Abstract
The issue of knowledge and documentation of cultural heritage is always central in the field of Drawing, which, through the survey, is always preliminary to the safeguarding and valorisation of artistic and architectural heritage. The paper aims at presenting some results of a survey realized following an approach that mix direct, laser and photographic techniques to obtain high quality ortophotos of the painted surfaces, useful to be the basis for the graphical analysis of the trompe l'oeil perspective. The phases are: survey campaign and data acquisition, postproduction of the RAW file, photo straightening, photo modelling, orthophoto output, CAD analysis, conclusive considerations. All these steps are referred to the experimentation on a concrete case study, one of the numerous frescoed rooms belonging to the rich iconography of Palazzo Arese Borromeo in Cesano Maderno (MB, Italy), a building that is an actual unicum in its kind.
TopIntroduction
From Survey, through Restitution, to Analysis
The broader meaning of the term “Survey” states that it is an act of knowledge1, in which all activities are directed to the reconstruction of a geometric-dimensional model of the building, but also to the production of boards, drawings, images and texts that contribute to the complete understanding of the architecture in its complexity. Decorative or pictorial apparatus, for which the direct survey is incomplete and ineffective, always enriches almost all the historical architecture, even with simple shapes. Commonly, one uses an expensive and complex tool as the Laser Scanner, which requires specialized operators and advanced instrumentation. The recent and rapid develop of Structure from Motion software and others technologies of non-contact survey can support, enhance and simplify this goal.
The decision to begin in recent years the survey of Palazzo Arese Borromeo in Cesano Maderno (MB) (overall view from painting in Figure 1) was determined, as well as by an incomplete documentation of its state of fact, even by the great historical and artistic value of this complex. The building was individuated in the area of interest of the project PRIN2010 Architectural perspective. Digital preservation, content access and analytic2. Most of the architecture and the decorations dates back to the Seventeenth Century, when it was founded and promoted by the client and first designer Count Bartolomeo III Arese. The architecture and the numerous painted rooms of the palace are one of the most important and best-preserved artistic expressions of the Seventeenth Century Lombard style (photos in Figure 2), on which scholars are proceeding with studies and valorisation activities. Because today a complete survey of the building is not available, a collaboration was established between the Politecnico di Milano and the management authorities of the property, in order to produce an architectural survey of the entire complex3. The building was proposed as a teaching exercise of direct survey conducted in teams through two measurements campaigns lasting a few days in 2014 and 20154. The palace turned out to be an excellent laboratory to field-test the architectural and photogrammetric survey, carried out with several different tools, direct and instrumental measuring, photo stitching and three-dimensional photomodelling, oriented to gather information on the state of the built and painted architecture.
Figure 1. Wall painting of Palazzo Arese Borromeo in the II half of the XVII Century, before the construction of the east wing and Genovese Loggia. Piazza Esedra still was a farmyard and there were many dervice buildings in the north side. This picture is depicted in a hall of the building itself.
Figure 2. Some photo of rooms and exteriors: the Genovese Loggia, Ninfeo with pebble mosaic, Sala delle Rovine, Galleria delle Arti Liberali
In consideration of various factors, such as its dimensions and characteristics, the pictorial quality of the decorative apparatus, the lack of architectural studies, etc., the building was later chosen as an upper level case study, also suitable for comparative tests between different methods.
The research presented in this paper is a part of the non-contact and photogrammetric survey of the painted architecture, focusing on the study of the optimal conditions of realization of the photos to be processed with automatic SfM Structure from Motion systems to obtain high quality orthophotos for the study of the perspective. The intention is to verify the ease of use, reliability and integration of some IBM Image Based Modelling software, establishing a rule of action required to optimize the result even by non-technical photographers. During the survey campaign, the use of quick methods of non-contact survey allows capturing large amounts of three-dimensional metric and chromatic data, which constitute heritage of knowledge about the status of the architecture and permits to postpone the choosing of sections and specific analysis. However, only a thorough knowledge of the instrumentation and its dynamics gives the preparation that allows evaluating and interpreting the output data, in order to get the wanted scientific representation.
The text is deemed to explain the survey procedures, in continuously monitoring the quality of the output, the graphic restitution and comparison between different tools. A workflow type is attempted to be defined: photographic campaign, calibration, photo postproduction, data processing with different SfM, extraction of orthophoto and their comparison, and finally the analysis of the illusionistic architecture.
This research also tries, through the study of the adopted techniques and rules of representation in relation to the units of measurement of the time, to reveal the operating stratagems and describe the painter’s geometric reasoning, theory and practice, to give the illusion of continuity between the real and virtual space represented by perspective.
Key Terms in this Chapter
Integrated Survey: Widespread term which means the restitution of an architectonic digital model (or object), 2D but also 3D, obtained overlapping and standardizing data (metric, colour, materials, quality, conservation) acquired from multiple measuring methods. For example, direct survey (carried out by operator with simple measuring instruments), laser scanner, photogrammetry or photomodelling, topographic survey, photography, thermography, etc.
Photogrammetry: Reverse process of perspective, which allows deriving actual measurements from a central projection thanks to the rules of projective geometry. The simplest form is flat, from single frame. The solid photogrammetry reconstructs the position of the points in space combining the same process on two or more frames differently oriented, in digital way. Before, the operator separately individuated each point.
Photomodelling: Branch of the solid photogrammetry that consists in the restitution from photographs of 3D dense point cloud and/or textured mesh models, using the principles of projective geometry. The first algorithm of automatic recognition of features (homologous points in multiple photographic images with different perspectives) dates back to 1999. It can be performed using commercial or freeware software, on oneself computer or by cloud computing. The workflows includes recognition of features, photo matching, orientation, dense matching (and polygonal reconstruction & texture mapping). One of its output is the orthophoto.
Orthophoto, Photoplan: Photoplan is a photographic image corrected and rectified so that the image plane represents the plan of the real object in scale and metrically measurably. Of intuitive understanding in the case of walls, floors etc., it become more complex in the case of curved surfaces or overhangs, where a virtual plane must be identified, to orthogonally project the surface without deforming it. This is impossible with a single photograph, where the central point of view produces perspective deformations. The solution is the orthophoto , or a parallel projection of a three-dimensional model view taken along a predetermined plan. Orthophotoplan is a product of mapping, namely the zenith photographic satellite image rectified and mosaicked.
HDR(I) High Dynamic Range (Imaging): Technique used in computer graphics and photography to obtain an image in which the dynamic range (range of luminance between the lightest and darkest areas) is the broader possible. It has detail in the whole scale from white to black. No sensor can capture in a single shot the entire DR of a high-contrast subject (its exposure latitude is less) and often it is necessary to perform at least three bracketing shots, by varying the exposure time and fixing the other parameters. This will compensate the loss of detail in underexposed or overexposed zones in each image. Dedicated software process and merge the series of shots to make a single photo with the correct exposure at each point. If exaggerated, the photo seems fake, like a painting.
Photo Straightening, Photo Stitching: Process by which a photographic image that takes a foreshortening of a flat surface is distorted according to the rules of projective geometry to correctly and measurably represent the surface itself ( photoplan ). The stitching, merging or mosaicking of more images is a next step of union of several rectified images to have a unique big image of high quality. It may be necessary if the surface is very large and with a single frame would be less detailed, or it is impossible to photograph it entirely due to physical obstacles. Panoramic photos or multi-images are obtained through the seam of images taken in parallel or rotating the camera around the focal point of the lens (sphere). It serves to overcome the limitations of the sensor/lens used.
Perspective: Representation method theorized in the Renaissance where the projection of the object on the picture plane ( quadro ) occurs from a central point of view. The different types of perspective are defined by the mutual position of observer, object and picture plane. It is the representation most similar to human vision and photography.
Quadratura: Painting genre that had the widest dissemination in the Baroque period and that, taking advantage of the rules of descriptive geometry, involves the decoration of wall surfaces with fake architectural apparatus drawn in perspective. It is a category of trompe l'oeil , or “trick the eye”. The similar term sfondato recalls a type of quadratura in which an architectural frame is open toward a landscape or the sky, animated or not with human figures.
IBM Image-Based Modelling, SfM Structure from Motion: Photography-based techniques cover a wide range of topics: animation, inverse lighting, light field rendering, layered depth images, etc. The acronym SfM indicates modelling techniques through automatized algorithms that use the principles of solid photogrammetry to process photographic datasets shot by moving the camera, and are able to return a dense point cloud, sometimes a textured mesh, of the object. From the sequence of frames, the algorithm obtains the parameters of internal and external orientation of the photo taken. Possible to use commercial photo cameras, software are often free.