Illuminant Adaptive Wideband Image Synthesis Using Separated Base-Detail Layer Fusion Maps

Abstract

In this study, we present a wideband image synthesis technique for day and night object identification. To synthesize the visible and near-infrared images, a base component and a detailed component are first decomposed using a bilateral filter, and the detailed component is synthesized using a local variance map. In addition, considering the difference in the near-infrared image characteristics between daytime and nighttime, the base components are synthesized using a luminance saturation region map and depth and penetration map using a joint bilateral filter. The proposed method overcomes the partial over- or under-exposure caused by sunlight and infrared auxiliary light, which is experienced variously in wideband imaging, and improves the identification of objects in various indoor and outdoor images compared with that achieved by existing methods by emphasizing detailed components.

1. Introduction

When a mobile camera is used for both indoor and outdoor object identification, the capturing conditions may include bright day and dark night outdoors, daylight and dark indoors, tunnels, basements, alleys, and roads with streetlights and vehicle headlights [1]. To ensure the accuracy and usefulness of object identification, it is necessary to secure sufficient image information in this range of spaces and lighting conditions. Because a near-infrared image has a lower scattering effect due to particles than that of a visible-light image, the boundary contrast is large, and more detailed information can be secured in fog, shade, and distant areas. Near-infrared images do not include color information [2]. Therefore, research on improving visibility using visible-light and near-infrared image synthesis is being actively conducted. Figure 1 shows a resulting image synthesized using visible-light and near-infrared input images. The near-infrared image can greatly improve visibility, which is lacking in the visible-light image, by providing information on regions that visible light cannot reach.

In the case of night image capture, IR LED lighting, which does not have a visual impact, is generally used for image acquisition for security or crime prevention. It can be difficult to adjust the brightness of the LED light according to the environment. Visible and near-infrared image synthesis algorithms proposed by Li and Vanmali et al. synthesized luminance channels using a weight map including local contrast and entropy, and synthesized images through post-processing of sharpening and color compensation [2,3]. However, detailed information of the near-infrared image cannot be reproduced accurately, and the boundary information is not well-defined after color compensation [1].

In this study, we produced an image with excellent color expression and boundary contrast of detailed information through a synthesis method suitable for both day and night environments, and with visible-light and near-infrared images taken temporally separated. At night, multiple images are acquired by repeatedly operating IR illumination at regular unit time intervals, and the synthesis algorithm adaptively adjusts the synthesis ratio. The proposed method can obtain matching of the viewing angles of the visible and near-infrared images and sufficient brightness from time-division image acquisition using IR lighting switching. It has advantages in terms of border contrast. The proposed image processing part consists of the base-detail layer decomposition, the base layer synthesis, the detail layer synthesis, and the color compensation and adjustment. Each part is processed in the LAB color space to preserve the color information of a visible-light image.

The base-detail layer decomposition part decomposes the input image into base and detail layers using the bilateral filter. The bilateral filter smooths the input image while preserving its main edges. The base layer is the bilateral filtered output of the input image, and the detail layer is the difference between the input image and the base layer. The proposed method applies different weight maps to the base image and the detail image to independently synthesize them, then sets the base and detail layers in the luminance channel. The base layer synthesis reduces the local contrast according to the near-infrared distribution characteristics of day and night. The detail layer synthesis acquires detailed information from the near-infrared image that cannot be expressed in the visible-light image. The process is carried out considering detailed information regarding the area where near-infrared rays do not pass depending on the medium. Then the fused luminance image is obtained by adding the synthesized base layer and detail layer. Finally, a resulting image is generated through a color compensation and adjustment step using the ratio of change between the luminance channel of the visible-light image and the fused luminance channel.

Recently, as object detection research using deep learning is actively conducted, the importance of clearly identifying objects in dark areas under the low-light condition is increasing. It is necessary to improve the important information in the images by synthesizing the visible and near-infrared image pairs. The distribution characteristics of infrared light during the day and night are different, and it is necessary to synthesize visible and infrared images adaptive to various environments. Existing methods focus on synthesizing the daytime visible and IR natural light images. However, at night, the multiplexed image synthesis should reflect the light-emitting IR characteristics. Therefore, in this study, we propose a day and night adaptive visible and near-infrared image synthesis method considering near-infrared band characteristics of both daytime and nighttime. The proposed method can be applied to fixed surveillance cameras such as CCTVs and mobile surveillance cameras used in autonomous vehicles.

This paper is organized as follows: Section 2 explains the significance of near-infrared and visible-light image synthesis and previous studies, Section 3 provides a proposed synthesis method. Section 3.1 introduces a visible-light and near-infrared image acquisition camera system, Section 3.2 describes the base-detail layer decomposition process. The base layer and detail layer synthesis processes are addressed in Section 3.3 and 3.4 with a comparison of the daytime and nighttime images. Section 3.5 represents the color compensation step for resolving color distortion and imbalance caused by luminance channel synthesis. Section 4 shows the results of visible-light and near-infrared image synthesis compared to the results of previous studies. Finally, a summary and conclusions are presented in Section 5.

2. Related works

2.1. Synthesis of Visible and Near-Infrared Images

The wavelength band of light that can be visually recognized is 400–800 nm, and the wavelength band of nighttime infrared auxiliary light is approximately 800–1500 nm. Near-infrared rays experience a low degree of scattering effect by particles in the atmosphere; therefore, there is a region with strong object contrast and sharpness. Considering these characteristics, both visible-light and near-infrared images can be captured and the improved images may be fused and reproduced. Visible-light and near-infrared image synthesis measures information visibility primarily through the local contrast of visible and near-infrared images and controls the synthesis ratio so that the boundaries and textures of the entire image are maximally expressed. In particular, it can be used to accurately judge the edge of an object by synthesizing visible and near-infrared images not only at night but also in fog, smoke, or shaded areas during the daytime [1,2,3,4,5].

Representatively, the visible-light and near-infrared image synthesis algorithm using latent low-rank representation (LatRR) decomposes the fusion process into low-rank parts and saliency parts, which presents global and local structure information. After decomposition, both low-rank and saliency parts are fused by weighted-average strategy with different weighting values. Finally, the fused result is obtained by adding fused low-rank and saliency parts [6]. However, the local edge information of the near-infrared image is not well-preserved, and the color expression is not properly presented, especially in nighttime images.

In addition, the synthesis algorithm adopted a multiresolution decomposition method, which uses the Laplacian–Gaussian pyramids to obtain the details of the near-infrared image, and measures local contrast, local entropy, and visibility to create a weight map to synthesize the image, followed by color and sharpness correction to synthesize the resulting image [2]. However, the details of the near-infrared image cannot be conveyed reliably during synthesis, and the color information is unnaturally distorted by excessive color compensation and boundary enhancement filters. A method using principal component analysis (PCA) has been proposed to transfer the details more effectively from the near-infrared image and to preserve better color chrominance. A weight map for the detail layer is created from the PCA analysis of the visible and near-infrared scenes including the PCA-skipping mask filter [7]. In addition, the enhanced fusion method for the base layer synthesis proposed the local background tone compression to emphasize the detail layer [8]. However, for night images, the algorithm fails to resolve the color distortion and block noise problems are caused by excessive tone scaling.

2.2. Joint Bilateral filter

In this study, a joint bilateral filter was used to detect luminance saturation. The bilateral filter, which is a low-pass filter with boundary-preservation characteristics, has excellent performance and is used in many fields. The bilateral filter uses two Gaussian functions for each distance difference and intensity difference from adjacent pixels [9]. The bilateral filter (B) is obtained as follows:

$$\begin{gathered} B\left[I\left(i,j\right)\right]=\frac{1}{W_p}{\displaystyle \sum }_{y=-hw}^{hw}{\displaystyle \sum }_{x=-hw}^{hw}I\left(i+x,j+y\right)g_r\left(\Vert I\left(i+x,j+y\right)-I\left(i,j\right)\Vert \right)g_d\left(\Vert x,y\Vert \right) \\ {W}_p={\displaystyle \sum }_{y=-hw}^{hw}{\displaystyle \sum }_{x=-hw}^{hw}{g}_r\left(\Vert I\left(i+x,j+y\right)-I\left(i,j\right)\Vert \right)g_d\left(\Vert x,y\Vert \right) \end{gathered}$$

(1)

$$\begin{gathered} g_r\left(x\right)=\exp \left(\frac{-x^2}{2{\sigma }_r{}^2}\right) \\ {g}_d\left(x\right)=\exp \left(\frac{-x^2}{2{\sigma }_d{}^2}\right) \end{gathered}$$

(2)

where $I\left(i,j\right)$ is the input image, $hw$ is the kernel size,$g_r$ is a Gaussian function for boundary preservation that adjusts the blur weight by color difference with adjacent pixels, and $g_d$ is a Gaussian function that adjusts the blur weight according to the distance difference from adjacent pixels in the spatial domain. In this case, the widths of the Gaussian functions $g_r$ and $g_d$ are determined by the standard deviations ${\sigma }_r$ and ${\sigma }_d$, respectively. $W_p$ stands for the normalization term.

However, the bilateral filter has limitations in denoising because it uses a single image. Eisemann and Petschnigg et al. used an additional flash image with a strong signal-to-noise ratio (SNR) adjacent to the existing two Gaussian functions. A joint bilateral filter that applies a term to the displacement difference with a pixel has been proposed [10,11,12]. This joint bilateral filter receives two images and applies the filter to the image by adjusting the blurring weight using the boundary information of the remaining image in which the boundary information is relatively well-preserved [13]. The joint bilateral filter was obtained as follows:

$$\begin{gathered} JB\left[I\left(i,j\right)\right]=\frac{1}{W_p}{\displaystyle \sum }_{y=-hw}^{hw}{\displaystyle \sum }_{x=-hw}^{hw}I\left(i+x,j+y\right)g_r\left(\Vert F\left(i+x,j+y\right)-F\left(i,j\right)\Vert \right)g_d\left(\Vert x,y\Vert \right) \\ {W}_p={\displaystyle \sum }_{y=-hw}^{hw}{\displaystyle \sum }_{x=-hw}^{hw}{g}_r\left(\Vert F\left(i+x,j+y\right)-F\left(i,j\right)\Vert \right)g_d\left(\Vert x,y\Vert \right) \end{gathered}$$

(3)

where $g_r\left(\Vert F\left(i+x,j+y\right)-F\left(i,j\right)\Vert \right)$, which adjusts the blur weight according to the color difference between adjacent pixels, uses the intensity difference of image $F$ instead of that of image $I$, unlike the bilateral filter. Image $F$ is an image in which the boundary information is relatively well-preserved.

3. Proposed method

Figure 2 shows the visible and near-infrared light image synthesis algorithm proposed in this study. The proposed method is composed of an image pair acquisition system, base-detail decomposition, base layer synthesis, detail layer synthesis, and color compensation. Image acquisition describes a method for acquiring visible and near-infrared image pairs, and decomposes each image into a base layer and a detail layer during the base-detail decomposition process. After that, each layer goes through the synthesis process independently, which is conducted in the base layer synthesis and the detail layer synthesis. The base layer is synthesized by adopting a different synthesis method for the day and night images using the joint bilateral filter and the near-infrared light penetration characteristics according to day- and nighttime images. Detail layer is synthesized using weight map generated by the image variance. Then, the fused luminance image is obtained by adding the synthesized base layer and detail layer. Finally, the final result can be derived through a color compensation step according to the luminance channel value changed in the synthesis process. In Figure 2, each subpart of Chapter 3 is separated by blocks of different colors.

3.1. Single Camera Module Time-Division Image Acquisition System

In this study, daytime images were obtained by temporally multiplexed capturing using an IR cut filer, and in the case of night images, a separately manufactured imaging device was used. A time-division image acquisition system using a single CMOS broadband camera module was applied to obtain visible and near-infrared images, thereby solving the problem of matching errors due to the optical path difference occurring in a dual camera system. However, if a time-sharing system is used, motion blur may occur because of a time-shifting error. To address this issue, different exposure times were set for the visible-light image and the near-infrared image, and when the amount of light was insufficient owing to a short exposure time, the amount of light was secured by using the IR auxiliary light. The decision to use the IR auxiliary light was made according to the surrounding environment.

The time-shifting error and motion blur both increase if the exposure time is increased to secure sufficient light while acquiring low-light images with minimized exposure time. The increased noise is unfavorable for high-quality image acquisition. Therefore, an image is obtained with minimized time-shifting error, motion blur, and noise by supplying a sufficient amount of light in a short time using the IR auxiliary light. In the case of daytime images, which experience luminance saturation when using IR-assisted light, images are acquired while limiting the use of IR-assisted light [11].

When acquiring visible-light and near-infrared images, it is difficult to adaptively adjust the brightness of the lighting according to environmental changes. Therefore, in this study, we proposed a time-division image acquisition system using a camera trigger signal and a flash-type IR auxiliary light switching signal, as shown in the flowchart in Figure 3. As shown in Figure 4b,c, by repeatedly operating the auxiliary light with the IR auxiliary light switching signal, a sufficient amount of light is first secured in the near-infrared image using a fixed IR flash of sufficient brightness, and the luminance channel visibility is then secured by adjusting the synthesis ratio with the visible-light image for the saturated area.

3.2. Base-Detail Layer Decomposition

For image processing of separate layers for the acquired images, first the luminance component to be synthesized is separated from the two input images through LAB color space conversion. An image in the LAB color space has an advantage in that the color information and luminance information of the image can be independently processed because the color difference and luminance channels are separated. Next, a bilateral filter is applied to decompose the two images into a base layer and a detail layer. The bilateral filter preserves the strong boundary components and blurs the detai to obtain a base layer image. In addition, the detail layer is obtained from the difference between the resulting image and the luminance channel image [13]. The image decomposition using a bilateral filter is shown in Figure 5.

$$L_B\left(i,j\right)=B\left[L\left(i,j\right)\right]$$

(4)

$$L_D\left(i,j\right)=L\left(i,j\right)-B\left[L\left(i,j\right)\right]$$

(5)

where $L_B$ is a base layer image obtained by applying a bilateral filter to the LAB color space luminance image $L$, and $L_D$. is a sublayer image obtained from the luminance image and base layer image. The kernel size of the bilateral filter for base-detail decomposition acts as the most critical factor that distinguishes the base and detail in the image. The larger the kernel size, the bigger the details that are included in the detail layer. In this study, the (21, 21) kernel was applied for decomposition.

3.3. Base Layer Synthesis

The base layers of the separated visible-light and near-infrared images are synthesized by forming different weight maps according to the infrared natural and auxiliary light characteristics. In the case of daytime images, the intensity and diffusion of IR natural light in the near-infrared images are secured compared to visible images in distant, fog, and shade areas. In contrast, at night, IR auxiliary light is used to compensate for the low-light conditions, long-range IR infiltration is weak in the nighttime infrared images, and the near-distance luminance saturation is relatively strong due to the IR flash.

The main purpose of base layer synthesis is local background tone compression through synthetic ratio control of visible-light images and near-infrared images. This compresses the local tone range of the base layer within the limited luminance range of the display, thereby improving the expression performance of the detailed layer and enhancing the clarity. Therefore, in low-light areas, such as backlight and shade, which inhibit visibility, the synthesis ratio of an infrared image with a relatively large luminance value must be increased, and in a high-light area, such as the anterior saturation area caused by night auxiliary lights, the ratio of visible images with a relatively small luminance value should be increased.

As shown in Figure 6a,b, the left-side visible-light luminance image has low-level shading information on average, and the right-side near-infrared luminance image is bright on average and has many saturated regions owing to the extended band component. The image synthesis ratio needs to be adjusted for these regions. A bilateral filter with edge preservation is used to divide local regions according to strong boundary components and compress tones from the synthetic map, from which details are removed. The structural formula of the bilateral filter used in each synthesis map and the VIS-IR joint bilateral filter blurred by the boundary information of the near-infrared image is as follows:

$$MA{P}_B\left(i,j\right)=N\left[B\left[L_{VIS}\left(i,j\right)\right]\right]$$

(6)

$$MA{P}_{JB}\left(i,j\right)=N\left[J{B}_{L_{IR}}\left[L_{VIS}\left(i,j\right)\right]\right]$$

(7)

$$N\left[I\left(i,j\right)\right]=\frac{I\left(i,j\right)-\min \left(I\left(i,j\right)\right)}{\max \left(I\left(i,j\right)\right)-\min \left(I\left(i,j\right)\right)}$$

(8)

where $MA{P}_B$ is a bilateral filter map, which is an image of applying a bilateral filter by substituting visible-light luminance images $L_{VIS}$ into the input image I in Equation (1). In addition, in Equation (2), $MA{P}_{JB}$ is a joint bilateral filter map obtained by substituting a visible-light luminance image $L_{VIS}$ into the input image I and a near-infrared luminance image into the input image $F$ to determine the blur weight using boundary information. $N$ is a min–max normalization function that normalizes the image to a value between 0 and 1.

However, when the two images are synthesized using a bilateral filter map to preserve the strong boundaries of the object, the results include distorted objects because the object details are reflected in the synthetic map, as shown in Figure 6c. In the low-brightness areas of visible light, the synthetic ratio of near-infrared images is increased, and the high-brightness areas of near-infrared images are excessively synthesized in dark letters. When creating a synthetic map using a joint bilateral filter, the strength of near-infrared images is reflected in the visible-light low-brightness areas, which increases the ratio of near-infrared images. Therefore, the VIS-IR joint bilateral filter, which includes the boundary information of the near-infrared image of Equation (7), was used.

Figure 6d shows the synthesis results using the joint bilateral filter map. It is confirmed that the internal area of the object of the synthetic map is blurred according to the strength of the neighboring pixel image of the near-infrared image, and the details of the information are well-synthesized as the local tone increases in the resulting image. Figure 7 shows the average synthesis results of the visible-light and near-infrared luminance base layer for comparison with the synthesis results of the visible-light and near-infrared image base layer.

Next, synthetic maps were added to reflect the characteristics of the nearest infrared images that appear differently between day and night, along with the joint bilateral filter. In the daytime visible-light image, a low-brightness area that inhibits visibility due to shade or backlight, such as the center of the tree in Figure 8b, is generated, and the boundary information is not expressed. In contrast, in the daytime near-infrared image, light penetration and diffusion phenomena caused by IR natural light in the tree shade area, as shown in Figure 8c, secures a large amount of light compared to visible light and increases visibility, including boundary information. The characteristics of this daytime near-infrared image are visible through the car image of visible-light and near-infrared rays in Figure 8d, and it can be seen that a large difference occurs in the area where IR penetration occurs.

In the nighttime visible-light image, the border information is weak for shaded areas, such as the box in Figure 9b, and has a low luminance value on average. In contrast, in the nighttime near-infrared Figure 9c, IR auxiliary light complements the lack of light in the box and secures the boundary information of the color chart in the box. However, unlike natural light, auxiliary light is weak in long-range IR infiltration and diffusion, and short-range luminance saturation caused by the front filter of the IR flash is relatively strong. This can be confirmed by the characteristics of the night image using the auxiliary light and the change in the luminance in the cup area where saturation occurs when comparing the box and the cup area in Figure 9d. Therefore, brighter daytime visibility and near-infrared car images reflect IR light penetration, and brighter visible-light and near-infrared car images show anterior luminance saturation by IR auxiliary light. To synthesize the characteristics of these day and night images, an IR depth compensation map was applied at night, and a penetration compensation map was applied during the day.

Because the night near-infrared image uses the IR assist light from the front, the difference between the visible-light image and the luminance value is oversaturated in the objects closer to the camera and shows the characteristics of the depth map, where the difference becomes weaker as the distance increases. Therefore, in the near-infrared image, it is necessary to increase the synthesis ratio of the non-saturated visible-light image so that the boundary information is well-preserved in the forward saturation region owing to the auxiliary light projection. In this case, the halo phenomenon that may occur during local tone mapping is minimized by using the difference image of the base layer image to which the bilateral filter is applied. The depth compensation map is obtained as follows:

$$MA{P}_{dep}\left(i,j\right)=N\left[L_{B_{IR}}\left(i,j\right)-L_{B_{VIS}}\left(i,j\right)\right]$$

(9)

where, as the value of the depth compensation map $MA{P}_{dep}$ increases, it becomes a near-saturation region and increases the ratio of the visible-light base layer. $L_{B_{VIS}}$ and $L_{B_{IR}}$ are the base layer images of the visible and near-infrared images, respectively.

Unlike nighttime images using auxiliary light, light diffusion and penetration by near-infrared natural light occur in daytime images. A visible-light image may have a high brightness level in the distant region, and an absolute value is used for the difference between visible light and near-infrared light. Because the regions with a large difference value are ones in which visibility can be improved by IR light penetration, it is necessary to increase the synthesis weight of the near-infrared image to secure boundary information. However, in the case of the daytime image, because the average luminance of the bright region of the visible-light image is higher than that of the night image, synthesis saturation may occur in the high-luminance region of the visible-light image. Therefore, the daytime base layer synthesis increases the ratio of the near-infrared base layer image as the difference value in the visible-light and near-infrared difference images increases, but this should be limitedly applied to the low-luminance region of the visible light image. The penetration compensation map is obtained as follows:

$$MA{P}_{pen}\left(i,j\right)=N\left[abs\left(L_{B_{VIS}}\left(i,j\right)-L_{B_{IR}}\left(i,j\right)\right)\right]\times \left(1-N\left[L_{B_{VIS}}\left(i,j\right)\right]\right)$$

(10)

where, as the penetration compensation map $MA{P}_{pen}$ value increases, it becomes a near-infrared penetration compensation region and increases the ratio of the base layer of the near-infrared image. The $abs\left(L_{B_{VIS}}\left(i,j\right)-L_{B_{IR}}\left(i,j\right)\right)$ term indicates the IR light penetration region, and the $\left(1-N\left[L_{B_{VIS}}\left(i,j\right)\right]\right)$ term is a function that limits the low-luminance region of the visible-light image. Using these two characteristics as an AND condition, the ratio of the near-infrared image was increased.

Using the previously obtained VIS-IR joint bilateral filter and the additional compensation map according to the day and night images, the final base layer synthesis map and synthesized image can be obtained as follows:

$$MA{P}_{base}=\left\{\begin{array}{c}\frac{MA{P}_{JB}+MA{P}_{dep}}{2} ,\\ \frac{MA{P}_{JB}+\left(1-MA{P}_{pen}\right)}{2} ,\end{array}\begin{array}{c}if night mode\\ \begin{array}{c}\\ if day mode \end{array}\end{array}\right.$$

(11)

$$L_{base }=MA{P}_{base} L_{B_{VIS}}+\left(1-MA{P}_{base}\right) L_{B_{IR}}$$

(12)

where the larger the value of the base layer synthesis map $MA{P}_{base}$, the higher the visible light base layer synthesis ratio. $L_{base }$ is the luminance base layer synthesis result image, and the base layers of the day and night images generated above are synthesized using the alpha blending method with the base layer synthesis map as a weight.

3.4. Detail Layer Synthesis

In the synthesis of detail layers, the main goal of synthesizing sublayers is to include and synthesize as much boundary information as possible according to the activity of the visible and near-infrared images. In the visible-light image, boundary information is lost in distant, foggy, or shaded areas. In the near-infrared image, the luminance saturated area due to the auxiliary light, or the boundary information below the water surface through which the near-infrared light does not pass, cannot be expressed. Therefore, for synthesis, the activity of detailed layers of visible-light and near-infrared images is calculated through local variance, and a variance map that can adjust the synthesis ratio according to the difference is used.

In this study, the mean deviated local variance, which has low computational complexity, was used to measure the activity of images. Because the mean deviated local variance uses the difference from the mean values of the pixels around the central pixel without considering each pixel value of the kernel, the distribution of changes in the pixel values in the kernel can be well-represented. The mean deviation local variance is calculated as follows:

$$V\left(i,j\right)=\frac{1}{N}{\displaystyle \sum }_{\left(i,j\right)\in \eta }{\left(I\left(x, y\right)-\mu \left(i,j\right)\right)}^2$$

(13)

$$\mu \left(i,j\right)=\frac{1}{N}{\displaystyle \sum }_{\left(i,j\right)\in \eta }I\left(x,y\right)$$

(14)

where $V$ is the mean deviated local variance, and $\mu$ is the local mean. $\eta$ is the size of the kernel, and a value of $\eta =5$ was used in the proposed method.

The local variance image contains detailed boundary information for each visible-light and near-infrared image. Because the square term of the luminance channel is included in the calculation of the local variance, the variance may have a value of 0 to ${255}^2$. In this case, because the range of values is very wide, the min–max normalization method is used, and the case where the variance value is small can be ignored. Therefore, we use a log function to adjust the scale. The difference image between the two dispersed images is then obtained and used as an index for determining which image among the visible-light and near-infrared images contains more detailed information in a specific area. The following equation shows the local variance log-scale difference images of the visible and near-infrared images.

$$Va{r}_{diff}\left(i,j\right)=\log \left(\frac{V_{VIS}\left(i,j\right)+1}{V_{IR}\left(i,j\right)+1}\right)$$

(15)

where $Va{r}_{diff}$ represents the log-scale local variance difference image. $V_{VIS}$ and $V_{IR}$ are the mean deviated local variance images of the visible and near-infrared regions, respectively.

The log-scale difference image including negative values is then converted to a value between 0 and 1 and normalized so that it can be used as a synthesis map. If the local variance difference image ($Va{r}_{diff}$) is negative, it indicates an area with a relatively large amount of detail in the near-infrared image, and if it is positive, it indicates an area with a relatively large amount of detailed information in the visible-light image. For this purpose, when the variance difference image is negative, it is normalized to a value between 0 and 0.5, and when it is positive, it is normalized to a value between 0.5 and 1.

$$MA{P}_{var}=\left\{\begin{array}{c}0.5\left(\frac{Va{r}_{diff}}{\max \left(Va{r}_{diff}\right)}+1\right) ,\\ -0.5\left(\frac{Va{r}_{diff}}{\min \left(Va{r}_{diff}\right)}-1\right) ,\end{array}\begin{array}{c}for Va{r}_{diff}\ge 0\\ \begin{array}{c}\\ for Va{r}_{diff}<0\end{array}\end{array}\right.$$

(16)

where $MA{P}_{var}$ is a distributed map in which $Va{r}_{diff}$ has a value between 0 and 1, and the proposed regularization expression is applied.

A final detailed layer composite map is created by applying a blur function to the variance map, as follows:

$$MA{P}_{detail}=G\left[MA{P}_{var}\right]$$

(17)

$$G\left[I\left(i,j\right)\right]=I\left(i,j\right){\displaystyle \sum }_{y=-hw}^{hw}{\displaystyle \sum }_{x=-hw}^{hw}\exp \left(-\left(\frac{{\left(i+x\right)}^2}{2{\sigma }_x{}^2}+\frac{{\left(i+y\right)}^2}{2{\sigma }_y{}^2}\right)\right)$$

(18)

where the $G$ function represents a two-dimensional Gaussian filter and ${\sigma }_x$, ${\sigma }_y$ are the standard deviations in the x and y directions, respectively. $MA{P}_{detail}$ is a detailed layer synthesis map that is blurred by applying a Gaussian filter to $MA{P}_{var}$ for natural synthesis. The larger the kernel size of Gaussian function, the more natural detail layer fusion is possible, but a tradeoff occurs due to a decrease in detail contrast and an increase in the amount of computation. Thus, in this study, the kernel size (7, 7) was applied for this Gaussian filter.

Using the generated detailed layer synthesis map ($MA{P}_{detail}$), the synthesis is performed as shown in the following equation:

$$L_{detail}=MA{P}_{detail}{L}_{D_{VIS}}+\left(1-MA{P}_{detail}\right)L_{D_{IR}}$$

(19)

where $L_{detail}$ is the luminance detailed layer synthesis result, and the detailed layers of the day and night images generated above are synthesized using the alpha blending method using the detailed layer synthesis map as a weight. $L_{D_{VIS}}$ and $L_{D_{IR}}$ are detail layer images of visible and near-infrared images, respectively.

Figure 10 shows the sublayers of the visible and near-infrared images and the results of the synthesis of the sublayers. The details of the cup and box parts are well-preserved.

The final synthesized luminance channel $L_{fused}$ is generated as follows by synthesizing the independently synthesized base layer and detailed layer using the above process.

$$L_{fused}=L_{base}+L_{detail}$$

(20)

3.5. Color Compensation and Adjustment

In general, color compensation is required to maintain the same color information when the luminance of an image is changed. The color information uses the ab channel of the visible-light image. By synthesizing the luminance channels of the two images, the luminance channel L of the existing visible-light image is changed, resulting in an imbalance with the color channel ab, making the color expression unnatural. Therefore, for the color channels ($a_{VIS}, b_{VIS})$ of the visible-light image, saturation compensation using the ratio of the luminance channel ($L_{vis}$) of the visible-light image to the luminance ($L_{fused}$) after synthesis is required, and the saturation compensation function is calculated as follows:

$$\begin{gathered} CC=MA{P}_{base}{\left(\frac{L_{fused}}{L_{vis}+0.001}\right)}^{\gamma }+\left(1-MA{P}_{base}\right) \\ a\prime =\left(a_{VIS}-128\right)\times CC+128 \end{gathered}$$

(21)

$$b\prime =\left(b_{VIS}-128\right)\times CC+128$$

(22)

where $CC$ is the chroma compensation ratio, and the chroma compensation is changed according to the change range of the luminance value through the change ratio between the visible-light image and the luminance synthesis result. At this time, 0.001 is added to the denominator to prevent divergence of the $CC$ compensation gain. In addition, the blending process is performed through the base layer synthesis map to prevent excessive saturation compensation at the boundary of the image. $\gamma$ is a constant value that controls the amount of color compensation, and in this study, $\gamma =0.6$ is used.

In Equation (21), $a$ and $b$ are color channels for which saturation is compensated, and $a_{VIS}$ and $b_{VIS}$ are color channels before saturation compensation of a visible-light image. $a_{VIS}$ and $b_{VIS}$ are multiplied by $CC$ to perform saturation compensation for luminance modulation. At this time, to make the median of the chroma channel range 0, 128 is subtracted, values are multiplied by $CC$, and 128 is added again to obtain a chroma channel with a median of 128 that is uniformly distributed. Using the obtained chroma $a$ and $b$ and the synthesized luminance $L_{fused}$, the final synthesized image is acquired through a color-space conversion process.

4. Simulation Results

To evaluate the performance of the proposed algorithm, the synthesis results using various synthesis maps were obtained and compared. In addition, the results obtained using the multilayer resolution synthesis method, which is a representative conventional visible-light near-infrared synthesis method, and a deep-learning-based synthesis method were compared. The daytime images used in the experiment were taken by alternately applying an IR cut filter and a VIS cut filter, and the dataset published by EPFL was used [14]. Therefore, the input images were acquired as visible-light and infrared images.

For the night image, an image captured by switching the IR auxiliary light with the IR cut filter removed using a broadband camera module with the image sensor light receiving band ranging from visible to near-infrared (approximately 1000 nm) was used. In this paper, a multiplex imaging system was fabricated to capture IR and visible-light images at night. The proposed system captures two types of IR images, high-intensity IR and low-intensity IR by alternating the intensity of the IR lamp. The IR lamps are synchronized with the capturing period. In addition to the IR LED array of CCTV at night, four 5W-1000 nm IR LEDs are used for IR flash, and are turned on only when capturing high-intensity images. When the IR flash is on, the luminance channel image is taken, and when the IR flash is off, the RGB 3 channel image is taken.

Figure 11 shows the sequence of NIR and visible image switching capturing at night. The input image was acquired as a visible-light image and a broadband image. A broadband image refers to an image in which visible-light and near-infrared images are combined, and because the color channel of the broadband image is not used in this study, it is used synonymously with the near-infrared image.

Figure 12 shows a comparison of the performance of the synthesis results according to the synthesis maps of the various base layers. It shows the resulting image using the bilateral filter map, joint bilateral filter map, and proposed synthesis map. As can be seen from the figure, the resulting image using the proposed base layer synthesis map contains detailed information inside the cup and box.

Figure 13, Figure 14 and Figure 15 show the synthesis results for the daytime images. Compared with the Densfuse synthesis method, it is confirmed that the proposed method has excellent visibility of detailed information in the area because the luminance range of the base layer is well-compressed in the shaded area of the visible-light image. In addition, in the Vanmali and PCA synthesis method, colors are shown differently than in the original image, such as a strong red color, over-saturation symptoms, and mismatched white balance throughout the image, including buildings and roofs. In contrast, in the proposed method, the original color of the visible-light image is naturally expressed without oversaturation. In addition, it can be confirmed that the proposed method expresses detailed information better than the two existing synthesis methods in the details of the mountain in Figure 13, the boundary information of the leaves in Figure 14, and the details of the wave in Figure 15.

Figure 16, Figure 17 and Figure 18 show a performance comparison for the night images. In the night image, compared to the existing Densfuse synthesis method, the proposed method improves visibility by increasing the brightness in the dark area of the visible-light image, such as the area inside the box in Figure 16 or the area outside the door in Figure 18. It can be confirmed that detailed information is better expressed in the letters on the cup in Figure 16 or in the text in the book in Figure 17 Thus, it can be confirmed that the identification is excellent. In addition, in the Vanmali and PCA synthesis method, block noise is generated by oversaturation compensation in the dark areas of the visible-light image; thus, the identification is reduced throughout the image. However, compared with the previous two methods, the proposed method shows excellent detail expression performance while expressing the natural color by revealing details without oversaturation symptoms and noise.

Figure 19 shows the resulting images according to the synthesis method.

Table 1. Comparison of the S3 image quality metrics.

	Half Mixed	Low Rank	Dense Fusion	Vanmali	PCA Fusion	Proposed
1	0.174178	0.181196	0.197622	0.233407	0.289241	0.298935
2	0.199256	0.209302	0.204024	0.242589	0.256526	0.324236
3	0.267765	0.293025	0.286023	0.312739	0.361716	0.442621
4	0.141611	0.153234	0.151603	0.184209	0.290325	0.246327
5	0.086545	0.099137	0.102088	0.118752	0.271392	0.146197
6	0.295408	0.313109	0.295229	0.326159	0.296745	0.450269
7	0.121372	0.198892	0.120727	0.191929	0.162646	0.197848
8	0.091421	0.149966	0.089868	0.134533	0.12752	0.149905
9	0.089133	0.148288	0.088384	0.141575	0.149946	0.139303
10	0.078376	0.156636	0.077333	0.136656	0.175766	0.129637
11	0.057808	0.1386	0.05744	0.16435	0.136583	0.126464
12	0.037551	0.131233	0.037351	0.13783	0.139274	0.088884
13	0.061953	0.141537	0.060919	0.150945	0.151045	0.110261
Average	0.130952	0.178012	0.136047	0.190436	0.216056	0.219299

,

Table 2. Comparison of the VIFF metrics.

	Half Mixed	Low Rank	Dense Fusion	Vanmali	PCA Fusion	Proposed
1	0.745553	0.70428	0.891771	0.747645	0.998658	0.993871
2	0.649207	0.67164	0.673752	0.694098	0.681961	0.853196
3	0.590613	0.590075	0.668474	0.673026	0.833315	0.923921
4	0.571183	0.628726	0.635068	0.799879	0.743241	0.936521
5	0.486169	0.430399	0.63737	0.580149	0.665135	0.711031
6	0.705222	0.741236	0.705064	0.760859	0.718529	0.911802
7	0.827273	0.738855	0.822458	0.869483	0.842554	0.958695
8	0.851614	0.786688	0.836366	0.856757	0.829625	0.940923
9	0.803353	0.710701	0.793571	0.829319	0.809903	0.907141
10	0.778532	0.66576	0.768575	0.745719	0.812205	0.871855
11	0.68633	0.637163	0.68143	0.808235	0.719192	0.814258
12	0.694986	0.57363	0.691165	0.690373	0.768138	0.802015
13	0.663064	0.645432	0.653592	0.752853	0.692664	0.799822
Average	0.696392	0.655737	0.727589	0.754492	0.778086	0.87885

,

Table 3. Comparison of the spatial frequency metrics.

	Half Mixed	Low Rank	Dense Fusion	Vanmali	PCA Fusion	Proposed
1	12.54686	14.14278	13.94147	15.9371	19.472	24.33865
2	12.4484	15.05581	12.68151	16.60634	17.24116	23.4287
3	17.34853	20.25724	18.68782	21.34845	26.16536	36.68218
4	10.14903	11.66165	10.66109	13.74866	19.43001	21.46389
5	7.238538	9.097301	8.040366	10.16365	18.85362	13.88883
6	20.75647	24.89999	20.74539	23.60679	21.74731	38.34033
7	16.09507	19.39597	15.99705	20.85826	18.50416	25.57275
8	13.67963	17.51154	13.48037	15.63094	16.24731	20.72835
9	12.45933	15.2962	12.35908	14.98069	14.04758	18.13125
10	14.35344	19.68436	14.16031	17.66891	20.58637	21.32011
11	11.42254	16.26538	11.313	16.53255	17.70956	18.81093
12	9.273672	16.03667	9.219986	13.74854	19.35755	13.9302
13	12.02791	20.40927	11.83664	17.52521	21.03067	19.64098
Average	13.06149	16.90109	13.31724	16.79662	19.26097	22.79055

,

Table 4. Comparison of the LPC SI metrics.

	Half Mixed	Low Rank	Dense Fusion	Vanmali	PCA Fusion	Proposed
1	0.955372	0.964734	0.960554	0.95496	0.960093	0.963412
2	0.951973	0.96309	0.952825	0.96345	0.962063	0.955613
3	0.940863	0.954856	0.945167	0.94834	0.94615	0.961987
4	0.933977	0.95032	0.937741	0.937994	0.940312	0.958902
5	0.912469	0.906715	0.922604	0.928372	0.937174	0.954875
6	0.947806	0.955256	0.94787	0.947966	0.947871	0.964539
7	0.963834	0.962881	0.96352	0.970573	0.96687	0.973125
8	0.956019	0.962156	0.955143	0.955262	0.952006	0.968308
9	0.956409	0.961442	0.95589	0.957297	0.955774	0.966371
10	0.973693	0.966751	0.973387	0.972475	0.972709	0.982896
11	0.958526	0.964662	0.958383	0.969902	0.968848	0.978657
12	0.95942	0.959802	0.95985	0.957279	0.94973	0.958169
13	0.955009	0.965001	0.954265	0.957888	0.950951	0.966305
Average	0.951182	0.956744	0.952861	0.95552	0.954658	0.965628

,

Table 5. Comparison of the edge intensity metrics.

	Half Mixed	Low Rank	Dense Fusion	Vanmali	PCA Fusion	Proposed
1	34.9795	38.2708	38.66148	42.32002	49.30071	64.73625
2	43.57379	51.91313	44.37411	55.37315	56.45746	77.99232
3	60.13544	67.86473	64.65341	73.72576	88.08473	121.8725
4	32.05627	34.73319	33.67106	44.62118	53.37883	65.84561
5	23.02605	26.08361	25.7862	30.6574	46.29512	41.06304
6	70.21422	79.1646	70.18694	81.35604	76.28592	127.8647
7	49.07052	61.13808	48.81656	63.21881	58.45182	73.80415
8	46.28894	57.17705	45.60989	53.38763	53.79862	66.33241
9	42.9718	53.02003	42.60357	53.82957	49.14437	58.22496
10	37.33375	51.14391	36.75772	47.23395	59.97219	58.27842
11	30.13547	44.43486	29.98567	51.42781	49.36001	50.71271
12	25.76822	42.30395	25.8431	42.59128	53.73518	41.73756
13	32.84225	52.98035	32.29235	51.9274	58.67464	56.20844
Average	40.64586	50.78679	41.48016	53.20538	57.91843	69.59024

and

Table 6. Comparison of the average gradient metrics.

	Half Mixed	Low Rank	Dense Fusion	Vanmali	PCA Fusion	Proposed
1	3.880816	4.153757	4.299817	5.010667	6.093581	7.418859
2	4.516674	5.255388	4.598725	5.873233	6.03323	8.274568
3	6.478177	7.300047	6.966816	7.961987	9.733417	13.46038
4	3.322676	3.596682	3.48355	4.629657	6.418822	6.852565
5	2.443045	2.811208	2.712147	3.256962	5.96494	4.292975
6	7.359794	8.171345	7.356862	8.552003	7.804351	13.56777
7	4.66935	5.926646	4.645744	6.104861	5.591356	6.992199
8	4.355526	5.472404	4.293732	5.063539	5.076561	6.208429
9	4.132718	5.207001	4.098049	5.251618	4.762244	5.555363
10	3.414311	4.782535	3.362169	4.379015	5.719537	5.342915
11	2.798207	4.212974	2.785703	4.854306	4.732941	4.743542
12	2.377223	4.019897	2.387551	4.012972	5.201723	3.872744
13	3.028285	5.025304	2.978305	4.864133	5.689062	5.233637
Average	4.059754	5.071938	4.151475	5.370381	6.063213	7.062765

and Figure 20 show the quality and sharpness scores of the resulting images according to the performance indicators. For quantitative evaluation, indices that evaluate the image quality and sharpness S3 (spectral and spatial sharpness), visual information fidelity for fusion (VIFF), spatial frequency (SF), local phase coherence sharpness index (LPC-SI), edge intensity (EI), and average gradient (AG) were used.

Spectral and spatial sharpness (S3) generates a sharpness map considering human cognitive characteristics, and a larger local value indicates a visually sharper area. This is used to evaluate the quality and sharpness of the images in general [15]. Visual information fidelity for fusion (VIFF) is a quality metric that measures the effective visual information of fusion in all blocks in each sub-band. This is obtained by using the VIF models to gather visual information from the source and fused image pair [16]. Spatial frequency (SF) is used to observe the distribution of gradients throughout the image by calculating vertical and horizontal gradients and to evaluate the amount of detail and texture that can be perceived by humans [17]. LPC-SI is an image sharpness evaluation index that uses the fact that strong local phase coherence occurs in objects or regions that visually stand out in the complex wavelet transform domain [18]. Edge intensity (EI) is an image evaluation method based on the idea that high edge intensity values are superior in terms of image quality and sharpness [19]. The average gradient (AG) metric quantifies the gradient information in the fused image and reveals the details and textures [20]. Regarding the evaluation indicators S3, VIFF, SF, LPC-SI, EI, and AG used in this evaluation, the higher the performance evaluation value, the better the performance in terms of image quality and sharpness.

The proposed synthesis method shows, on average, fine improvements of 1.48% for S3, 11.47% for VIFF, 15.49% for SF, 0.92% for LPC-SI, 16.77% for EI, and 14.15% for AG, compared to the method with the highest performance among the existing methods.

5. Conclusions

In this study, for effective synthesis of visible and near-infrared images under various ambient light conditions during the day and night, the images were decomposed into a base layer and a sublayer using a two-way filter, and synthesized independently by applying a synthesis map reflecting the light characteristics of day and night, respectively. By dividing the daytime and nighttime images according to the use of an IR assist light, we proposed different base layer synthesis maps reflecting the propagation characteristics of near-infrared rays, thereby synthesizing the base layer for image detail expression and enhancing the detailed information expression performance to secure excellent visibility. Color compensation was performed using the luminance ratio between the luminance synthesized image and the visible-light input image.

Compared with the existing low-rank, Densfuse, Vanmali, and PCA algorithms, the proposed algorithm showed improved sharpness by increasing the local contrast through compressing the luminance range of the base layer for the dark or detailed area in the daytime image synthesis result. In addition, the color of the original image was well-expressed without oversaturation throughout the image. The night composite image showed excellent detail expression performance in color by securing identification through base layer tone mapping in the low-luminance region and solving the block noise problem. To improve robustness of performance in the daytime and also the nighttime, the different weight maps of base layer synthesis are presented to consider the visual properties of near-infrared band. As a result, compared with conventional methods, the proposed method showed better performance, particularly in terms of the sharpness and visibility of the images.