Expanding the Detection of Traversable Area with RealSense for the Visually Impaired
The introduction of RGB-Depth (RGB-D) sensors into the visually impaired people (VIP)-assisting area has stirred great interest of many researchers. However, the detection range of RGB-D sensors is limited by narrow depth field angle and sparse depth map in the distance, which hampers broader and longer traversability awareness. This paper proposes an effective approach to expand the detection of traversable area based on a RGB-D sensor, the Intel RealSense R200, which is compatible with both
... door and outdoor environments. The depth image of RealSense is enhanced with IR image large-scale matching and RGB image-guided filtering. Traversable area is obtained with RANdom SAmple Consensus (RANSAC) segmentation and surface normal vector estimation, preliminarily. A seeded growing region algorithm, combining the depth image and RGB image, enlarges the preliminary traversable area greatly. This is critical not only for avoiding close obstacles, but also for allowing superior path planning on navigation. The proposed approach has been tested on a score of indoor and outdoor scenarios. Moreover, the approach has been integrated into an assistance system, which consists of a wearable prototype and an audio interface. Furthermore, the presented approach has been proved to be useful and reliable by a field test with eight visually impaired volunteers. Sensors 2016, 16, 1954 2 of 20 through triangulating algorithms. However, they fail to return an efficient depth map in sunny environments because projected speckles are submerged by sunlight. As a result, approaches for VIP with light-coding sensors are just proof-of-concepts or only feasible in indoor environments          . ToF cameras, such as CamCube  (developed by PMD Technologies based in Siegen, Germany), DepthSense  (developed by SoftKinetic based in Brussels, Belgium), and SwissRanger (developed by Heptagon based in Singapore)  resolve distance based on the known speed of light, measuring the precise time of a light signal flight between the camera and the subject independently for each pixel of the image sensor. However, they are susceptible to ambient light. As a result, ToF camera-based approaches for VIP show poor performance in outdoor environments    . Stereo cameras, such as the Bumblebee  (developed by PointGrey based in Richmond, BC, Canada), ZED  (developed by Stereolabs based in San Francisco, USA), and DUO  (developed by DUO3D based in Henderson, NV, USA) estimates the depth map through stereo matching of images from two or more lenses. Points on one image are correlated to another image and depth is calculated via shift between a point on one image and another image. Stereo matching is a passive and texture-dependent process. As a result, stereo cameras return sparse depth images in textureless indoor scenes, such as a blank wall. This explains why solutions for VIP with stereo camera focus mainly on highly-textured outdoor environments     . The RealSense R200 (developed by Intel based in Santa Clara, CA, USA) uses a combination of active projecting and passive stereo matching  . IR laser projector projects static non-visible near-IR patterns on the scene, which is then acquired by the left and right IR cameras. The image processor generates a depth map through an embedded stereo-matching algorithm. In textureless indoor environments, the projected patterns enrich textures. As shown in Figure 1b ,c, the texture-less white wall has been projected with many near-IR patterns which are beneficial for stereo matching to generate depth information. In sunny outdoor environments, although projected patterns are submerged by sunlight, the near-IR component of sunlight shines on the scene to form well-textured IR images as shown in Figure 1g . With the contribution of abundant textures to robust stereo matching, the combination allows the RealSense R200 to work under indoor and outdoor circumstances, delivering depth images though it has many noise sources, mismatched pixels, and black holes. In addition, it is possible to attain denser depth maps pending new algorithms. Illustrated in Figure 1 , the RealSense R200 is quite suitable for navigational assistance thanks not only to its environment adaptability, but also its small size.