Occlusion Removal in Terahertz Frequency-Modulated Continuous Wave Nondestructive Testing Based on Inpainting

Terahertz frequency-modulated continuous wave is an effective tool for nondestructive testing. Occlusion is one of the bottlenecks of current imaging techniques for the nondestructive testing of complex objects. In reflective imaging, surface objects can cause information occlusion of deep defects in the detection direction, thus decreasing the effectiveness of nondestructive testing. In this article, a method of occlusion removal is proposed by using the layered imaging capability of frequency-modulated continuous wave systems. A specific mask is generated from the image of the surface layer, which can cover the shading on the image of the layer behind it. After an isophote-driven, exemplar-based synthetic inpainting process, the effect of the occlusion can be eliminated and the images of each layer can be restored. It is important to note that deep objects cannot be completely occluded and the deep image should have enough feature information for successful restoration. To demonstrate this, we successfully restored the occluded images from overlapping multilayer samples and validated the method in real scenes. The quantitative computational results show that the proposed method can effectively remove the occlusion and restore the images of each layer. The method provides a practical solution to the problem of occlusion that arises in the application field of nondestructive testing for any objects with multiple layers.