance Yth by a predetermined value.

In addition, f (Y) may be changed by lowering the maximum level of the luminance value of the pixel that is equal to or greater than the threshold luminance Yth by a predetermined value and scaling the luminance value of the pixel that is equal to or greater than the threshold luminance Yth according to the changed maximum level. For example, when using 0 to 255 gray scale, the maximum gray scale may be changed to 180, and the luminance value may be scaled as in Equation 2 below.

In addition, f (Y) may be to fix a luminance value of a pixel (clip pixel) having a threshold luminance Yth or more to a constant value.

3 is an example in which Equation 3 is implemented by applying a characteristic curve change of gamma correction. When the number of clip pixels is less than a certain number (N <N _th ), the normal gamma characteristic curve is followed, and the number of clip pixels is equal to or greater than a certain number ( If N≥N _th , the fixed luminance value f (Y) is fixed to pixels having an input luminance level equal to or higher than the threshold luminance Y _th along the changed gamma characteristic curve.

When setting Y ₁ = 0 in Equation 3 to fix all luminance values of pixels equal to or greater than the threshold luminance Yth to zero level, an effect in which the number of pixels fixed to zero level is removed from all pixels is generated. do. If automatic exposure adjustment is performed on the basis of the reduced number of pixels, some pixels may be saturated in areas other than the clip pixels, resulting in overexposure. Therefore, in this case, it is more preferable to perform autoexposure adjustment after re-scaling the luminance by the following equation (4).

FIG. 4 is a view for explaining an example in which an image scaling method is performed to prevent overexposure after luminance fixing is performed at a zero level with respect to a clip pixel. FIG. 4 shows results of remaining regions except for the clip pixel areas CLIP1 and CLIP2. The luminance adjustment is a result of the following equation (5). At this time, the clip pixel areas CLIP1 and CLIP2 were fixed at zero level (black).

FIG. 5 is a flowchart illustrating a backlight compensation method of a photographing apparatus according to another exemplary embodiment of the present invention, in which an additional step S200 is provided in addition to FIG. 2.

First, it is determined whether or not a low light condition is executed under the condition of BLC (S200). In this case, the determination of whether or not the low light situation may refer to, for example, the AGC level of the CDS / AGC / ADC unit (104 in FIG. 1). Step S200 is a second feature of the present invention for determining the backlight compensation condition. That is, the backlight compensation is performed selectively by determining the condition of the input image. By the step S102, the conventional BMB technique causes unnecessary image distortion because the pixel information above a specific luminance level is clipped to a constant luminance value even when the BLC is not necessary because the ambient illumination is sufficiently high or the size of the peripheral illuminant is small. Can be improved.

According to the step S200, even if there is sufficient ambient illumination and daytime, even if there is some backlight, it is easy to identify the image and thus does not perform the BLC. The BLC is only performed when the environment is relatively dark due to a low light condition and a backlight such as a headlight of a car at night.

When it is determined in the step S200 that the lighting conditions are low, the number N of pixels (clip pixels) having the luminance Y equal to or greater than the threshold luminance Yth in the input image is counted (S202). Then, it is determined whether the number N of clip pixels is equal to or larger than the predetermined threshold number Nth (S204).

If it is determined that the number N of clip pixels is equal to or larger than the predetermined threshold number Nth, the luminance value of the pixel equal to or greater than the threshold luminance Yth is lowered by a predetermined rule (Y? F (Y) <Y) (S206). If it is determined that the number N of clip pixels is less than the predetermined threshold number Nth, the luminance clip process of step S206 is not performed. Then, automatic exposure adjustment is performed according to the changed luminance value (S208).

Steps S202 to S208 may be continuously performed until the BLC end command S210 is issued.

The best embodiments have been disclosed in the drawings and specification above. Although specific terms have been used herein, they are used only for the purpose of describing the present invention and are not used to limit the scope of the present invention as defined in the meaning or claims. Therefore, those skilled in the art will understand that various modifications and equivalent other embodiments are possible from this. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

As described above, according to the backlight compensation method of the present invention, since the backlight compensation is performed to determine the appropriate backlight compensation condition of the image input through the image pickup device and to adjust the input brightness level, automatic exposure adjustment by conventional unconditional backlight compensation is performed. This can improve the distortion of the image unnecessarily.

The invention is not limited to the examples described above and represented in the drawings. Those skilled in the art taught by the above-described embodiments, many modifications to the above-described embodiments are possible by substitution, erasure, merging, etc. within the scope and object of the present invention described in the following claims.

Claims (6)

(a) counting the number N of pixels (hereinafter referred to as clip pixels) having luminance Y equal to or greater than a predetermined threshold luminance Yth in the input image; (b) determining whether the number N of the clip pixels is equal to or greater than a predetermined threshold number Nth; (c) if it is determined that the number N of the clip pixels is equal to or greater than a predetermined threshold number Nth, lowering the luminance value of the clip pixels by a predetermined rule; And (d) determining that the number N of clip pixels is less than a predetermined threshold number Nth, or after step (c), performing automatic exposure adjustment according to the lowered luminance value. Backlight Compensation Method. The method of claim 1, wherein the predetermined rule of step (c) And a luminance value of the pixel equal to or greater than the threshold luminance is decreased by a predetermined value. The method of claim 1, wherein the predetermined rule of step (c) And changing the maximum level of the luminance value of the pixel above the threshold luminance by a predetermined value and scaling the luminance value of the pixel above the threshold luminance according to the changed maximum level. The method of claim 1, wherein the predetermined rule of step (c) The backlight compensation method of the camera, characterized in that to fix the luminance value of the pixel above the threshold luminance to a constant value. The method of claim 4, wherein the predetermined rule of step (c) The luminance value of the pixel equal to or greater than the threshold luminance is fixed to zero level (black), The luminance values of all the pixels in the frame with respect to the threshold pixel fixed at zero level

And a step of scaling by a ratio. The method according to any one of claims 1 to 5, And continuously determining whether or not the low light condition is performed before the step (a), and proceeding to the step (a) if the low light condition is determined.

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