# Realization of Impression Evidence with Reverse Engineering and Additive Manufacturing
## 3 Results
### 3D Reconstructions of Plastic Footwear Impression
#### 3.1.3 Quantitative Comparison of Impression Models  
Using the geometrical performance results from the previous section's morphological comparison, the structured-light scanner's model served as the benchmark reference for the quantitative evaluation of the reconstruction quality of the other four scanning modalities.  
Table 3 displays the comparative deviation analysis results of 3D reconstructed footwear outside impression models. Lower standard deviation and RMS values of the distances indicate reduced errors and better scanner performance. As anticipated, the handheld LS performed the best, with the lowest standard deviation and RMS values (SD: 0.0682, RMS: 0.0684), followed by SP photogrammetry (SD: 0.198, RMS: 0.199), then Kinect v2 (SD: 0.522, RMS: 0.620), and finally, the iLiDAR (SD: 0.696, RMS: 0.706).  
While the RGB-D Kinect v2 sensor and iLiDAR technologies are fast and cost-efficient, they exhibit the lowest accuracy in capturing fine impression details. However, Kinect v2 outperformed the iLiDAR, which produced the poorest results in impression reconstruction, displaying the highest mean, RMS, and min/max deviation between points across all data percentage levels. This finding aligns with the visual qualitative comparison. The global error outcomes of SL scanning, LS, SP photogrammetry, Kinect v2, and iLiDAR align with those reported by ([PERSON] et al., 2019, 2020; [PERSON] et al., 2019, 2020).  
Figure 20 illustrates color maps and distance plots of impression point clouds generated from the five scanning methods. The color scale is set to display the range of the deviation with colors saturation to dark red at max positive deviation and dark blue at max negative deviation. As color coded scales show, the largest absolute deviations occurred for iLiDAR (2.481 mm), followed by Kinect v2 RGB-D (2.382), due to their limited ability to precisely capture the full depth information of the impression compared to SL, LS and SP photogrammetry. Thus, their performance is deemed unsatisfactory.  
A histogram showing a global overview of the comparison analysis results for impression model acquired by HandySCAN 3D handheld LS is shown in Figure 21 as a representative example. In this instance, 88% of the CAD model acquired by the scanner is within 0.1 mm compared to the reference structured-light model. The blue color on the left side of the histogram represents data points with negative deviation, while the red color on the right side of the histogram indicates data points with positive deviation.  
In contrast to traditional POP casts which are bulky to store and difficult to carry out and compare geometrical measurements on its surface, not only deviation analysis can be carried out on digital files but also 3D local topographical analysis, wear amount quantifying, digital biometric measurements, and advanced 3D visualization. Furthermore, the requirement for creating multiple test impressions of one shoe, maybe under various circumstances, must be a part of the forensic procedure for making test impressions and this is very exhaustive to be created by casting [38]. Therefore, 3D models can also be easily compared to 3D model of a test impression made with either a known suspect's footwear or directly to an 3D model of the outside of a suspect's footwear especially remotely. These operational differences make portable SL and LS scanners as strong proposals as alternative to traditional casting.  
Figure 21: A histogram of the comparison analysis results for impression model acquired by HandySCAN 3D handheld laser scanner.  
Figure 20: Color maps and distance plots of the impression point clouds generated by: (**a**) EinScan-SP structured-light 3D scanner (reference mesh); (**b**) HandySCAN 3D handheld laser scanner; (**c**) smartphone photogrammetry; (**d**) MS Kinect v2 RGB-D sensor; and (**e**) iPhone’s LiDAR sensor, contracted against a photograph of the plastic footwear impression in dry sand (**f**). Distance values are in millimeters.