# Realization of Impression Evidence with Reverse Engineering and Additive Manufacturing
## 1 Introduction  
Forensic impression evidence represents powerful visualizations for intelligence gathering, police investigations, and courtroom purposes to help law enforcement agencies better engage with and understand the circumstances of crimes or accidents [1, 2]. The examiner's ability to conduct precise forensic comparisons relies heavily on the quality of impressions documented and collected at the crime scene (CS). Therefore, impressions require meticulous documentation, handling, and storage for evidence integrity. Impressions left at CS can be classified into three categories, namely plastic, latent, and visible. Figure 1 illustrates examples of these impression evidence types. Plastic impressions, depicted in Figure (a)a, are three-dimensional marks left on a flexible surface like sand, clay, putty, or wax. The most common plastic impression evidence encountered at outdoor crime scenes includes footprints and tire tracks. Footwear impressions alone are discovered in approximately 35% of all crime scenes [3] and comprise the primary workload of tread examiners who analyze both footwear and tire marks [4]. Despite advancements in forensic technologies, the reconstruction of plastic impressions lags behind, with traditional casting and 2D photography remaining prevalent methods to reconstruct plastic impressions. Casting, a multi-step procedure lasting two days, involves creating replicas using materials like Plaster of Paris (POP), dental stone, or foam and requires various tools. Despite its popularity for recovering plastic imprints, casting presents significant drawbacks [5,6]. It is time-consuming, messy, and results in fragile casts prone to cracking and drying out. Air bubbles are common, and casts have low durability against friction. Moreover, it only produces actual-size molds, demands skilled practitioners, and poses challenges in sharing and storing. Post-casting cleaning is necessary, and processing parameters like water-to-powder ratio and mixing time are sensitive. Additionally, traditional 2D images, even with high resolution, lack sufficient depth information for spatial visualization [7]. Unlike plastic impressions, latent impressions are typically 2D and invisible to the naked eye. However, they can be rendered visible using various methods such as chemicals, special powders, light sources, or electrostatic lifting. Latent impressions are commonly found indoors on hard surfaces like glass or concrete, with fingerprints being the most prevalent type (refer to Figure 1b). In contrast, 2D visible impressions (depicted in Figure 1c) are marks that can be transferred onto substrates such as dust, floors, or blood. These impressions are typically encountered indoors and are often documented using perpendicular 2D photography. Bloody fingerprints and shoeprints are among the most frequently encountered types of 2D visible impressions. Computational pattern recognition systems are commonly used for comparison and analysis of 2D latent and visible impressions [8,9]. However, the extensive variability of impressions and the multitude of manufactured and organic patterns pose significant challenges for their computational analysis and presentation in court [4]. In addition, it does not give enough information to represent a spatial visualization due to loss of depth information [7,10,11]. In contemporary digital forensics, it is necessary to reconstruct both 2D and 3D impression types digitally and physically in 3D as this will add much value for forensic analysis. 3D reconstruction is vital in computer vision and is gaining importance in future metaverse forensic frameworks that combine virtual reality, augmented reality, and mixed reality [13, 14, 15]. The spatial representation of evidence may enhance personal identification accuracy, aligning with current technologies and shaping future impression analysis procedures.