This paper presents a depth-based defocus map estimation method from a single camera with multiple off-axis apertures. The proposed estimation algorithm consists of two steps: (i) object distance estimation using off-axis apertures and (ii) defocus map estimation based on the object distance. The proposed method can accurately estimate the defocus map using object distances that are well-characterized in a color shift model-based computational camera. Experimental results show that the proposed method outperforms the state-of-the-art defocus estimation methods in the sense of both accuracy and the estimation range. The proposed defocus map estimation method is suitable for multifocusing, refocusing, and extended depth of field (EDoF) systems.
© 2015 Optical Society of America
Recent advances in various digital imaging systems have increased the demands on high-quality images over the past decades. To remove degradation factors in the image, an accurate analysis of blur parameters plays an important role in various applications of image enhancement such as robot vision, satellite image processing, video surveillance, and the image signal processing (ISP) chain in modern digital cameras [1, 2]. Image degradation occurs in various types of non-ideal imaging conditions. With a constant exposure time, the size of out-of-focus blur increases if the size of an aperture increases or an object is located farther away from the in-focus position. For this reason, the acquired image with multiple objects with different distances has, in general, a spatially varying blur.
Most conventional defocus estimation methods use multiple input images acquired from the scene with different focal planes [3–5 ]. Tai et al. estimated the defocus map using the MRF propagation based on the local contrast prior . On the other hand, Zhou et al. estimated the amount of defocus blur using the gradient of a re-blurred input image . Shen et al. combined the local contrast prior and a guided filter to derive a blur map, and then removed the out-of-focus blur using ℓ 1 or ℓ 2 optimization in . These algorithms are sensitive to the accuracy of tuning parameters, and cannot utilize the depth information because of the intensity-based framework.
Willert et al. proposed a single camera-based three-dimensional particle imaging to analyze the flow field of a vortex ring impinging on a wall . They used two small apertures to determine the object distance from the lens by measuring two separate defocused particle images. Since only isolated particles can be analyzed, Willerts work cannot be used to measure both three-dimensional information and RGB data at the same time. To overcome such limitation, the proposed work can use wider apertures separated by color filters with a novel, color-based registration algorithm.
The fundamental theory of range estimation is based on measuring the change in a two-dimensional image sensor that results from the change in the three-dimensional world. As an alternative to traditional approaches to this measurement including binocular stereo and structure from motion, Farid et al. proposed a direct measurement technique for the viewpoint derivative using a single camera at a fixed location . More specifically, a single stationary camera and a pair of Gaussian optical attenuation masks were used to measure the derivatives. Instead of using optical masks, the proposed work uses a single lens with three different color-filtered apertures, and measure the derivative information using region-based image registration algorithm. The multiple color-filtered aperture system can also provide a normal photographic RGB data as well as the depth or range information.
Recently, computational cameras attracted increasing attention to acquire additional information such as the distance and velocity of an object [11, 12]. In this paper, we estimate the depth-based defocus map by using an off-axis aperture camera. The proposed method can be applied to either a classical stereo camera or the color shift model-based computational camera proposed by Kim et al. . More specifically, Kim’s method acquires both a color image and depth information using a computational camera equipped with multiple color-filtered apertures (MCA). The image acquired by the MCA camera contains the distance information of an object from the plane of focus in the form of disparity between color channels. To estimate the distances of multiple objects and to make all of them focused at the same time, a set of fundamental image processing algorithms are needed such as image segmentation for clustering object regions, color shift model-based registration and fusion, and image restoration. The disadvantage of Kim’s method is the high computational load and the limited quality of multi-focused image.
In this paper, we present a novel defocus map estimation method by analyzing the relationship among multiple off-axis apertures. In order to estimate the object distance, we analyze color shifting vectors which are generated according to the optical property of the off-axis apertures. The defocus map is then estimated using the distance and specifications of apertures, such as the shape of each aperture and the relative distance between apertures. Since the proposed method uses the real object distances in a single input image, we can efficiently estimate the defocus map without the problems of conventional methods.
This paper is organized as follow. Section II presents a computational camera using multiple off-axis apertures. Section III presents the object distance estimation method by analyzing off-axis apertures, and section IV presents the circle of confusion estimation method by analyzing the object distance and aperture specifications. Section V summarizes experimental results, and Section VI concludes the paper.
2. Properties of an off-axis aperture
Various computational cameras have been developed to obtain additional information which cannot be acquired in the conventional camera [11,12]. Additional information is used to solve fundamental problems of a conventional camera system such as depth estimation, motion de-blurring, refocusing, and high dynamic range [13, 14]. Recently, computational cameras using multiple off-axis apertures were proposed for a new type of computational camera [15–19 ].
An aperture of an optical system is the opening that can adjust the amount of incoming light. The center of an aperture in a normal camera is aligned to the optical axis of the lens, and the convergence pattern on the image plane forms either a point or a circular region whose radius is determined according to the distance of the object from the focus plane as shown in Fig. 1(a). On the other hand, if the center of the aperture is not aligned to the optical axis, the convergence location of the incoming light is deviated from the optical axis as shown in Figs. 1(b) and 1(c). The MCA system is the extension of this approach using three apertures with different color filters. The main advantage of the MCA camera is that it provides both color and depth information that can be estimated from the direction and amount of color deviation from the optical axis. Maik et al. have originally proposed color shift model-based auto-focusing using the multiple color-filtered apertures system , and the rigorous analysis of the MCA system was given in . Lee et al. have proposed a distance estimation method using dual off-axis color filtered apertures . However the original contribution of this work is twofold: i) the proposed three different color-filtered apertures can provide better color reproduction, and ii) it can estimate color shifting vectors more robustly using three pairs of apertures. In the following section, we derive the relationships between color shifting vectors and circle of confusions.
3. Object distance estimation by multiple off-axis aperture
The color deviation of the image acquired by a color shift model-based computational camera is characterized by a function of the distance of an object from the plane of focus. To analyze the color deviation, we consider a single-aperture model that has an off-axis aperture away from the optical axis as shown in Fig. 2.
In the thin lens model, the focal length f has the relationship as
To compute v, we analyze the relationship between the object distance and the corresponding shifting vector as shown in Fig. 3.
If a circle of confusion is sufficiently small and the length of the shifting vector is determined according to the distance of the image plane from the lens, it can be computed using the property of similar triangles asEq. (3) can be expressed as
As a result, the object distance s is inversely proportional to the shifting vector Δ. If an optical system has multiple apertures, the imaging sensor also has the same number of multiple images and shifting vectors as shown in Fig. 1(c). We can separate the multiple images and shifting vectors in the image plane by using a different color filter in each aperture. As shown in Fig. 4, the incoming lights passing red and blue color-filtered apertures are focused in the corresponding color sensors, respectively, and the position of each image is determined by the single-aperture model. In the multiple off-axis aperture model, the direction of shifting vector is determined by the in-focused object distance sin. Therefore, we can estimate s by analyzing the direction and magnitudes of the shifting vector according to Eq. (5).
4. Depth-based defocus map estimation based on color shifting vectors
We estimate the object distance using color shifting vectors of off-axis apertures under the assumption that each circle of confusion is sufficiently small. In this section, a rigorous analysis of the circle of confusion is presented according to color shifting vectors.
The size of a circle of confusion is determined by the distance of an object away from the plane of focus. If the object is placed on the in-focused plane, the circle of confusion ideally converges to a point, whereas it grows bigger if the object moves away from the in-focused plane as shown in Fig. 5.
Using the property of similar triangles, the diameter of a circle of confusion is computed as
If sizes of the image and sensor are m × n and p × q, respectively, the shifting vector Δ is computed asEq. (5) and Eq. (9) as
If centers of three apertures are located at the vertices of an equilateral triangle, the shifting vector can be expressed asFig. 6.
Equation (12) states that both the object distance and depth-based defocus map can be estimated from the color shifting vectors. Although the ideal color shifting vectors have the same magnitude, we use the median value of three color shifting vectors that were estimated in RGB color channels..
The proposed defocus map estimation algorithm is summarized in Algorithm 1.
5. Experimental results
In this section, experimental results of the proposed defocus map estimation method using multiple off-axis apertures are presented. Hardware specifications used in the experiment are shown in Table 1.
In order to measure the relationship among the object distance, shifting vector, and circle of confusion, we acquired multiple images with different distances as shown in Fig. 7, and then color shifting vectors are estimated using the phase correlation method in .
Blue curves in Figs. 8(a) and 8(c) show the ground truth of object distances from the camera versus the magnitude of the shifting vectors, and red crosses show the estimated object distances according to Eq. (5). As shown in Figs. 8(a) and 8(c), the proposed method can accurately estimate the object distances.
Blue curves in Figs. 8(b) and 8(d) show the ground truth of circle of confusion sizes according to the object distance, and red crosses show the estimated size of circles of confusion according to Eq. (11). As shown in the figure, the proposed method can accurately compute the size of circles of confusion corresponding to the object distance.
Experimental results of the proposed defocus map estimation algorithm are shown in Fig. 9 and Fig. 10. Figure 9(a) and Figs. 10(a)–10(b) show input images containing spatially varying blur according to the distances. Zhuo’s and Shen’s methods can estimate the amount of defocus blur only in the neighborhood of edges since they use intensity-based estimation as shown in Figs. 9(b)–9(c) and Figs. 10(c)–10(f). On the other hand, the proposed method successfully estimates the spatially varying blur that varies according to the object distances as shown in Fig. 9(d) and Figs. 10(g)–10(h).
Figure 11 shows the result of spatially adaptive image restoration using defocus maps of Zhuo’s, Shen’s, and the proposed method. Since the proposed method estimates the defocus map using the correct distance information, it can successfully remove spatially varying out-of-focus blur.
Table 2 summarizes quantitative analysis of Zhuo’s, Shen’s, and the proposed methods. To analyze the accuracy, the defocused image was restored using defocus maps estimated by the three methods. We used L1-2 optimization  and Wiener filter  as image restoration methods. For the quality assessment, we used natural image quality evaluator (NIQE)  and spectral and spatial sharpness(S 3) . It is to note that the proposed method produces the best result in most NIQE and S 3 values.
In order to estimate the defocus map using a color shift model-based computational camera, a combined optical and mathematical model of off-axis apertures is presented. Based on the assumption that an image has multiple objects at different distances from the camera, each object region is out-of-focused by the corresponding circle of confusion that is determined from the object distance. Misaligned regions among color channels are registered for computing color shifting vectors, and the object distance and the corresponding circle of confusion are estimated from the color shifting vectors. Since the propose method uses the real object distance, it can correctly estimate the spatially varying blur without manually tuning parameters in a single input image. Although the proposed approach primarily focuses on the estimation of the defocus map using the image acquired by the color shift model-based computational camera, it can also be used for any types of color misaligned and/or spatially varying out-of-focused images. To acquire the highly accurate defocus map, an improved registration method will be developed in the future.
This work was supported by Institute for Information & communications Technology Promotion(IITP) grant funded by the Korea government(MSIP) ( B0101-15-0525, Development of global multi-target tracking and event prediction techniques based on real-time large-scale video analysis) and the Technology Innovation Program (Development of Smart Video/Audio Surveillance SoC & Core Component for Onsite Decision Security System) under Grant 10047788.
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