Advances in non-mechanical frequency-diverse apertures and reconstruction algorithms have made real-time millimeter-wave data acquisition and volumetric imaging possible. Fast frame rates allow imaging people in motion, which represents a tremendous opportunity to increase security screening throughput over existing solutions where subjects must individually strike and hold a pose. However for non-mechanical systems specularity coupled with limited viewing perspectives diminish coverage for individual images.
To mitigate these issues, a system can leverage relative motion between the aperture and subject for a diversity of perspectives across several images. Such an image set offers overlapping and complementary swaths of subject coverage. By stitching together these images a composite image of the subject can be produced with much better overall coverage.
Of course, people change shape as they move, which significantly complicates the image stitching registration and blending process. A deformable geometric model of a person suitable for real-time stitching is required. Drawing from the field of computer animation, we introduce a deformation model of a person based on Shape Key Deformation (SKD) and Skeletal Subspace Deformation (SSD). SKD blends shapes together, while SSD utilizes a simplified “skeleton” to guide deformation and modulate SKD. Assuming the pose of the skeleton is known, the deformation model is able to map any arbitrary image of a person onto a single rest image for stitching. The model is simple, fast, and robust. We go on to demonstrate image stitching of a simulated person in motion using software that models a massively multistatic MIMO metasurface computational imaging system.
Computational imaging is a proven strategy for obtaining high-quality images with fast acquisition rates and simpler hardware. Metasurfaces provide exquisite control over electromagnetic fields, enabling the radiated field to be molded into unique patterns. The fusion of these two concepts can bring about revolutionary advances in the design of imaging systems for security screening. In the context of computational imaging, each field pattern serves as a single measurement of a scene; imaging a scene can then be interpreted as estimating the reflectivity distribution of a target from a set of measurements. As with any computational imaging system, the key challenge is to arrive at a minimal set of measurements from which a diffraction-limited image can be resolved. Here, we show that the information content of a frequency-diverse metasurface aperture can be maximized by design, and used to construct a complete millimeter-wave imaging system spanning a 2 m by 2 m area, consisting of 96 metasurfaces, capable of producing diffraction-limited images of human-scale targets. The metasurfacebased frequency-diverse system presented in this work represents an inexpensive, but tremendously flexible alternative to traditional hardware paradigms, offering the possibility of low-cost, real-time, and ubiquitous screening platforms.