DStretch Tattoo Protocol: Full step-by-step protocol for identification and visualization of tattoos on preserved archaeological remains using the ImageJ plugin DStretch v1
2022-12-31

Archaeological preservation of intentional skin modifications such as tattoos is rare but can yield important insights into the human past with respect to the belief systems, culture, and art of past societies. Tattooing can be briefly described as an invasive process of inserting pigments into the skin with the intention of creating permanent motifs. Preserved tattoos have been identified on naturally and deliberately mummified human remains from archaeological sites of many cultures across the globe, dating back at least 5,000 years before present (Deter-Wolf 2022; Deter-Wolf et al. 2016). Preserved tattoos on mummified remains are often documented using standard photography. However, taphonomic processes contributing to preservation may darken or shift natural skin color, rendering tattoos indistinct or invisible to the naked eye. Multispectral imaging using specialized camera equipment has been applied with great success to bypass this problem (e.g., Alvrus et al. 2001; Austin and Goebil 2017; Barkova and Pankova 2005; Friedman 2017; Gaber et al. 1995; Hansen et al. 1991; Samadelli et al. 2015; Smith and Zimmerman 1975), but comes with the disadvantage of requiring specialized equipment that, even with the recent increased availability of consumer-grade digital imaging technology, may be prohibitively expensive. The DStretch® software plugin presents another cost-effective and easy tool to visualize ancient, preserved tattoos. This low-cost software plugin for the open-source image processing and analysis freeware ImageJ© (Schneider et al. 2012) also functions as a stand-alone app for mobile Android and iOS devices. The DStretch® algorithm was initially developed in remote sensing (Gillespie et a. 1986) and was later adapted in 2005 by Jon Harman (2005) for the visualization of faded or ephemeral rock art (e.g., Evans and Mourad 2018; Harman 2015; Gunn et al. 2014; Gunn et al. 2010; Harman 2005), and has subsequently been used on other archaeological subjects (e.g., Emmitt et al. 2021; Gonzales et al. 2019). The program has also recently been used by Anne Austin and colleagues (Austin 2022; Austin and Arnette 2022; Austin and Gobeil 2017) to visualize tattoos preserved on archaeological remains from ancient Egypt and 19th century Europe. The mathematical background of the decorrelation-stretch transformation has been well documented (e.g., Kaur and Sohi 2017; Campbell 1996; Gillespie et a. 1986). However, to our knowledge, detailed descriptions of its applicability, especially with respect to the DStretch® software plugin, are so far widely lacking. Since 2017 the senior author (DG) has experimented with new ways of implementing DStretch® in recording archaeological subjects, including preserved tattoos and bone discolorations (Göldner in preparation), while Deter-Wolf has used the technology to evaluate tattoos on mummified remains from the Andes region of South America (Deter-Wolf et al., in preparation). This protocol summarizes the results of our work in a reproducible and understandable framework for other researchers to use when visualizing preserved tattoos on mummified remains in both archaeological and museum settings. It represents the first complete guidelines for applying this method, covering image collection and post-processing, decorrelation stretch transformation, and subsequent documentation, including drawings, descriptions, and reconstruction drawings. This protocol can also be used in the same or slightly modified manner for other types of color-decorated objects and specimens and even on colored surface texture files derived from laser- or light-based three-dimensional (3D) scans and photogrammetry (e.g., Gonzáles et al. 2019; Emmitt et al. 2021; Keppler et al. in preparation). With this effort, we aim to establish a standard for visualizations of preserved tattoos using a reproducible and citeable method in order to improve scientific practice in the field of ancient tattoo analysis. We hope that interested scientists will adopt this protocol as a standard for their work in visualizing, identifying, reconstructing, and documenting their findings.