ABSTRACT:This paper investigates the use of different greyscale conversion algorithms to decolourize colour images as input for two Structurefrom-Motion (SfM) software packages. Although SfM software commonly works with a wide variety of frame imagery (old and new, colour and greyscale, airborne and terrestrial, large-and small scale), most programs internally convert the source imagery to singleband, greyscale images. This conversion is often assumed to have little, if any, impact on the final outcome. To verify this assumption, this article compares the output of an academic and a commercial SfM software package using seven different collections of architectural images. Besides the conventional 8-bit true-colour JPEG images with embedded sRGB colour profiles, for each of those datasets, 57 greyscale variants were computed with different colour-to-greyscale algorithms. The success rate of specific colour conversion approaches can therefore be compared with the commonly implemented colour-to-greyscale algorithms (luma Y'601, luma Y'709, or luminance CIE Y), both in terms of the applied feature extractor as well as of the specific image content (as exemplified by the two different feature descriptors and the various image collections, respectively). Although the differences can be small, the results clearly indicate that certain colour-to-greyscale conversion algorithms in an SfMworkflow constantly perform better than others. Overall, one of the best performing decolourization algorithms turns out to be a newly developed one.
<p><strong>Abstract.</strong> In the second half of the 19th and early 20th century, sheep shepherds have built dry-stone shelters all over the Slovene Kras (or Karst) region. Despite being made out of stones that are interlocked without the use of any binding material, many of these vernacular constructions survived &ndash; even though sometimes only partially &ndash; the ravages of time. The fact that over one hundred fifty shepherd shelters are currently known is mainly due to the craftsmanship of their builders and thanks to (and even despite) their present location. A majority of these stone constructions can be found in areas that are nowadays forested, thus shielding them from weather-related or anthropogenic damage (because they are difficult to spot). This paper reports on the geometric documentation of those shelters using a photogrammetric computer vision pipeline, thereby mainly focussing on the difficulties that were encountered during this process. However, such image-based modelling approaches merely yield digital three-dimensional (3D) approximations of the shelters’ surface geometry (along with some sub-optimal colour data). Although these 3D surface models might be suitable to digitally preserve vulnerable vernacular buildings to some extent, they do not magically advance our understanding of them. The second part of this article focuses, therefore, on the extraction of archaeological information from these digital 3D constructions. More specifically, the total amount of stones, the total building time and the building cost regarding caloric energy expenditure are estimated for each of the digitised shelters. Although this assessment of architectural energetics provided useful insight into the building efforts and nutrient uptake of the shepherds, it also revealed many assumptions and shortcomings that often characterise archaeological information extraction from digital 3D models of buildings.</p>
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