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Dynamic Perception of Transparency In Architecture: Mechanisms of the Proper Recognition of Light Permeable Surfaces

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Page 1: Dynamic Perception of Transparency In Architecture: Mechanisms of the Proper Recognition of Light Permeable Surfaces

COMPREHENSIVEPSYCHOLOGY

Comprehensive Psychology is an Open Access peer-reviewed publication and operates under the CC-BY-NC-ND Creative Commons License. The Author(s) retains copyright to this article and all accompanying intellectual property rights.

Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.

NonCommercial — You may not use the material for commercial purposes.

NoDerivatives — If you remix, transform, or build upon the material, you may not distribute the modified material.’

Additional Information about the terms and conditions of this Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Public License can be found at www.CreativeCommons.org

ORIGINALITY | CREATIVITY | UNDERSTANDING

www.AmSci.com

Comprehensive Psychology

Page 2: Dynamic Perception of Transparency In Architecture: Mechanisms of the Proper Recognition of Light Permeable Surfaces

Dynamic perception of transparency in architecture: mechanisms of the proper recognition of light

permeable surfaces1

Marcin Brzezicki

Wroclaw University of Technology, Poland

AbstractThis paper examines the issue of transparency recognition in architecture from a dynamically changing point of view. A simple and intuitive pictorial model of the transparent surface appearance is presented, followed by a discussion of the transparency processing (mid- or high-level). Two zones of perception that determine the proper perception of light-permeable surfaces are distinguished, each with their own phenomena responsible for the formation of transparency perception cues (transmittance/absorbance or reflectance). The main visual pro-cessing mechanisms are discussed, and some parallels to machine vision are found. Finally, a short checklist is presented to provide practical advice for ar-chitects dealing with large surfaces of glass in their projects.

Contemporary architectural transparency seems to go beyond all previously formulat-ed definitions. It involves new materials and technologies, as well as new ways to assess the visual perception of architecture. With the introduction of the study of the dynamic perception of architecture, previously neglected qualities of transparent materials (e.g., reflectance and refraction) have become important factors in the perception of space, and these factors are processed on different levels in the recognition of transparent ma-terials. The approach presented here stems from architecture-based experience, but is approached from the perspective of optics and vision science. The general principles are illustrated via diagrams and individual case studies.

It must be noted that the issue known as motion transparency, where “two overlap-ping surfaces move transparently over each other” (Snowden & Verstraten, 1999, p. 369), in other sources defined as “perception of more than one velocity field” (Qian, An-dersen, & Adelson, 1994, p. 7357) is beyond the scope of this paper (Stoner, Albright, & Ramachandran, 1990).

Optics of TransparencyTransparent materials are perceived differently than opaque materials. In the case of opaque objects, the phenomenon of occlusion occurs: “visible contours of the more dis-tant object terminate at the outer boundary of the nearer one” (Wilson & Keil, 1999, p. 844). Panes—large flat thin surfaces—of transparent materials are usually so smooth that the luminous fluxes that strike them are partially reflected, partially transmitted, and partially absorbed (absorption/transmission are linked). Each transparent object absorbs part of the luminous flux energy, but simultaneously also generates specular re-flections upon its surface. The optical model that approaches transparency from the po-sition of reflected vs transmitted luminous flux balance (meaning: real and virtual im-ages con-fused) seems to be sufficient and appropriate in consideration of optical cues distinctive for the visual system in the kinetic perception of transparency. A real/virtual image model is adopted for the sake of simplicity, as—due to the scale of the pane in ar-chitecture—other phenomena like “total internal reflection, and wavelength related ef-fects (chromatic aberrations) caused by the refractive index of the object” (Ben-Ezra & Nayar, 2003, p. 1025) are invisible for human observers. A more complex and more ac-curate perforated model has been recently developed by the author, but seems to be re-dundant in the scope of current research (Brzezicki, 2013).

Ammons Scientificwww.AmmonsScientific.com

COMPREHENSIVE PSYCHOLOGY2014, Volume 3, Article 7ISSN 2165-2228

DOI: 10.2466/24.CP.3.7© Marcin Brzezicki 2014Attribution-NonCommercial- NoDerivs CC-BY-NC-ND

Received November 11, 2013Accepted April 22, 2014Published May 15, 2014

CITATION Brzezicki, M. (2014) Dynamic

perception of transparency in architecture: mechanisms of the proper recognition of light permeable surfaces. Comprehensive Psychology, 3, 7.

1Address correspondence to Marcin Brzezicki, Prusa 53/55, 50-317 Wroclaw, Poland or e-mail ([email protected]).

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The condition of motion and modification of the ob-server's point of view affects the resulting con-fused im-age, as the pane's parameters change dynamically ac-cording to the rules described by Fresnel's equations (Westin, et al., 1992). In the practice of observation, a smooth light-transmitting pane is generally believed to be seen “through reflection from its surface” (Patterson, 2011, p. 31) and glass is considered to be “primarily a re-flective material” (Elsener, 2005, p. 152), but this varies according to the observation angle. As within 45° from normal (a line perpendicular to the pane), reflectance is more or less constant, the transmittance/absorbance phenomena produce conclusive cues in the perception of transparency. When the threshold of approx. 45° is exceeded, the influence of virtual images rises. The rea-son for this is the reflectance “increasing dramatically above 70°, to almost 100% reflection at glancing angle” (Wigginton, 1996, p. 71). Thus, cues seem to be differ-ent: depending on the prevailing optical phenomena, two perception zones are created (see Fig. 1). Specular reflection and the resulting virtual image become con-clusive for the visual system to recognize transparen-cy unequivocally for viewing angles exceeding 45°. The condition of motion influences the mutual change of both parameters affecting the retinal image produced.

All surfaces making up the observer's environment (distal stimuli) are transformed by the optical and neu-ral systems of the observer's eye into proximal stimuli. Processing of these stimuli is divided into “three basic stages” (Wilson & Keil, 1999, p. lvii), which could be re-

ferred to as levels. The analysis of sensory input “oc-curs in steps, each of them processing the information received from the previous level” (Lindsay & Norman, 1977, p. 136). The initial stage of perception is sensation, which takes place in the data reception phase called low-level processing. A “coherent representation of sur-faces and materials” (Anderson, 2011, p. R978) is estab-lished at the second stage of processing, also known as mid-level processing. The third stage in the process of stimulus processing—high-level—is perception, i.e., identifying actual objects in the observer's environment. It involves the most complex cognitive processes and many high-level mechanisms, due to the constant flow of information between the mid- and high-level stages. The processes appear to be organized as “a set of recur-rent loops, not a simple linear chain” (Anderson, 2011, p. R978). Assigning transparency to the surface is “often considered to be a higher-level visual process that (…) utilizes stereo and motion information to separate the transparent from the opaque parts” (Kersten, Bülthoff, Schwartz, & Kurtz, 1992, p. 573).

The perception of transparency is based “upon con-text to determine the most likely interpretation.” (Wil-son & Keil, 1999, p. 845). In most standard, uncompli-cated situations, perceiving transparency in architecture occurs at the mid-level stage and is based on transmit-tance/absorbance, as every light-transmitting object “perturbs the visibility of the underlying layer” (Ander-son, Singh, & O'Vari, 2008, p. 1146). The visual system relies upon the changes in luminance of the background caused by transmittance/absorbance of the pane and the particular configurations of adjacent regions of the retinal image. Since retinal images are not unambigu-ous and the same image can be interpreted in numerous ways (e.g., see Anderson, 2003, p. 793), the visual input unrecognized at the earlier stages is processed further. Should any dubious circumstances occur, it is necessary for the visual system to upgrade to high-level process-ing, when “identification and classification on the basis of previous experience” is carried out (Wilson & Keil, 1999, p. 64). In the context of transparency perception, the basic difference between mid- and high-level cog-nitive processes is believed to lie in the processing of optical phenomena occurring at the interface of materi-als with different refractive indices: specular reflection and light ray refraction (Ben-Ezra & Nayar, 2003). In the absence of basic transmittance-based cues, other opti-cal phenomena exist that could be utilized by the visual system. These include reflectance and resulting specu-lar highlights.

Transmittance-based Kinetic Perception and Occlusion

The visual system recognizes transparency as a “special case of superposition” (Arnheim, 1971, p. 257). As a re-sult of a ray of light being transmitted through the trans-

observer O2

observer O1

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reflectancezone

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tran

spar

ent p

ane

a sh

eet o

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anda

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Fig. 1. Perception zones: transmittance zone within the 45° angle from viewing axis, and reflectance zone beyond the 45° threshold. In general, transmittance remains conclusive for frontal observation (observer O1), while reflectance prevails for viewing angles exceeding 45° (observer O2). Schematic drawing by the author.

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parent pane, both the luminance and the contrast of the image behind it change. Since a homogenous transpar-ent pane absorbs a constant amount of light, the lumi-nance of the obscured part of the image changes at the same rate within its entire area: various regions of the background are attenuated to the same extent, creat-ing junctions at the edges, comprehensively classified by Kanizsa (e.g., X-junction: Kanisza, 1979). Based on specific cues—the unique relation between intensities of neighboring regions of the retinal image—a scission process is performed by the visual system, causing the transparent layer and the background to become spa-tially discriminated (Metelli, 1974). As a result, depth can be recognized: two planes—“one transparent and one, i.e., located in the background” (Anderson, 2003, p. 794). In transparency studies, since the beginning of the 20th century, this mechanism has been called percep-tual decomposition (Metelli, 1974; Kepes, 1995). Research “suggests that the brain, in some sense, ‘understands' the physics of transparency” (Anderson, 2011, p. 980). A vital discussion is still taking place on the issue of what is the “critical image variables in order to assign surface attributes to transparent layers” (Singh & Anderson, 2002, p. 533); or “what it is that the brain understands” (Anderson, 2011, p. R980). The first mathematical mod-el of this process was presented by F. Metelli (Metelli, 1974) in 1974, but was later questioned by numerous researchers. This yet unresolved problem continues to present a challenge (Masin, Tommasi, & Da Pos, 2007; Masin, 2011), but lies beyond the scope of this paper.

Kinetic occlusion occurs “when nearby objects hide (or reveal) others beyond them as a result of observer or object motion” (Wilson & Keil, 1999, p. 255). In the case of superposition of transparent objects perceived in motion, elements of the see-through background are systematically attenuated by the occluding transpar-ent object in the sequence, resulting from the direction of motion of the observer or the object. This motion af-fects the point of view and, according to the laws of perspective projection, creates an apparent mutual dis-placement of observed objects within the field of view (Norman, Todd, & Phillips, 1995). This process is called ‘structure from motion’ (SfM; Loomis & Eby, 1989) and describes the human ability “to perceive the 3-D shapes of moving objects on the basis of a succession of 2-D projections” (Loomis & Eby, 1988, p. 383) on the retina. If the elements of the background are not attenuated in the right sequence (as in parallax), the loss of the notion of transparency is inevitable, as the visual system tends to recognize the scene as flat, rather than 3-dimension-al (no scission). Experiments with various grey-level patches were previously reported as producing the illu-sion of “perceptual transparency” (Singh & Anderson, 2002, p. 532); see Fig. 2.

The change in luminance of the background be-hind the transparent pane is the main indicator of light

transmission; however, as R. Arnheim observed, “if the shape of physically transparent surface coincides with the shape of the ground, no transparency is seen” (Arn-heim, 1971, p. 258) , or “in order to recognize any figure, one must first perceive the background, from which this figure emerges” (Dominiczak, 2003, p. 57). This condi-tion seems rare, but is in fact common in the practice of architecture observation. Large smooth panes often fill the entire field of view, especially in the proximity of buildings. The transparent panes in shopping win-dows, doors, and freestanding screens might serve as a good example of this. The evaluation of transparency is very difficult, especially within a 45° angle from nor-mal. The lack of an unoccluded reference surface and the absence of a virtual image causes the entire pane to ‘vanish.’ The glass simply ceases to be noticed by the observer (see Fig. 3). Only the observer's motion, fol-lowed by the change in point of view, could provide in-formation about the reference area or the virtual image, and contribute to the correct recognition of its trans-parency. This is the main reason why, in certain coun-tries, there are building regulations that oblige a build-ing's owners to put clear visual manifestations “at two heights, 850–1000 mm and 1400–1600 mm above the fin-ished floor level” (Glazing – safety in relation…, 2000, p. 6) on glazed doors and other transparent surfaces. This is crucial, especially in the case of transparent sur-faces arranged in large screens, with edges located out-side of the observer's field of view.

In high-level processing, the visual system uses a number of comparative mechanisms to identify trans-parent surfaces. The most important mechanisms are those that perform content analysis of the retinal im-age; e.g., detect alterations of the image transmitted through the pane. In case of distortion, image identifica-tion is performed in the final stages of perceptual clas-sification of the observed object. It is at this stage that “sensory data is confronted with mental data” (Necka, Orzechowski, & Szymura, 2006, p. 279); i.e., with the mental representation of the object retained in the ob-server's memory. Two existing theories, the template (e.g., see Bülthoff & Edelman, 1992) and the feature-based theory (e.g., Lindsay & Norman, 1977) lie beyond the scope of this paper.

Light ray refraction is usually caused by an un-evenness in the transparent pane's relative thickness (pane defect) or a deliberately figured pane structure (e.g. in ornamental glazing etc.). As “transparent ob-ject does not have features of its own” it simply maps “features that exist in its environment onto the image plane” (Ben-Ezra & Nayar, 2003, p. 1026). Thus, refrac-tion-based perception of transparency is the result of in-terpreting a sequence of distorted images of the back-ground, observed through light-refracting media. This sequence results from the observer's or object's motion. A single (static) distorted image is insufficient, as the

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pattern front view

top view

H H H

H H H

illus

ion

oftr

ansp

aren

cyge

nuin

etr

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aren

cy

checkerboard plate

transparent plate

O3

O2

O1

O1 oberver’s view O2 oberver’s view O3 oberver’s view

O1 oberver’s view O2 oberver’s view O3 oberver’s view

Fig. 2. Dynamic perception destroys the illusion of transparency produced by various correctly arranged grey-level patches, as the background attenuation does not correspond to the expected motion parallax. Schematic drawing by the author.

Fig. 3. High transmittance (low absorbance) and frontal perception exclude all possible cues for the perception of transparency. The entire glazed wall seemingly vanishes (A). The perpendicular pane (the same type of glass) is thus perceived based on an almost exclusively virtual image (B). Case study: Conservatories of Cité des sciences et de l'industrie in Paris, France (A. Fainsil-ber & P. Rice, 1986). Photo by the author.

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apparent distortion could have been an intrinsic feature of the background. Along with the processing of the distorted background, a hypothesis is made that there exists a transparent medium generating this distortion, and—most notably—that its physical form is being re-constructed simultaneously (e.g., negative or positive lens). The mechanism of this activity is complicated, but was recently modeled as part of the research concerning machine vision (Ben-Ezra & Nayar, 2003). It is assumed that the visual system relies on the observation that all rays associated with a single feature of the background “are parallel to each other before they are scattered [re-fracted] by the transparent object” (Ben-Ezra & Nayar, 2003, p. 1027). This allows the visual system to recon-struct the shape of the refracting object by measuring the parallelism of perceived rays. Simply put, the man-ner in which the background changes confirms the exis-tence of transparent media.

Image distortions occur in both the real image trans-mitted through the pane (background distortions – see above) and the virtual image (specular image distor-tions – see below). In both cases, the transparency per-ception mechanisms based on deformation are similar and lead to the awareness of the presence of a medium that refracts rays of light.

Reflectance-based Kinetic PerceptionMaterials with rough surfaces bounce the light off mul-tidirectionally—reflected light is scattered due to the unevenness of the surface. Smooth materials interact with light in a different manner: they reflect rays of light unidirectionally, in a process known as specular reflection (speculus in Latin meaning: mirror). A single incident ray of light is “reflected into a single outgoing direction” (Shah, 2007, p. 43) resulting in the occurrence of a laterally inverted virtual image. The visual system relies on virtual images in the process of identifying smooth (glossy) objects.

In the case of transparent objects, the real image transmitted through the pane is overlaid with the vir-tual image reflected off the pane. Flat panes generate specular (mirror-like) reflections, while curved shaped glossy objects generate highlights. Thus, select areas of the retinal image are dominated by the real or the vir-tual image, while other areas will form “an inextricable blend of the two” (Downing, Liu, & Kanwisher, 2001, p. 1334). These distinctive features of the retinal image are not strong enough to ensure the recognition of trans-parency in stasis, as what seems to be the virtual/real image superposition could have been a pattern applied onto the object (Nayar, Ikeuchi, & Kanade, 1989). In the condition of motion, the virtual reflected image and the real transmitted image would shift in relation to each other and according to the observer's displacement (see Fig. 4). The individual ratio of this shift would depend only on the object-pane distance, regardless whether

the image is real or virtual (although the latter is later-ally inverted).

Kinetic changes of the pane's parameters can also lead to an unequivocal interpretation of incoming su-perposed images. According to the previously men-tioned Fresnel's equations, the rays reflected off the sur-face increase with the rise of the observation angle. In the condition of motion, the ratio of superposed im-ages visibly changes, as “reflection and depth are the two poles of the perception spectrum of glass” (Elsener, 2005, p. 151). A gradual transition of the pane from re-flection to transmission serves as a good example of this. When the observation of a pane begins at an oblique an-gle, the pane appears to be reflective; from this point of view, the observer cannot tell the difference between a mirror and a transparent surface, as both look alike (left panel of Fig. 5). As the angle of viewing decreas-es, reflection is reduced and the transparent quality of the pane progressively appears (right panel of Fig. 5). In this case, transparency cannot be properly recognized through static perception; a change of the viewpoint is necessary to establish whether the pane is a flat mirror or a transparent surface. Without motion on part of the observer, proper identification would not be possible.

In the case of dynamic transparency perception, a segregation of superposed images is vital to perform the scission and achieve spatial discrimination. In 1999,

O3 oberver’s view O2oberver’sview

O1oberver’sview

real tree

real house

virtual tree

transparent pane

I2

I1

I2´

O3 O2 O1

Fig. 4. Both virtual and real images would overlap and shift relative to each other and according to the observer's motion. Observer O3 sees the tree on the left side whereas the observer O2 sees the tree on the right side of the house.

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an interesting experiment was performed by Downing, et al., in which two images were superposed, so that they were “transparently overlapping at the same lo-cation” (Downing, et al., 2001, p. 1333). One of the im-ages was dynamic, while the other remained static, thus achieving an effect similar to a real life kinetic sce-nario. The observer was to direct his attention to one of the stimuli, thus activating the appropriate areas in the temporal cortex: FFA (Fusiform Face Area) when ob-serving faces, or PPA (Parahippocampal Place Area) when observing buildings. The experiment shows that the vi-sual system is able to perform a mental switch from re-ceiving one type of information to another, thus making the spatial discrimination of surfaces possible, in order to perceive transparency correctly. According to the au-thors, the process of image segregation proceeds as fol-lows: the object (real or virtual), being in the focus of attention, is recognized via high-level processing and the information underlining the features correspond-ing to the recognized object goes back to low-level pro-cessing (where accommodation occurs). The top-down processes guide attention in such a way as to “allow selection and grouping of the unambiguous features” (Downing, et al., 2001, p. 1334) of the objects fitting the already-recognized object, based on a pre-existing men-tal representation in the visual system. This exchange of information is performed repeatedly, until proper rec-ognition is achieved.

Virtual images of the environment that appear on smooth surfaces are used by the visual system as a cue for recognizing the type of material: e.g., the transparen-cy. As specular reflections (virtual images) are “simply images of the environment (…) distorted by the geome-try of the reflecting surface” (Anderson, 2011, p. 980), the

virtual image can be perceived and processed just like a real image would be. For smooth surfaces, “the light re-flected toward the point of observation (…) produces the appearance of specular highlights” (Norman, et al., 1995, p. 629). Within the retinal image, these highlights—areas of higher luminance—are distinctly different than the observed object. When curved smooth objects are con-sidered, the image contained within these particular ar-eas is difficult to process due to the significant distortion of the image. In return, their location alone can consti-tute an important cue for the visual system, as the high-lights make it easier to build a three-dimensional mental model of the object. The highlights also “affect observ-ers' judgment of surface gloss” (Blake & Bülthoff, 1990, p. 165; Blake & Bülthoff, 1991, p. 245), making it easi-er not only to build a three-dimensional mental mod-el of the object (see Veeraraghavan, Tuzel, & Agrawal, 2010), but also to “provide a general constraint for the estimation of material properties” (Motoyoshi, 2010, p. 8). It seems reasonable to theorize that transparency rec-ognition is the result of the perception of highlights ac-cording to the mechanism, where shine and gloss equal transparency. As already discussed, the particular “rela-tionship between specular highlights and non-specular shading patterns is a robust cue for the perceived trans-lucency and transparency of three-dimensional objects” (Motoyoshi, 2010, p. 1). In the condition of motion, these specular highlights “tend to move very rapidly over rel-atively flat regions of a surface and to cling more stably in regions of high curvature” (Norman, et al., 1995, p. 629). This velocity of the highlight's displacement might be used by the visual system to reconstruct the shape and the spatial orientation of an object (Blake & Bül-thoff, 1991), based on highlights only “in the absence of

Fig. 5. From an oblique point of view, the observer cannot tell the difference between a mirror and a transparent surface, as both look alike (left). With the change in viewpoint, the transparent quality of the pane progressively appears (right). Case study: Tetragon Office Building in Frankfurt, Germany (Bundschuh/Baumhauer Gesellschaft von Architekten mbH, 2003). Photo by the author.

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any identifiable features.” (Norman, et al., 1995, p. 635). Highlights, especially during the nighttime, serve as a relatively reliable cue for the assessment of the type of material in architecture (see Fig. 6).

ConclusionsDynamic perception is essential for the proper percep-tion of transparency. It enables the visual system to col-lect more data before the recognition is concluded. Due to the ambiguity of the retinal image, kinetic perception is required to distinguish whether the object perceived is a flat pattern (e.g., patch composition) or a bona fide transparent element. Motion is not crucial for the scis-sion process to begin, but facilitates the detection of any potential illusions.

In architectural scale, two vital zones of perception are created, each with its distinct phenomena respon-sible for the formation of transparency perception cues: absorbance/transmittance and ray refraction in fron-tal observation, reflectance and its local changes (high-lights) at oblique viewing angles. As the observer of architecture actively moves during perception, the phe-nomena described should be carefully studied if the ar-chitect's intent is to design a truly transparent and safe building. The understanding of the mechanism ana-lyzed above helps to determine the circumstances in which a building would be perceived and recognized according to the designer's intentions.

The following checklist should serve as practical ad-vice for designers dealing with large surfaces of trans-parent materials:

1. Try to assess the luminous fluxes that will affect the transparent panes in the building, both during daytime and nighttime. Consider the sun path, backlighting, and reflections from surrounding buildings. Create a light map, depicting the pos-sible shaded and well illuminated areas.

2. Study different points of view, as many as possi-ble, equivalent to real observers' viewpoints. Con-sider any possible prevailing optical phenomena that could potentially influence perception.

3. Identify the areas from which the building's transparent surfaces are perceived at oblique an-gles, and where frontal observation occurs. An-ticipate any possible perceptual mistakes that could generate a potential hazard, advocate for application of proper manifestations on transpar-ent panes.

4. Try to visit the site. L. Mies van der Rohe ana-lyzed glazing appearance using small scale mod-els of buildings exhibited on the building site it-self (Colomina, 2009, p. 82). Only in this way can real luminous fluxes and the resulting reflections of the surroundings be observed.

Fig. 6. Visible connection of gloss and transparency (translucency). A visible relation of specular highlights and non-specular shading patterns. A glossy, shining surface that communicates the transparent type of material during the nighttime. Case study: Kunsthaus in Graz, Austria (P. Cook & C. Fournier, 2003). Photo by the author.

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