## 2 Light Field Camera Structure and Depth Extraction
### Depth Extraction Based on Optical Flow for Light Field Camera
Thus, the pixels that are located near the edges of lens images can still converge at a physically plausible location. On the contrary, a certain optical axis of microlens and the main axis of LFC converge at a physically impossible position because the optical axes of microlenses are tilted out of the main optical axis according to their positions. As the optical flow value increases, the extracted depth gradually increases and soon diverges, as shown in Figure 2b. This phenomenon occurs at a smaller optical flow value when nearer sub-image to center sub-image is used. In other words, the available optical flow range is determined according to the view interval from the center sub-image. Figure 3a represents the depth on optical flow value by DOLF for corresponding points on center sub-image and another sub-image. Since two pixels that converge above a certain optical flow are located at the position that cannot be physically allowed, it is necessary to use a mask to neglect a range of optical flow value in forbidden area. In this paper, the mask for the specified range of optical flow value is applied to a case where the convergent depth position is completely unrealistic, that is, when the depth information is negative. Figure 3b shows the maximum optical flow value of what is available for the center sub-image in the order of the sub-images. The dotted line indicates the middle index of the sub-image. The optical flow values that are shown here are absolute values. Though, for example, the third and the fifth sub-image have the same optical flow value, the calculated data flows in different directions. Figure 4a,b shows simulated light field image and Figure 4c represents the result of depth information that was extracted from light field by optical flow calculation using Equation (1). We simulated the raw light field that was photographed by the LFC and extracted the light field information through the modified optical flow method. The specification for light field image is presented in Table 1. Figure 4d represents detailed depth information along the white dotted line in Figure 4c. Figure 4d compares the results that were obtained by DOLF and optical flow for InIm with ground truth data. To extract a precise depth, it is necessary to consider the geometrical arrangement of the light field camera, as shown in Figure 4d. This comparison is meaningful when considering that depth extraction algorithms for light field cameras extract relative depth rather than actual depth. When extracting the relative depth, it is necessary to fit the extracted value to the actual distance according to the many parameters of imaging system. In this case, the optical configuration of the light field camera must be fully considered to obtain the correct depth extraction result. As a result, the proposed depth extraction method for LFC is verified. It is noted that the data of ground truth is more discontinuous than the result of depth extraction on the center of graph because the original depth map of sub-image has been quantized after 8-bit conversion process. Figure 3: (**a**) Extracted depth of center sub-image with seventh sub-image (among seven sub-images) according to optical flow (**b**) Maximum optical flow on location of sub-image (among seven sub-images).