Patent Publication Number: US-9842382-B2

Title: Method and device for removing haze in single image

Description:
This application is a National Stage of International Application No. PCT/KR2014/003144, filed Apr. 11, 2014, and claims priority from Korean Patent Application No. 10-2013-0060600 filed May 28, 2013, the disclosures of which are incorporated herein in their entirety by reference. 
     TECHNICAL FIELD 
     The present invention relates to processing an image with a video quality degraded due to a haze included therein, and improving a video quality thereof. 
     BACKGROUND ART 
     In general, the quality of an image captured outdoors is degraded by a turbid medium such as atmospheric particles and trickles. In particular, the video quality of an image is influenced by weather, which influences the identification of the original color and shape of an object included in the image. 
     In particular, since haze-induced video quality degradation occurs throughout the image and is maintained relatively uniformly and continuously, various techniques are proposed to remove the haze-induced video quality degradation. 
     Hazy image improving methods are classified into a model-based method using an atmospheric physical model and a nonmodel-based method without using an atmospheric physical model. 
     Unlike the model-based method, the nonmodel-based method does not estimate an atmospheric parameter and may be simply implemented. However, the nonmodel-based method has a problem of distorting a color fidelity and a haze removing effect. Therefore, recently, it is common to improve a hazy image by using the model-based method using a piece of image. 
     On the other hand, since the model-based method uses an atmospheric physical model, the model-based method may effectively improve a color tone and a contrast degraded by scattered light. However, the model-based method has difficulty in accurately estimating a parameter and has a drawback of taking a long operation time. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
     Recently, as an example of the model-based method for improving a hazy image, there is a method of using a dark channel prior (DCP) in a piece of hazy image. However, in the case of a DCP method, since the color information feature of an RGB image is estimated on a patch basis, an operation time increases. Also, when an image is generally white or dark, a distortion occurs in the image due to the misestimation of a transmission. 
     Also, in the case of other model-based methods for improving a hazy image, since there is no image texture information compensation and brightness correction due to the subtraction of scattered light, there is a problem in that the restored image has a weak texture and low brightness. 
     An exemplary embodiment of the present invention improves the problem of the DCP method and restores/improves haze-induced image color damage and contrast degradation by using a single image. Also, the present invention provides a method for improving an image texture and solving a brightness degradation problem occurring in the conventional hazy image improving process. 
     Technical Solution 
     The present invention provides a method for removing a haze in a single image. In the present invention, a transmission is estimated by using a dark channel prior obtained from a hazy input image. The estimated transmission includes a block artifact. In an exemplary embodiment of the present invention, in order to preserve an edge and remove the block artifact, a refined transmission value is obtained by performing weighted least square (WLS) filtering by using an estimated transmission value and a morphologically-processed input image, the image is restored based on the refined transmission value, and then multi-scale tone manipulation image processing is performed. 
     Advantageous Effects of the Invention 
     The present invention has an advantageous effect of solving a problem occurring in a hazy image processing process by using a dark channel prior (DCP) in a hazy image. For example, the present invention has an advantageous effect of solving a block artifact problem occurring in an image restored through a transmission obtained by estimating the color information feature of an RGB image on a patch basis. 
     Also, the present invention has an advantageous effect of improving the texture and brightness of a restored image by performing image texture information compensation and brightness correction due to the subtraction of scattered light in a hazy image improving process. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a hazy image restoring method. 
         FIG. 2  illustrates a single image haze removing device according to an exemplary embodiment of the present invention. 
         FIG. 3  illustrates an estimated transmission image  320  obtained from a hazy input image  310 , a morphologically-processed input image  330 , and a refined transmission image  340  obtained by using the estimated transmission image  320  and the morphologically-processed input image  330 , according to an exemplary embodiment of the present invention. 
         FIG. 4  illustrates a restored image when various smoothing control constant values λ are applied to an input image  410  including a haze, according to an exemplary embodiment of the present invention. 
         FIG. 5  illustrates a restored image according to a change in an edge control constant α of a morphologically-processed hazy image, according to an exemplary embodiment of the present invention. 
         FIG. 6  illustrates a restored image of a hazy image and an image-processed final image of the restored image according to an exemplary embodiment of the present invention. 
         FIG. 7  illustrates a flow diagram of a method for removing a haze in a single image according to an exemplary embodiment of the present invention. 
     
    
    
     Best Mode 
     According to an exemplary embodiment of the present invention, a device for removing a haze in a single image includes: an image analyzing unit configured to generate a dark channel prior representing a distance-dependent haze density in an input image including a haze component; a transmission estimating unit configured to calculate an estimated transmission by estimating an amount of light reaching a photographing device based on the dark channel prior; a transmission refining unit configured to calculate a refined transmission by applying weighted least square (WLS) filtering based on the estimated transmission and morphologically-processed input image information, wherein the refined transmission represents information obtained by preserving edge information and removing a block artifact in the estimated transmission; and an image restoring unit configured to generate a restored image by removing the haze component in the input image based on the refined transmission. 
     For example, the device may further include an image processing unit configured to perform image processing for improvement of a local contrast of the restored image by dividing the restored image into a base layer, a medium layer, and a fine layer. 
     According to another exemplary embodiment of the present invention, a device for removing a haze in a single image includes: a transmission estimating unit configured to calculate a first transmission by estimating a transmission value based on a dark channel prior generated from a hazy input image; a transmission refining unit configured to calculate a second transmission for minimizing a block artifact generated in an image restoration process based on the first transmission by applying WLS filtering based on the first transmission and morphologically-processed input image information; an image restoring unit configured to generate a restored image by removing a haze component in the hazy input image based on the second transmission; and an image processing unit configured to perform multi-scale tone manipulation image processing on the restored image. 
     According to another exemplary embodiment of the present invention, a method for removing a haze in a single image includes: generating, by an image analyzing unit, a dark channel prior representing a distance-dependent haze density in an input image including a haze component; calculating, by a transmission estimating unit, an estimated transmission by estimating an amount of light reaching a photographing device based on the dark channel prior; calculating, by a transmission refining unit, a refined transmission by applying WLS filtering based on the estimated transmission and morphologically-processed input image information, wherein the refined transmission represents information obtained by preserving edge information and removing a block artifact in the estimated transmission; and generating, by an image restoring unit, a restored image by removing the haze component in the input image based on the refined transmission. 
     According to another exemplary embodiment of the present invention, a method for removing a haze in a single image includes: calculating, by a transmission estimating unit, a first transmission by estimating a transmission value based on a dark channel prior generated from a hazy input image; calculating, by a transmission refining unit, a second transmission for minimizing a block artifact generated in an image restoration process based on the first transmission by applying WLS filtering based on the first transmission and morphologically-processed input image information; generating, by an image restoring unit, a restored image by removing a haze component in the hazy input image based on the second transmission; and performing, by an image processing unit, multi-scale tone manipulation image processing on the restored image. 
     Mode of the Invention 
     A method for referring to an elementary stream located in an SAF session according to the present invention for achieving the above-described object will become apparent and more readily appreciated from the following detailed description taken in conjunction with the accompanying drawings. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  illustrates a phenomenon in which an artifact occurs in a conventional hazy image restoring process. In the conventional hazy image restoring process, since a transmission is estimated on a block-by-block basis, an artifact phenomenon occurs in a final haze-free image. In detail, there is a phenomenon in which an artifact occurs in a block of a restored final image  130  after a transmission is estimated in a hazy image  110  through Equations 1 to 4. 
     In general, a Koschmider model such as Equation 1 is widely used to represent an image including a haze.
 
 I ( p )= J ( p ) r ( p )+ A (1− r ( p ))  [Equation 1]
 
     In Equation 1, I(p) denotes an input image including a haze, J(p) denotes a scene radiance representing a haze-free state, A denotes a global atmospheric light source, and r(p) denotes a transmission estimated corresponding to each pixel of the input image. The transmission refers to the amount of light reaching a photographing device without dispersion. 
     The Koschmider model such as Equation 1 is represented for each of the R, G, and B color channels of the image. By using Equation 1, the global atmospheric light source A and the transmission r(p) are calculated from the input image I(p) including a haze, and then the scene radiance J(p) is acquired. 
     As the transmission r(p) decreases in Equation 1, the input image I(p) becomes a hazier image. Also, as the transmission r(p) increases, a distance to a subject decreases or a haze-induced distortion decreases. 
     According to an exemplary embodiment of the present invention, a dark channel prior (DCP) is used to estimate the transmission r(p) in Equation 1. However, in a conventional hazy image processing method, since only a DCP is used, there is a problem in that an artifact occurs in a restored image (see  131  in  FIG. 1 ). 
     In order to solve this problem, according to an exemplary embodiment of the present invention, a DCP is used to obtain an estimated transmission ( 120  in  FIG. 1 ), and weighted least square (WLS) filtering is additionally applied to the estimated transmission value to obtain a refined transmission, thereby solving the problem of artifact occurrence. This will be described  1  in detail with reference to  FIG. 2 . 
     As for the DCP, as a method for removing a haze in an image, some pixels in a block of a predetermined size around each pixel of a haze-free clean image are implemented such that at least one color channel has a very small near-zero value. This is represented as Equations 2 and 3. 
     
       
         
           
             
               
                 
                   
                     
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     In Equation 2, J dark  denotes a DCP, J c  denotes an input image of each color channel, and Ω denotes a block (or patch) around a P pixel. 
     When Equation 3 is used, the transmission r(p) in Equation 1 may be modified as Equation 4. 
     
       
         
           
             
               
                 
                   
                     
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     In Equation 4, μ is a constant value of about 0.95. 
     In the conventional art, a transmission {tilde over (r)}(p) in Equation 4 is estimated on a block-by-block basis ( 121 ,  122 ,  123 ) ( 120  in  FIG. 1 ). In this case, there is a problem in that an artifact ( 131 ,  132 ) such as a jaggy phenomenon occurs in a block boundary line when a haze-free image J(p) ( 130  in  FIG. 1 ) is restored. 
     As a scheme for solving this problem, a soft matting algorithm or the like is used. However, since the soft matting algorithm is complex and has a large operation amount, the soft matting algorithm has another problem of the impossibility of processing due to the lack of memory capacity when the image resolution is high. 
     An exemplary embodiment of the present invention provides a scheme for solving the problems of a complex operation and an artifact phenomenon occurring in the hazy image processing process. 
     For this purpose, according to an exemplary embodiment of the present invention, a block artifact may be removed by applying WLS filtering to the transmission {tilde over (r)}(p) obtained in Equation 4, and then brightness control image processing may be performed in a base layer image and image processing for improving a local texture may be performed in a medium layer image and a fine layer image. 
     Hereinafter, in an exemplary embodiment of the present invention, for prevention of a confusion between the terms, {tilde over (r)}(p) denotes an estimated transmission (see  320  in  FIG. 3 ) obtained through Equation 4, and r(p) denotes a refined transmission (see  340  in  FIG. 3 ) obtained finally by WLS filtering. 
       FIG. 2  illustrates a single image haze removing device according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , a single image haze removing device  200  includes an image analyzing unit  210 , a transmission estimating unit  220 , a transmission refining unit  230 , and an image processing unit  250 . 
     The image analyzing unit  210  generates a DCP representing a distance-dependent haze density in an input image including a haze component. A method of generating the DCP refers to Equations 2 and 3. 
     The transmission estimating unit  220  calculates an estimated transmission {tilde over (r)}(p) by applying the DCP generated by the image analyzing unit  210 . A method of calculating the estimated transmission refers to Equation 4. In this case, since the estimated transmission {tilde over (r)}(P) is estimated on a block-by-block basis, a block artifact (see  121 ,  122 , and  123  in  FIG. 1 ) is included therein. 
     The transmission refining unit  230  calculates a refined transmission r(p) by using a morphologically-processed input image ( 330  in  FIG. 3 ) and an estimated transmission value {tilde over (r)}(P) ( 320  in  FIG. 3 ) calculated by the transmission estimating unit  220 . 
     For example, the transmission refining unit  230  calculates the refined transmission r(p) by removing the block artifact included in the estimated transmission by applying WLS filtering to the estimated transmission value {tilde over (r)}(p) calculated by the transmission estimating unit  220 . 
     The transmission refining unit  230  sets an edge value of the morphologically-processed input image ( 330  in  FIG. 3 ) as a weight in the process of applying the WLS filtering. This refers to Equations 5 to 7. 
     Referring to  FIG. 3 , the transmission refining unit  230  calculates a refined transmission r(p) by preserving an edge value of an input image ( 310  in  FIG. 3 ) and removing (smoothing) a block artifact (e.g.,  321  in  FIG. 3 ) generated in the estimated transmission {tilde over (r)}(p), in order to minimize the block artifacts that may be generated when the image is restored by using the estimated transmission {tilde over (r)}(p). 
     For this purpose, in the estimated transmission {tilde over (r)}(p), the edge value of a region corresponding to the edge value (e.g.,  331  in  FIG. 3 ) of the morphologically-processed input image is preserved, and the block artifact of the other region is smooth-processed. 
     In an exemplary embodiment of the present invention, this process is referred to as a WLS filtering process, which uses Equations 5 to 7. 
     The image restoring unit  240  generates a restored image J(p) by removing the haze component in the input image based on the refined transmission value r(p), as represented in Equation 8. 
     The image processing unit  250  performs multi-scale tone manipulation image processing on the restored image J(p). As represented in Equation 10, a multi-scale image is generated by the difference between the images smoothed through a WLS filter. 
     In detail, for improvement of a local contrast of the restored image J(p) obtained in Equation 8, as represented in Equation 9, image processing is performed by dividing the restored image J(p) into a base layer, a medium layer, and a fine layer. 
     The image processing unit  250  is implemented to perform brightness control image processing in the base layer image and perform image processing for improvement of a local texture in the medium layer image and the fine layer image. 
     The image restoring unit  240  and the image processing unit  250  will be described later in detail with reference to  FIG. 6 . 
       FIG. 3  illustrates an estimated transmission image  320  obtained from a hazy input image  310 , a morphologically-processed input image  330 , and a refined transmission image  340  obtained by using the estimated transmission image  320  and the morphologically-processed input image  330 , according to an exemplary embodiment of the present invention. 
     
       
         
           
             
               
                 
                   
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     In Equation 5, p denotes a position of each pixel, r p  denotes a refined transmission ( 340  in  FIG. 3 ), {tilde over (r)}(p) denotes an estimated transmission ( 320  in  FIG. 3 ) calculated in Equation 4, and λ denotes a smoothing control constant. 
     An output image r p  ( 340  in  FIG. 3 ) is further smoothed as a λ value increases. Referring to  FIG. 4 , it may be seen that the output image r p  is further smoothed as the λ value increases. 
     Also, 
                 (       ∂   r       ∂   x       )     p     ,       (       ∂   r       ∂   y       )     p           
denotes a horizontal-axis and vertical-axis first-order differential operator of the output image r p  ( 340  in  FIG. 3 ); and w x,p (h), w y,p (h) denotes a smoothing weight value of the morphologically-processed hazy image ( 330  in  FIG. 3 ), which is represented as Equation 6. Also, the smoothing weight value w x,p (h), w y,p (h) is used to control the smoothing degree of the artifact of the estimated transmission including a block artifact.
 
     The smoothing weight value is applied to an estimated transmission image  320  of  FIG. 3  to preserve a region corresponding to an edge region  331  of a morphologically-processed hazy image  330  and smooth the other region thereof in the estimated transmission image  320 . 
     According to an exemplary embodiment of the present invention, a refined transmission value for minimizing an object function of Equation 5 is obtained. For this purpose, a pixel value difference between the input image (the estimated transmission, {tilde over (r)}(p), and  320  in  FIG. 3 ) and the output image (the refined transmission, r p , and  340  in  FIG. 3 ) is minimized ((r p −{tilde over (r)} p ) 2 ), and the block artifacts generated in an image restoration process is minimized by the estimated transmission by using the edge of the morphologically-processed input image 
     
       
         
           
             
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     In Equation 6, h denotes a morphologically-processed hazy image ( 330  in  FIG. 3 ), and α denotes an edge control constant of the morphologically-processed hazy image h. As α increases, the other region other than the region corresponding to the edge region  331  of the morphologically-processed hazy image  330  is further smoothed. This refers to  FIG. 5 . 
     Also, ε denotes a constant value for preventing a denominator from being divided by 0, and may have a value of about 10 −4 . Also, 
                 δ   ⁢           ⁢   h       δ   ⁢           ⁢   x       ,       δ   ⁢           ⁢   h       δ   ⁢           ⁢   y             
denotes the horizontal-axis and vertical-axis first-order differential absolute value of the morphologically-processed hazy image ( 330  in  FIG. 3 ). By differentiating Equation 5, a refined transmission r is obtained as Equation 7.
 
 r =( I +λ( D   x   T   A   x   D+D   y   T   A   y   D   y )) −1   {tilde over (r)}   [Equation 7]
 
     In Equation 7, Ax, Ay denotes a matrix of the smoothing weight values w x,p (h), w y,p (h); and Dx, Dy denotes a matrix of 
                 (       ∂   r       ∂   x       )     p     ,         (       ∂   r       ∂   y       )     p     .           
In this case, A may be implemented to use a value of the pixel having the greatest pixel value among the upper 0.1% in the DCP value obtained through Equation 2.
 
     When the refined transmission r(p) is obtained through Equations 5 to 7, a haze-free restored image J(p) may be obtained as Equation 8 according to an exemplary embodiment of the present invention. In this case, A may be implemented to use a value of the pixel having the greatest pixel value among the upper 0.1% in the DCP value obtained through Equation 2. This will be described later with reference to  FIG. 6 . 
       FIG. 4  illustrates a restored image when various smoothing control constant values λ are applied to an input image  410  including a haze, according to an exemplary embodiment of the present invention. 
     As the smoothing control constant λ increases in Equation 5, the output image r p  ( 340  in  FIG. 3 ) is further smoothed. An exemplary case of restoring an image by applying Equations 5 to 7 to an input image  410  including a haze is illustrated as an example. In this case, an edge control constant value α=1.2 of a morphologically-processed hazy image is used. 
     In  FIG. 4 , after an estimated transmission  410  is obtained, a refined transmission  430  and a restored image  431  in the case of λ=0.01, a refined transmission  440  and a restored image  441  in the case of λ=0.1, and a refined transmission  450  and a restored image  451  in the case of λ=1 are illustrated. It may be seen that the refined transmission value is further smoothed as the λ value increases. 
       FIG. 5  illustrates a restored image according to a change in an edge control constant α of a morphologically-processed hazy image, according to an exemplary embodiment of the present invention. In  FIG. 5 , a smoothing control constant value λ=1 is used. 
     Herein, α is the edge control constant of the morphologically-processed hazy image. As α increases, the other region other than the region corresponding to the edge region  331  of the morphologically-processed hazy image  330  is further smoothed. 
     After an estimated transmission  520  of an input image  510  is obtained, a refined transmission  530  and a restored image  531  in the case of α=0.01, a refined transmission  540  and a restored image  541  in the case of α=0.1, and a refined transmission  550  and a restored image  551  in the case of α=1 are illustrated. As α increases, the other region other than the region corresponding to the edge region of the morphologically-processed hazy image is further smoothed. 
       FIG. 6  illustrates a restored image of a hazy image and an image-processed final image of the restored image according to an exemplary embodiment of the present invention. 
     When a refined transmission is obtained from a hazy input image  610 , a restored image J(p)  620  is obtained based on Equation 8. 
     
       
         
           
             
               
                 
                   
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                     ⁢ 
                     
                         
                     
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     In Equation 8, r 0  denotes a constant value for preventing a denominator from being divided by 0, which may be implemented to use a value of about 0.1. 
     In order to improve the local contrast of the restored image J(p)  620  obtained in Equation 8, the image processing unit ( 250  in  FIG. 2 ) obtains an output image V  630  by performing image processing as Equation 9 by dividing the restored image J(p) into a base layer, a medium layer, and a fine layer.
 
 Vp=B+S (δ, d   p   1 )+ S (δ, d   p   2 )  [Equation 9]
 
     In Equation 9, V p  denotes an output image, B denotes a base layer image, and S(δ,d p   1 ) and S(δ,d p   2 ) denote a medium layer image and a fine layer image respectively. Also, S( ) denotes a sigmoid function, and δ denotes a boosting factor and represents a reverse transmission. In an exemplary embodiment of the present invention, the reverse transmission may be implemented to use a value modified from 0 to 10. As the boosting factor increases, the texture information survives strongly in the corresponding layer. 
     In Equation 9, d i  denotes an image generated in an i-th step, which is obtained as Equation 10.
 
 d   i   =u   i-1   −u   i   ,i −1,2, . . . , k  ( K : Number of Layers)  [Equation 10]
 
     In an exemplary embodiment of the present invention, K=2 may be set. 
     Brightness control image processing such as gamma correction is performed in the base layer image, and texture improvement is performed in the medium layer image and the fine layer image. For this purpose, a difference value between the images smoothed through a WLS filter is used as represented in Equation 10. 
     In detail, according to an exemplary embodiment of the present invention, in order to improve the texture information and the local contrast of the restored image J(p), image processing is performed to enhance a band-pass signal component in the medium layer and enhance a high-pass signal component in the fine layer. 
       FIG. 7  illustrates a flow diagram of a method for removing a haze in a single image according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 7  together with  FIG. 2 , the image analyzing unit  210  generates a DCP representing a distance-dependent haze density in an input image including a haze component (S 710 ). A method of generating the DCP refers to Equations 2 and 3. 
     The transmission estimating unit  220  calculates an estimated transmission based on the DCP (S 720 ). A method of calculating the estimated transmission refers to Equation 4. The transmission represents the amount of light reaching a photographing device. The estimated transmission calculated by the transmission estimating unit  220  includes a block artifact because it is estimated on a block-by-block basis in Equation 4. 
     Thereafter, the transmission refining unit  230  calculates a refined transmission by applying WLS filtering based on the estimated transmission and morphologically-processed input image information (S 730 ). According to an exemplary embodiment of the present invention, the refined transmission represents information obtained by preserving edge information and removing a block artifact in the estimated transmission. For this purpose, the transmission refining unit  230  performs WLS filtering. By using the edge information of the morphologically-processed input image in a WLS filtering process, the edge information of the morphologically-processed input image is preserved in the estimated transmission, and the other region other than the edge region of the morphologically-processed input image is smooth-processed. 
     Thereafter, the image restoring unit  240  generates a restored image by removing the haze component in the input image based on the refined transmission (S 740 ). In this case, the restored image may be generated by using Equation 1 or 8. 
     Thereafter, the image processing unit  250  generates a final image by performing multi-scale image manipulation image processing on the restored image (S 750  and S 760 ). This refers to Equations 9 and 10. 
     The present invention may also be embodied as computer-readable codes on a computer-readable recording medium. The computer-readable recording medium may be any data storage device that may store data which may be thereafter read by a computer system. Examples of the computer-readable recording medium may include ROMs, RAMs, CD-ROMs, magnetic tapes, floppy disks, and optical data storages. The computer-readable recording medium may also be distributed over network-coupled computer systems so that the computer-readable codes may be stored and executed in a distributed fashion. 
     The exemplary embodiments have been described above with reference to the drawings. The terms herein are only used to describe the present invention and are not intended to limit the scope of the present invention as defined by the following claims. Therefore, those of ordinary skill in the art will understand that various modifications and other equivalent embodiments are possible therefrom. Thus, the scope of the present invention should be defined by the appended claims.