Abstract
The capsule endoscopy robot can only use monocular vision due to the dimensional limit. To improve the depth perception of the monocular capsule endoscopy robot, this paper proposes a photometric stereo-based depth map reconstruction method. First, based on the characteristics of the capsule endoscopy robot system, a photometric stereo framework is established. Then, by combining the specular property and Lambertian property of the object surface, the depth of the specular highlight point is estimated, and the depth map of the whole object surface is reconstructed by a forward upwind scheme. To evaluate the precision of the depth estimation of the specular highlight region and the depth map reconstruction of the object surface, simulations and experiments are implemented with synthetic images and pig colon tissue, respectively. The results of the simulations and experiments show that the proposed method provides good precision for depth map reconstruction in monocular capsule endoscopy.
Highlights
The capsule endoscopy robot is a novel endoscopic device that can implement non-invasive digestive tract inspection
The main contributions of this paper are: (1) we combine the specular property and Lambertian property of the object surface to estimate the depth of the specular highlight point, and (2) the estimated depth of the specular highlight point can serve as the boundary condition of the subsequent depth map reconstruction; no extra depth measurement devices are needed for the capsule endoscopy robot
For the centroid of the specular highlight region, i.e., the specular highlight pixel with minimal depth estimation error, the reconstructed depth map of the object surface almost has the same shape of the ground truth; the Root Mean Squared Error (RMSE) of the depth reconstruction of the object surface is less than 0.1 mm; and the relative RMSE of the depth reconstruction of the object surface is less than 0.5%, which is relatively precise for the capsule endoscopy application
Summary
The capsule endoscopy robot is a novel endoscopic device that can implement non-invasive digestive tract inspection. To reconstruct the depth map of the whole object surface with photometric stereo, the ground truth of the depth of at least one object surface point is needed to serve as a boundary condition To solve these problems, in this paper, we study the reflection property of the specular highlight on a non-ideal Lambertian surface and propose a method that estimates the depth of the specular highlight point and reconstructs the depth map of the digestive tract surface by photometric stereo. The main contributions of this paper are: (1) we combine the specular property and Lambertian property of the object surface to estimate the depth of the specular highlight point, and (2) the estimated depth of the specular highlight point can serve as the boundary condition of the subsequent depth map reconstruction; no extra depth measurement devices are needed for the capsule endoscopy robot.
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