## 1 Introduction  
Archaeological cultural heritage is a precious asset that must be preserved and passed on to future generations. Digital 3D recording of objects, monuments, and sites has proven to be a reliable solution. This requires the design and implementation of adequate procedures for a correct and accurate 3D digitization of the asset. The documentation of prehistoric art has a triple purpose: (1) to serve as a research and knowledge tool, (2) to create records which will allow further investigations towards the implementation of conservation interventions, and (3) the production of replicas that will maintain the shape and materiality of the originals--which may significantly deteriorate over time [1]. In the field of rock art, we generally investigate paintings and/or engravings. To obtain a faithful representation of the preserved traces, its final tracing must record, on the one hand, the engraving groove and, on the other, faithfully the pigment. This article presents the methodology followed to document a piece of Paleolithic portable art, using the macro photogrammetry as a basis. Photogrammetry is considered one of the most successful passive sensing techniques for measuring and reconstructing 3D objects from multiple images, regardless of the study scale and the platform (static/mobile, terrestrial/aerial/underwater/underground) used to acquire the images [2]. Digital photography has been used for years to document rock art. Specifically in the study of engravings, macro photography, and reflectance transformation imaging (RTI) have been used for two decades [3]. RTI technology is based on processing multiple images of an illuminated object from different directions (azimuth) and angles (elevation), while the camera remains in a fixed position. In the last five years, researchers who have used the RTI to better visualize engravings in portable art [4] and petroglyphs [5;6] have also opted for the 3D digitization of the artefacts to trace the engravings on the surfaces. To increase the visibility of the engravings in the 3D model, they use rendering techniques in MeshLab that allow, by modifying the light intensities, to represent the shape through shading, improving details, such as concavities and convexities [4; 5; 6; 7]. In the case of [PERSON] et al. [5], they also use a deviation map created in Geomagic, comparing the original model with another modified by smoothing it. In all cases, the authors conclude that combining RTI with 3D digitization offers adequate results for the rock art documentation needs. Two methodologies that complement each other to create a unique result. However, it is important to keep in mind that the quality of the 3D model is essential for optimal results. In recent years, other researchers have proposed methods that use the digital elevation model (DEM) and morphometric approaches applied through geographic information system (GIS) implementations, aiming to improve the identification of engravings [8; 9]. This investigation assumes that etching is an anthropogenic depression in the surface topography. On the other hand, the ImageJ DStretch complement designed by [PERSON] [10] for the image enhancement of cave paintings has also given good results to study fine engravings [9; 11; 12]. DStretch is one of the most efficient tools for deciphering faint paintings [13; 14; 15]. It is based on the application of the decorrelation stretching algorithm to the main components of a digital color image. Image enhancement techniques were first introduced to remote sensing. There are many custom color spaces employed in producing digital replicas. Digital image enhancement is immediate, nearly operator-independent, and reproducible (thanks to prerecorded settings), enhancing objectivity [12]. According to [PERSON] [11], the software proved to be equally useful for visualization (enhancement) of linear incisions and scraping (scratches).