# Realization of Impression Evidence with Reverse Engineering and Additive Manufacturing
## 2 Materials and Methods
### Reconstruction of Plastic Footwear Impression
#### 2.1.1 3D Digital Reconstructions of a Footwear Impression
This includes taking a minimum of at least 20 photographs of from varying oblique angles of \(\approx\)0\({}^{\circ}\), 20\({}^{\circ}\) and 40\({}^{\circ}\) and directly above the sandbox, ensuring all photos overlaps. A scaled ruler was placed next to the impression. Photogrammetry performs well in area of interest with color variability which allowing features to be acquired easily. So, a textured mats were placed around the sandbox to increase the acquisition quality. In total, 226 photos with resolution 1080 \(\times\) 2400 pixels were captured by Samsung Galaxy A70 smartphone equipped with 32 MP main camera resolution. Following photography, the photos were processed in Autodesk Recap Photo (Autodesk Inc., One Market Plaza, San Francisco, CA, USA) (Figure 10b). The photos were first uploaded to Recap Photo and the project was submitted to Autodesk cloud server and data were then processed on the cloud server. Recap built the mesh model, removed excess geometry before building the final impression model, then the model was scaled to the real dimensions based on the scale ruler. Finally, the high-density triangulated mesh file was downloaded from the server in (.STL) format (Figure 10c). The acquisition lasted for 3 min, processing requires 90 min, and the monthly cost for Recap Pro is USD 50. Microsoft Kinect v2 Depth Sensor (Kinect v2, Kinect Fusion Explorer)  
Steps of 3D reconstruction of footwear impression by Microsoft Kinect v2 depth sensor illustrated in Figure 11. Microsoft Kinect v2 depth camera (Microsoft Inc., Redmond, WA, USA) was used to scan the impression (Figure 11a). It features an HD color camera with RGB resolution of 1920 \(\times\) 1080 pixels and a 512 \(\times\) 424 pixels ToF depth sensor resolution, horizontal field of view of 70.6\({}^{\circ}\) and 60\({}^{\circ}\) and frame rate of 30 frames per second (fps) which acquiring 217,088 colored 3D points every frame. The stand-off distance between the impression and Kinect sensor was kept about 0.5 m due to the limitation of minimum range of Kinect and impression was placed in center in the frame of view of the Kinect sensor. The data collected from Kinect device were processed simultaneously using Kinect fusion explorer application in the free Kinect for Windows Development Kit (SDK) 2.0 software (Figure 11b) and saved in (STL) format (Figure 11c). The acquisition and processing time was 2 min, and the market price of the Microsoft Kinect v2 is approximately USD 200. Figure 10: 3D reconstruction of footwear impression by photogrammetry using Autodesk Recap Photo: (**a**) Taking a series of overlapping photos; (**b**) image processing; (**c**) impression model. iPhone's LiDAR (iPhone 13 Pro, Polycam)  
iPhone 13 Pro with iOS 15.4.1 was used in combination with the Polycam 3D Scanning App for the scanning of the impression. The iPhone 13 Pro released on September 2021 and features a 12 MP camera system that contains three different cameras (1x telephoto, 1x wide, and 1x ultrawide camera), a flashlight and a LiDAR sensor on the backside of the phone (Figure 12a). Polycam app processes data in cloud and the user has control over Depth Range, Voxel Size, and Simplification in the processing options. A maximum depth range of 0.5 m, a voxel size of 2 mm, and a simplification 0% were chosen to generate a high-quality point cloud. Polycam is free to use but it requires a paid license (costs USD 7/month) to allow exporting the scan data in different formats. Figure 12 shows the main features of iPhone 13, 3D reconstruction scene of footwear impression by in Polycam app and the final impression model. The acquisition and processing time was 4 min. After completing scans with the five different techniques, the resulting 3D models in (STL) format were imported into Vxelements. To ensure consistency in engineering measurement units, the models from Kinect v2 and iLiDAR were scaled by a factor of 1000.