Quantifying Spatial Openness and Visual Perception in Historic Urban Environments

Abstract

With accelerating urbanization, the preservation and adaptive renewal of historic urban environments have emerged as critical challenges in the field of urban science. Among various morphological attributes, spatial openness plays a fundamental role in shaping visual perception and influencing human well-being, but remains insufficiently examined within the context of historic streetscapes. This study investigates the spatial configuration of Tangchang Ancient Town in Chengdu, China, to elucidate the relationship between spatial openness and perceptual responses. A mixed-methods approach was employed, integrating semantic differential (SD) surveys with a suite of spatial analysis techniques, including GIS-based viewshed analysis, depth-to-height ratios, building density, and street curvature metrics. The empirical findings reveal that increased spatial openness is positively associated with visual comfort, while reduced openness contributes to a heightened sense of enclosure and psychological stress. Mediating factors, such as sky visibility and natural lighting conditions, were identified as significant, with elevation angle and curvature further enriching the explanatory framework. Drawing on these insights, this study proposes a set of context-sensitive spatial design strategies tailored to varying degrees of openness. These include enhancing vertical openness through building form regulation, improving lighting and sky access, integrating vegetation more effectively, and activating corner spaces to support spatial legibility and visual interest. This research contributes to the growing discourse on evidence-based urban design by linking quantifiable spatial parameters with perceptual and affective outcomes. The proposed framework offers practical guidance for the sustainable conservation and transformation of historic urban areas undergoing contemporary urbanization pressures.

1. Introduction

Historic alleyways are an integral part of traditional urban forms, and serve as crucial bearers of cultural identity and historical continuity. Rapid urban regeneration, however, presents serious obstacles to their preservation because of these areas’ complicated shapes and limited spatial capacity [1,2,3]. Historic alleyways, in contrast to contemporary streets, produce unique visual experiences that are influenced by their intimate character, enclosed perspectives, and small scale [4]. Therefore, the interaction between spatial morphology (such as building height, openness, and enclosure) and human perceptual responses must be considered in efforts to conserve historic alleyways. The inherent connection between spatial openness and visual perception is particularly noteworthy because it serves as a link between an alleyway’s morphological characteristics and users’ psychological reactions, in addition to having a direct impact on the quality of users’ experiences. It is crucial to comprehend this relationship because it clarifies how physical arrangements influence human experience and offers a theoretical understanding of environmental psychology and spatial cognition. In practice, this information guides sustainable urban renewal plans and context-sensitive design, which helps strike a balance between the protection of cultural assets and modern urban demands.

Spatial openness has long been recognized as a critical factor connecting urban morphology and environmental performance, influencing both microclimatic comfort and visual perception [5,6]. The majority of research, however, has focused on these aspects independently, largely ignoring the interrelated processes of perception and openness. Certain spatial characteristics are captured by traditional measures like the sky view factor and Depth-to-Height (D/H) ratio [7,8,9], but they do not take into consideration multifaceted factors that influence perceived openness, such as building density and road curvature. Furthermore, although indexes, such as Ashihara’s D/H ratio [10] and the sky openness index [11,12,13], have impacted urban design methodologies, they usually overlook the complex psychological responses that take place in diverse spatial contexts. This lack of integrative analysis has created a research gap that limits both theoretical understanding and practical recommendations in heritage conservation.

Recently, GIS-based viewshed analysis has emerged as a robust tool for quantifying spatial openness in urban environments. When combined to traditional metrics such as the D/H ratio, and supplemented by variables including building density (BD) and the street curvature index (SCI), it enables the development of a more comprehensive framework for assessing spatial openness [14]. Notably, BD and SCI have been shown to influence walkability and perceptual outcomes through their interaction with other environmental factors [15,16,17,18,19]. However, these interaction effects remain underexplored in the specific context of historic alleyways.

In order to bridge this gap, this study investigates the interdependent relationship between visual perception and spatial openness in historic alleyways, promoting dialog among heritage studies, urban morphology, and environmental psychology. In order to create an expanded spatial openness model, a hybrid methodology is used: (1) Semantic differential (SD) surveys are used to measure perceptual responses. This method captures the complex spatial forms and lived experiences of historic alleyways more effectively, linking objective morphological indicators with subjective perceptual evaluations in real urban contexts. (2) GIS-based viewshed analysis is combined with traditional indices (D/H ratio, building density, and street curvature index). (3) Kendall’s τ correlation is applied to examine the association between spatial openness and visual perception. (4) Tangchang Ancient Town in Chengdu, China, was chosen as the case study. The town retains characteristic narrow historic alleys and a distinctive visual environment, making it an appropriate setting to examine the interactive relationship between spatial form and visual perception in historic urban contexts.

In addition to providing a more thorough explanation of how spatial openness influences perceptual experience, this integrated paradigm offers practical insights for the sustainable conservation and revitalization of historic alleyways.

2. Overview of the Study Area

Tangchang Ancient Town, a historic locale in Chengdu, has been selected as the focus of this research. Situated in Pidu District, Sichuan Province, at the intersection of Pidu, Dujiangyan, and Pengzhou, Tangchang is renowned for its long history and well-preserved ecological environment [20]. With relatively limited exposure and underdeveloped infrastructure, it remains the only existing case in the Chengdu Plain that preserves the characteristic the traditional “cross-axis layout “of wide streets and narrow alleys.

The town contains a high density of ancient alleyways, and its spatial configuration, defined by courtyard compounds and narrow alleys measuring 2–3.5 m in width, differs from that of other ancient towns, which usually feature commercial buildings along wider streets ranging from 4 to 6 m. Moreover, the spatial openness of Tangchang’s alleyways is markedly distinct from that of ot3her historic towns, offering visitors a unique visual perception during their exploration (Figure 1).

With the growing emphasis on urban renewal, the conservation and enhancement of Tangchang’s extensive alleyway network, together with strategies to attract tourism, have become key objectives in regional development. Effective protection of these ancient streets, however, requires more than improvements to physical infrastructure; it must also ensure the preservation of their distinctive visual qualities.

Hence, examining how spatial openness shapes the visual perception of Tangchang’s alleyways is essential for achieving a balance between economic development and the conservation of traditional streetscapes. This understanding can provide critical guidance for the strategic revitalization of historic urban environments.

This study selected Dachun Alley, Xinan Alley, and Wenchang Alley in Tangchang Town as representative cases. Both Dachun Alley and Wenchang Alley run parallel to each other and intersect with the West Street, while Xinan Alley connects South Street and Little South Street, historically known as the Xinan Horse Path (

Table 1. Ancient alleyways statistics [21].

Alleys	Placement	Specificities	Length (m)	Width (m)
DaChun Alley	Intersection with West Street	lit. a winding path leads an apparition	192	2~3
Wenchanggong Alley	Parallel to Toon alley, Intersection with West Street	Folk houses are simple and rustic	187	1.5~2.5
Xinan Alley	Connecting South Street with Little South Street Intersected by Wenchanggong alley	Anciently known as the Xinan Horse Road, the street is lined with small shops for horseback riding	180	3~5

). To isolate the effects of spatial openness on visual perception, extraneous visual elements were controlled in the site selection. Depth, shape, color, and motion are recognized as the four fundamental attributes of visual perception [21]. Field surveys revealed minimal differences in color among the three alleys. The surfaces were primarily composed of gray bricks, with painted wood and gray tiles as secondary materials, resulting in a relatively uniform color perception. In terms of spatial morphology, all three alleys exhibited a longitudinal strip-like configuration with relatively narrow spaces. Regarding depth and motion, all three alleys measured approximately 190 m in length, with minimal spatial variation. Therefore, these three alleys were chosen to enable a precise analysis of spatial openness as the primary variable (Figure 2).

3. Methodology

The first step of this study was to apply the SD method to design a visual perception evaluation, which examined individuals’ visual experiences in ancient alleyways. Within the framework of the urban panoramic spatial openness [22], this study combined GIS line-of-sight analysis and the D/H ratio to quantify spatial openness. After combining qualitative and quantitative data, Kendall’s τ correlation model was used to investigate the relationship between spatial openness and visual perception, as well as the underlying factors that contribute to it (Figure 3).

3.1. Visual Perception Evaluation Methods

3.1.1. Visual Perception Evaluation Based on the SD Method

This study employs the SD method to evaluate the visual perception of ancient alleys in Tangchang Town. A set of adjective pairs that reflect the characteristics of the sample environment is developed, and participants rated these adjectives to quantify visual perception in spaces with different degrees of openness. This approach allows for the analysis of differences in visual perception attributes among various alley spaces.

The SD method, introduced by C.E. Osgood in 1957, is a psychological measurement technique designed for semantic analysis. This method utilizes linguistic scales in psychological experiments, allowing researchers to quantify and analyze the concepts and structures of subjects based on predetermined rating scales [23]. In such studies, the SD method primarily expresses subjective perceptions and experiences of environmental content. It typically employs pairs of antonymous adjectives as the two endpoints of an evaluation scale, presented on an odd-numbered rating scale [24].

3.1.2. Survey Questionnaire

To investigate the influence of spatial openness on visual perception in historic alleyways, a structured questionnaire survey and SD evaluation were conducted. The questionnaire was designed based on environmental perception theory and previous studies on urban heritage spaces [25,26]. Three representative alleyways were selected for this study: Dachun Alley, Xinan Alley, and Wenchang Alley. Two representative photographs were chosen from each alleyway to capture typical views for evaluation.

In total, 100 valid responses were collected from the three alleyways. Respondents aged between 20 and 50 years and included both local residents and tourists. The survey was conducted between January and March 2024 through a combination of on-site face-to-face interviews (50 participants) and online distribution via the WeChat platform (50 participants).

During the evaluation, participants engaged in two modes: those surveyed on-site conducted direct observations of the alleyways, while those participating online viewed the representative photographs of these alleyways. In both cases, participants were subsequently asked to complete an SD evaluation form.

The survey comprised three sections:

• Demographic information, including age, gender, residency status, and length of stay;
• Perceptions of spatial openness and related environmental attributes, such as sky visibility, building density, sense of interest, and sense of hierarchy, evaluated on a seven-point Likert scale (−3 = very low, 3 = very high) (

  Table 2. SD evaluation: seven-level evaluation scale.

  Very Poor	Poor	Below Average	Neutral	Good	Very Good	Excellent
  −3	−2	−1	0	1	2	3

  );
• Overall assessments of comfort, historical atmosphere, and spatial preference.

Each question was designed to correspond to the quantitative indicators of the spatial openness model (

Table 3. Evaluation of project and adjective pairs.

Evaluation Projects	Adjective-to-Adjective Ratio
Sense of Scale	Unreasonable scale–Reasonable scale
Spatial Continuity	Discontinuous–Continuous
Sense of Depression	Oppressive–Non-oppressive
Space Atmosphere	Noisy–Tranquil
Spatial Openness	Cramped–Spacious
Sense of Hierarchy	Blurred Hierarchy–Clear Hierarchy
Visibility of the sky	Overexposed–Shaded
Landscape Uniqueness	Common Landscape–Unique Landscape
Landscape Diversity	Monotonous Landscape–Diverse Landscape
Sense of Interest	Uninteresting–Interesting
Attractiveness	Unattractive–Attractive

). For example, questions on sun exposure relate to the sky view factor, while questions on spatial openness and sense of scale correspond to the D/H ratio. This structured design ensured that perceptual data could be applied to validate the statistical model.

3.2. Analysis of Spatial Openness of Alleyways

Prior studies mainly explore the relationship between spatial openness and building height to improve urban streetscapes and residential comfort, often using the D/H ratio proposed by Yoshinobu Ashihara as a key metric. Experimental findings show that a D/H ratio of 1 is generally optimal, and confirm that spatial openness substantially influences visual perception [26]. However, the D/H ratio relies heavily on expert judgment and applies only when building heights on both sides are uniform, which limits its accuracy in representing actual spatial openness. To overcome these limitations, researchers have increasingly adopted supplementary techniques such as virtual reality simulations, visual assessments, and GIS line-of-sight analysis. Despite these advances, existing approaches still fail to capture the complex interplay between openness and the nuanced psychological and physiological responses it generates in diverse urban contexts. To fill this gap, this present study develops an integrated framework that combines SD analysis and advanced spatial metrics, including GIS viewshed analysis, D/H ratios, building density (BD), and street curvature index (SCI), to comprehensively assess spatial openness and its perceptual impacts.

GIS have become essential tools in urban planning, commonly using digital elevation models (DEMs) to analyze spatial openness. Viewshed analysis, line-of-sight analysis, and visibility analysis are among the various spatial analytical tools available in ArcGIS 10.4. However, these tools cannot fully replicate the actual field of vision perceived by the human eye. To address this issue, Perry Yang introduces a virtual hemispherical model to calculate urban spatial openness [27]. Zhang Xia et al. develop a Human Visual Openness Index (HVOI) based on 3D GIS to assess spatial openness [28]. Du Xingyu’s proposes the concept of urban panoramic spatial openness [14], which is based on GIS line-of-sight analysis and includes panoramic urban perspective, area openness, and volumetric openness, providing a conceptual foundation for assessing actual spatial openness in this study [22]. Among these, area openness and volumetric openness serves as the two core quantitative indicators for evaluating the spatial openness of ancient alleyways. Specifically, area openness measures to the extent to which urban space provides an unobstructed view for observers, while volumetric openness represents the proportion of visible spatial volume within a given urban area [14].

In recent years, scholars increasingly adopt interdisciplinary approaches to evaluate spatial openness in street environments, advancing research depth and providing quantitative foundations for this study’s framework. Building on the concept of panoramic spatial openness, this research constructs detailed street models in GIS and CityEngine to calculate actual spatial openness with precision. The study then applies the optimal D/H ratio to assess spatial experience and incorporates BD and SCI as supplementary metrics to comprehensively evaluate spatial openness. This approach aims to clarify the relationships among spatial openness, BD, SCI, and visual perception.

3.2.1. Modeling the Spatial Openness of Alleyways

A cylindrical viewing surface with a 200 m radius was employed as a supporting tool for measure spatial openness in observed alleyways, in accordance with the results of earlier investigations [14,22]. In this study viewing surfaces were constructed in the GIS and City Engine for the three alleys, with the midpoint of each alley serving as the observer’s reference position. With a 30° vertical viewing angle, the cylindrical viewing surface at a 200 m distance was modeled to reach 167.8 m in height. A 7 × 7 m grid was applied to calculate the area-based spatial openness of the alleyways. Furthermore, a conical viewing grid model was implemented to compute the volumetric openness of the visible space.

This study quantifies spatial openness using ArcGIS line-of-sight analysis. Two indicators are defined: area openness and volume openness. Area openness is defined as the ratio of visible area to the total cylindrical façade area. Volume openness refers to the ratio of visible volume to the total field-of-view volume. In consideration of the fact that perceived volume diminishes with increasing spatial distance, distance-weighted corrections are applied to both indicators. The calculation is expressed in Formula (1):

$$F=K^D$$

(1)

here, F represents the attenuation rate of volume openness. D is the spatial distance between the observer’s eye and a given spatial unit and K is a constant parameter.

According to existing studies [14], the influence coefficient for volume openness is assigned a value of 0.85. This coefficient (2) is used to calculate the final spatial openness index:

$$I=0.15I_a+0.85I_v$$

(2)

In this equation, I is the spatial openness, I_a refers to area openness, and I_v represents volumetric openness.

3.2.2. Integrated Analysis of BD, SCI, and D/H Ratio with Panoramic Spatial Openness

This study introduces BD and SCI as supplementary metrics for spatial openness, aiming to comprehensively quantify the morphological characteristics of historic alleyways.

• BD Calculation

Building density is defined as the ratio of building footprint area to the total planar area of the street buffer zone within the study area, calculated as follows:

$$BD={\displaystyle \frac{A_b}{A_t}}\times 100\%$$

(3)

where A_b represents the total building footprint area (m²) within the street buffer zone and A_t denotes the total area (m²) of the buffer zone.

2.

SCI Calculation

The street curvature index quantifies the degree of linear curvature of street morphology, reflecting its dynamic influence on visual perception. Following established road curvature calculation methods [9], the SCI is computed as the sum of absolute changes in direction angles per unit length along the street centerline:

$$SCI={\displaystyle \frac{\sum \left|\Delta {\theta }_i\right|}{L_i}}$$

(4)

where ∆θ_i is the directional angle difference (degrees) between adjacent line segments and L_i is the total length (m) of the street centerline.

All calculations were performed within the ArcGIS software. Building density was obtained via spatial overlay analysis between building footprint polygons and street buffer zones, while street curvature was measured by densifying street centerline vertices and measuring segment-wise directional angles.

4. Results and Analysis

4.1. Visual Perception Evaluation

4.1.1. Reliability Analysis of Evaluation

The reliability of the perceptual evaluation was tested using Cronbach’s alpha in SPSS 25.0. Based on 11 questionnaire items, the coefficient reached 0.981, which is well above the conventional threshold of 0.7 [29], indicating excellent internal consistency and strong reference validity.

4.1.2. Evaluation Results

Based on the statistical analysis of the SD evaluation (

Table 4. SD factor evaluation table.

Evaluation Item	Adjective Pairs (Negative-Positive)	DaChun Alley	Wenchanggong Alley	Xinan Alley	Adjective Pairs-Positive
Sense of Scale	Unreasonable Scale	0.73	−0.20	1.13	Reasonable Scale
Spatial Continuity	Discontinuous	0.67	0.27	0.60	Continuous
Sense of Depression	Oppressive	0.53	−0.13	0.93	Non-Oppressive
Degree of Quietness	Noisy	0.87	1.73	0.13	Tranquil
Spatial Openness	Cramped	0.33	0.73	0.8	Spacious
Sense of Hierarchy	Blurred Hierarchy	0.67	−0.13	0.73	Clear Hierarchy
Sun Exposure	Overexposed	0.47	0.87	−0.53	Shaded
Landscape Uniqueness	Common Landscape	0.67	0.60	0.40	Unique Landscape
Landscape Diversity	Monotonous Landscape	0.53	0.33	0.47	Diverse Landscape
Sense of Interest	Uninteresting	0.47	0.40	0.40	Uninteresting
Attractiveness	Unattractive	0.53	0.53	0.93	Attractive
(Grand) Total		0.58	0.32	0.54

; Figure 4). Wenchang Alley achieved the highest score for quietness (1.73), but consistently received negative scores on spatial openness, with the greatest number of negative items due to its high enclosure and shadowed corners. In contrast. Xinan Alley benefited from greater natural light and a lively atmosphere created by food stalls, yielding the highest attractiveness rating (0.93), although its overall evaluation was only marginally higher than Dachun Alley. Subgroup analysis (n = 96) showed that younger participants (20–35) demonstrated stronger preferences for alleys with greater openness (M = 0.62 vs. 0.47 among participants aged 36–50), suggesting higher tolerance for spatial density. Gender differences were also observed: female respondents reported a significantly lower sense of safety in Wenchang Alley (−1.2 vs. −0.5), consistent with prior findings on gender-based environmental perception. Overall, the results underscore the critical role of spatial openness in shaping user experience and highlighted the need for more diverse samples in future studies.

After deriving spatial openness values via panoramic viewshed analysis, these values were calibrated against specific D/H ratios to establish thresholds for visual perception evaluation. According to Yoshinobu Ashihara [10], a D/H ratio of 1 represents a visual balance point: values equal to 1 produce spatial harmony and optimal visual comfort; values below 1 generate feelings of compression and claustrophobia; and values above 1 create sense of spatial expansiveness, which may also be perceived as emptiness or helplessness. Thus, the D/H = 1 ratio marks to a peak of positive visual perception.

By converting this D/H threshold into panoramic spatial openness metrics through modeling, the equivalent spatial openness is approximately 20% [14]. Thus, 20% is used in this study as the threshold for how spatial openness affects visual perception. The visual experience often feels expansive when spatial openness above this threshold; when it falls below, it could create a feel of oppression.

4.2. Spatial Openness Analysis

4.2.1. Model Analysis Based on Urban Panoramic Spatial Openness

Following the framework of urban panoramic spatial openness, GIS and CityEngine technologies were utilized to construct models of area and volumetric openness for the studied alleys, as shown in Figure 5.

Using the alley spatial openness analysis model and Formula (1), this study conducted a comprehensive evaluation of alley spatial openness, as detailed in

Table 5. Spatial openness analysis of alleyways.

Model Name		H1 DaChun Alley	H2 Wenchanggong Alley	H3 Xinan Alley
Maximum Elevation Angle (°)		40	70	60
Open Spatial Area /Sky View Factor	Sample points Collected	22,400	22,400	22,400
	Visible points	6672	2308	3392
	Area openness (%)	29.7	10.3	15.1
Volumetric Openness of Space	Spatial Points	19,720	19,720	19,720
	Visible spatial points	3086	1330	2642
	Volumetric openness	15.6	6.7	13.4
Spatial Openness (%)		17.7	7.3	13.7

. The results indicate that spatial openness ranked from highest to lowest as follows: Dachun Alley, Xinan Alley, and Wenchang Alley. Moreover, the spatial openness of the studied alleys exhibits an inverse relationship with their maximum elevation angles. The was because spatial openness reflected the degree to which alleyway buildings obstructed the observer’s field of vision. When spatial openness was high, the obstructive effect of buildings was reduced, enabling observers to perceive the alley’s full view without excessive upward head movement. Conversely, when buildings were closer to the observer, greater upward head movement was required to fully appreciate the architectural scenery of the alleyway.

4.2.2. Integrated Analysis of Panoramic Spatial Openness Under the Synergistic Influence of Multiple Indicators

The quantified spatial openness results are summarized in

Table 6. Comprehensive analysis of spatial openness of ancient alleyways.

Study Object	H1 DaChun Alley	H2 Wenchanggong Alley	H3 Xinan Alley
Spatial Openness (%)	17.7	7.3	13.7
Maximum Elevation Angle (°)	40	70	60
D/H	0.9	0.4	0.95, 0.8
Number of Corners in Historic Alleyways	3	1	2
Street Curvature Index (SCI)	0.29°/m	0.25°/m	0.36°/m
Alleyway Building Density	55.36%	65.1%	52.2%

. A comprehensive comparison of urban panoramic spatial openness and the D/H ratio yields the conclusions below.

This study defined 20% as the critical threshold for assessing the influence of spatial openness on visual perception. However, the spatial openness values for Dachun Alley, Wenchang Alley, and Xinan Alley all fell below 20%, indicating a generally enclosed spatial configuration. It was therefore inferred that the distinct visual experience of Tangchang Town’s alleyways resulted from their limited spatial openness. The confined vertical enclosures and steep elevation angles contributed to a heightened sense of discomfort among visitors. In addition, the substantial height disparity between the two sides of Xinan Alley, with one side formed by the Tangchang Ancient City Wall, blocks approximately 50% of the view, resulting in lower spatial openness. These finding demonstrate that the D/H ratio alone should not be solely relied upon when evaluating spatial openness.

4.3. Impact of Spatial Openness on Visual Perception

This section presents the results of the empirical analysis. First, the overall correlation between spatial openness and visual perception was examined through the integration of visual assessment scores with spatial openness indicators and the application of Kendall’s τ correlation analysis. The findings suggest that spatial openness appears to exert an important influence on visual perception across multiple interrelated dimensions. The relative importance of particular morphological indicators and their implications for perceptual outcomes are further examined in the subsections below.

4.3.1. Correlation Analysis of Spatial Openness Effects on Visual Perception

To examine the relationship between spatial openness and perceptual evaluation, we conducted an exploratory Kendall’s tau correlation analysis using the three case alleyways—a non-parametric method well-suited for small samples and robust in assessing monotonic relationships. For each alleyway, spatial openness was quantified by the integrated openness percentage derived from GIS-based viewshed analysis (Table 5), while perceptual scores were aggregated from the SD evaluation (Table 4).

Kendall’s tau, a non-parametric rank-based statistic, gauges how strongly two variables are associated in a monotonic fashion. It is defined as follows:

$$\tau ={\displaystyle \frac{C-D}{\frac{1}{2}n(n-1)}}$$

(5)

where C denotes the number of concordant pairs and D denotes the number of discordant pairs.

The computational results presented in

Table 7. Kendall’s τ correlation between spatial openness and visual perception.

Variable	τ
Spatial Openness (%)	1.0
D/H	1.0
Street Curvature Index (SCI)	0.33
Alleyways Building Density	−0.33

Note: τ values range from −1 (perfect negative association) to +1 (perfect positive association). Results are based on n = 3 alleyways and should be interpreted as exploratory and descriptive rather than inferential.

further demonstrate that spatial openness is the most influential morphological factor in shaping the visual experience of historic alleyways.

The results indicate that spatial openness and the D/H ratio were positively correlated with visual perception, whereas building density showed a slight negative association and street curvature exhibited a weaker positive one. Given the very small sample size (n = 3), the test could not achieve conventional statistical significance, with the minimum attainable p-value being 0.333. Accordingly, the findings should be regarded as exploratory rather than conclusive. Nevertheless, the consistent patterns suggest that openness-related factors exert the strongest influence on perceptual experience in historic alleyways. Overall, the exploratory correlation analysis highlights the potential importance of spatial openness in shaping visual perception. Future studies will broaden the dataset to include additional alleyways and case towns, thereby enabling more rigorous statistical testing and model validation.

4.3.2. Analysis of Spatial Openness Effects on Visual Perception

By integrating the results of visual perception evaluations and spatial openness analyses, as summarized in Table 7, and a comparative analysis of alleyways’ spatial openness and visual perception (

Table 8. Comparative analysis of alleyways spatial openness and visual perception.

Study Object	H1 Dachun Alley	H2 Wenchanggong Alley	H3 Xinan Alley
Spatial Openness (%)	17.7	7.3	13.7
D/H	0.9	0.4	0.95, 0.8
Maximum Elevation Angle (°)	40	70	60
Visual Perception Score	0.58	0.32	0.54
Number of Corners in Historic Alleyways	3	1	2
Street Curvature Index (SCI)	0.29°/m	0.25°/m	0.36°/m
Alleyway Building Density	55.36%	65.1%	52.2%

), this study provides a comprehensive assessment of the mechanisms and interrelations by which spatial openness shaped visual perception in historic alleyways. The key findings are as follows:

• Positive Correlation and Psychological Response

The analysis revealed a positive correlation between spatial openness and visual perception scores (Kendall’s τ = 1.0), suggesting that greater openness was associated with more favorable visual experiences. For example, Wenchang Alley, with a low openness value of 7.3%, was found to trigger negative physiological and psychological responses. In terms of perception, over half of the participants experienced a “compression misjudgment” when D/H ratio dropped to 0.4. This occurred because building heights were perceived as taller than their actual dimensions, suggesting that low spatial openness distorted spatial scale recognition. Such distortions tended to evoke feelings of anxiety and confinement, which in turn lowered participants’ overall evaluations of the alley environment.

2.

Mediating Role of the Lighting Environment

The lighting environment played a key mediating role in the relationship between spatial openness and visual perception. The analysis showed that spatial openness was correlated with illuminance levels within alleys: lower openness corresponds to weaker natural lighting. Poor lighting environments have been reported to increase crime rates and reduce people’s sense of safety [30,31]. Thus, the lighting environment further shaped the influence of spatial openness on visual perception by affecting both perceived safety and visual comfort [32,33].

3.

Physiological and Psychological Regulation by Sky Visibility

Sky visibility exerted significant physiological and psychological effects on visual perception. Experimental results show that increasing spatial openness from 7.3% in Wenchang Alley to 17.7% in Dachun Alley correspondingly reduced the sense of enclosure caused by surrounding buildings and improved visual perception scores. These findings suggest that enhancing sky visibility helped alleviate stress and increased psychological comfort [13].

4.

Physiological Constraints of Viewing Angle Dynamics

Viewing angle dynamics imposed physiological constraints on visual perception. According to the comfortable visual cone theory, the upper limit of the natural head tilt angle is approximately 25°, within which objects can be viewed without neck movement. In Wenchang Alley, however, the viewing angle reached 70°, forcing most participants to tilt their heads backward to observe the surroundings [34].

5.

Linear Oppressive Effect of Building Density

Building density exerted a linear influence on visual perception. High-density buildings limited pedestrians’ field of vision, causing visual obstruction [35]. Reduced lighting and poor ventilation further weakened pedestrians’ instinctive need for an “escape view” [36]. These factors combined increased psychological stress for pedestrians [37]. For instance, Wenchang Alley’s high building density (65.1%) resulted in a visual oppression score of −0.13 and a scale rationality score of −0.20. In contrast, alleys with lower building densities showed decreased oppression and higher-scale evaluations, indicating a positive correlation between building density and perceived visual oppression. Density gradient experiments further confirmed that increased building density reduced pedestrian walking speed by approximately half and sharply decreased willingness to stay [38]. These findings indicate that excessive building density induced strong feelings of oppression, adversely affecting spatial behavior and psychological experience, thereby negatively influencing visual perception.

6.

Compensatory Effects of Curvature and Corners

Curvature and corners exerted compensatory effects on visual perception. Spatial syntax analysis showed that Dachun Alley’s curvature of 0.29°/m enhanced its “interest” score by 0.47. This effect was attributed to dynamic changes in sightline occlusion and the element of surprise provided by corners. Highly curved, winding streets obscure views and create a “progressive revelation” effect, stimulating pedestrian exploration. This finding aligns with the Prospect-Refuge Theory in environmental psychology [39]. Therefore, appropriate curvature and corners increase spatial exploration interest and enhance psychological pleasure, partially compensating for the negative effects of inadequate spatial openness on visual perception [16].

In summary, the impact of spatial openness on visual perception is a complex multi-factor coupling process involving physiological, psychological, and spatial morphological dimensions. Urban spatial design and planning should carefully consider the effects of spatial openness and related factors on visual perception. By appropriately adjusting spatial morphology, optimizing lighting, enhancing sky visibility, controlling building density, and strategically employing curvature and corners, designers can create a more comfortable, safe, and engaging visual environment, thereby improving people’s spatial experience and quality of life.

5. Discussion

5.1. Practical Design Strategies

This study quantifies the spatial openness of three historic alleys in Tangchang Town and examined their correlation with visual perception by integrating a panoramic spatial openness framework, GIS-based viewshed analysis, D/H ratio calculations, and SD-based perceptual evaluation. The results confirm that spatial openness was a key factor shaping visual perception in ancient alleyways, primarily through its effects on natural lighting, sky visibility, and elevation angles. Therefore, careful adjustment of spatial openness should be prioritized in the conservation and renewal of historic alleys to enhance both visual comfort and environmental quality.

Based on these findings, this study proposes several practical design recommendations to guide the conservation and revitalization of historic alleyways with varying levels of spatial openness.

For alleys with spatial openness below 10%, we propose the following:

• Improve Natural Lighting and Sky Visibility

To mitigate visual oppression and psychological stress experienced by pedestrians in high-density built environments, systematic spatial interventions are proposed: (1) remove visual barriers such as overhead pipelines and external equipment through underground installation or facade integration to enhance sky visibility, and (2) regulate building interfaces with setback designs or height restrictions to reduce shadowing and ensure natural light penetration. This mechanism operates on two levels: First, expanding visible sky areas enhances spatial orientation in line with the Prospect-Refuge theory [36], fulfilling the evolutionary demand for escape vision. Second, increasing direct light flux regulates circadian rhythms via the retina–hypothalamus pathway, thereby alleviating perceived environmental stress [40]. The Cheonggyecheon Urban Renewal Project in Seoul illustrated this approach: by systematically eliminating visual barriers, including the demolition of elevated viaducts and the thinning of dense vegetation, it effectively improved street-level sky visibility. Empirical evidence further demonstrates that this intervention not only enhances pedestrian satisfaction with environmental perception, but also stimulates the revitalization of surrounding commercial activities.

2.

Increase Resting Spaces within Alleys

The narrow width of ancient alleys often restricted opportunities for visitors to pause and appreciate the environment. Introducing small plazas, seating areas, or viewing platforms can enrich spatial experiences and mitigate perceptions of confinement. Two strategies were particularly effective: (1) incorporating micro-squares or viewing platforms to provide resting points, and (2) installing facilities that integrated with the historical landscape. The mechanism operated on two levels. First, the creation of “pause spaces,” as articulated in Jan Gehl’s theory of interaction spaces, interrupted the monotony of linear environments and satisfied pedestrians’ exploratory needs [41]. Second, locally open areas served as visual buffer zones, consistent with the connectivity theory in spatial syntax, thereby reducing the oppressive effects associated with high alley height-to-width ratios [42]. A representative example was the renovation of pocket squares in Suzhou’s Pingjiang Road Historic District, where the introduction of resting spaces with recreational and cultural functions encouraged visitors to linger, increased pedestrian flow, and contributed to the district’s revitalization [43]. Overall, this strategy achieves a balance between preservation and functional use through micro-spatial restructuring, offering technical support for the sustainable renewal of historic districts.

For alleys with spatial openness between 10% and 20%, we propose the following:

3.

Enhance Greenery and Color Perception

Climbing plants and façade greenery can soften spatial enclosures and improve the microclimate, while diverse exterior color schemes enriched visual experience and enhanced environmental attractiveness [44]. Accordingly, an ecological color intervention strategy was proposed: (1) implement vertical greening systems, such as climbing plants or modular green walls, to reduce the rigidity of hard surfaces; and (2) selectively incorporate flowers or colorful foliage plants. The mechanism operated on two levels: First, green, as a natural symbolic color, activated the parasympathetic nervous system and lowered blood pressure and cortisol levels, as confirmed by neuroenvironmental studies and supported by the bio-affinity theory [45]. Second, enhanced color contrast improves environmental legibility and alleviates cognitive load in narrow spaces [40]. Empirical studies show that vertical greening reduces wall surface temperatures by more than 15.0 °C and decreases ambient air temperature by up to 1.5 °C, thereby improving visitors’ environmental preferences [46]. This strategy integrates principles of color psychology and microclimate regulation, providing a low-intervention yet high-benefit ecological design pathway for the sustainable renewal of historic districts.

4.

Introduce Interactive Architectural Features and Commercial Elements

A lack of transparent or interactive façades often decreased pedestrians’ willingness to traverse alleys and undermined their perceived safety [47,48]. Without compromising historical authenticity, the introduction of windows, display showcases, or semi-open commercial spaces fostered interaction and revitalized alley environments. To address the limited pedestrian engagement caused by closed façades in traditional alley buildings, two interventions were proposed: (1) enhance façade transparency by enlarging door and window openings and incorporate glass surfaces to strengthen visual connectivity, and (2) introduce small-scale commercial functions, such as cultural and creative shops or specialty stalls. The mechanism operated at two levels. Behaviorally, transparent interfaces generate the “edge effect,” as described in Jan Gehl’s theory of social spaces, transforming static buildings into dynamic social interfaces [42]. Psychologically, “readable interfaces,” consistent with Kevin Lynch’s theory of urban imagery, increase visual permeability, enhance environmental control, and alleviate defensive anxiety [49]. This strategy reconciles heritage preservation with functional revitalization through targeted micro-scale interventions, offering a replicable framework for the sustainable renewal of historic districts.

5.

Enhancing the appeal of corners

At curved turning points, mirrored or perforated landscape walls can alleviate the sense of spatial confinement through physical intervention. The curvature itself generated natural ‘visual focal points,’ which can be enhanced with artistic installations to strengthen spatial appeal through psychological intervention. To counter visual fatigue caused by the continuous linearity of traditional alleys, a dual-mode intervention was proposed: (1) physically, mirror-reflection or perforated screen walls expanded perceived spatial depth; (2) psychologically, themed art installations created narrative visual focal points, guided by the anchor theory in environmental behaviorism [50].

Mechanistically, mirror elements produced a virtual extension effect, making a three-meter-wide alley appear wider, while the color saturation and morphological complexity of art installations stimulated dopamine release, enhancing environmental pleasantness [51]. In Nanjing’s Laomen Dong historic district, the installation of visually striking cultural decorations and glass railings at corners and alleyways preserved the district’s fabric while increasing its popularity through emerging cultural capital [52]. This dual physical–psychological intervention thus provides a non-expansion-based strategy for spatial optimization in high-density historic districts.

5.2. Theoretical Implications and Methodological Contributions

This study contributes to theoretical insights by clarifying the various ways that people’s visual perception in ancient alleyways was influenced by spatial openness. Prior studies on historic alleyways had primarily focused on architectural preservation, streetscape beauty, or the ways that spatial shape affects pedestrian flow [10,53]. Traditional measures, such as the D/H ratio, were widely employed to assess enclosure and openness [22], but they often overlook the subtle aspects of how variables in spatial openness affected users’ psychological and physiological responses. By showing how spatial openness altered visual perception by modulating natural lighting, sky visibility, and elevation angles, this work contributes the theoretical discourse on human–space interaction in heritage urban environments. The results provide useful information for heritage managers and urban planners who have to strike a balance between conservation and modernity. The improvement strategies suggested in this study show how minor spatial interventions, like improving sky visibility, building interfaces, or visual focal points at corners, can improve environmental quality and user comfort without requiring extensive demolition, in contrast to large-scale redevelopment projects that frequently disturb historic contexts [54]. By offering distinct suggestions for alleyways with varied degrees of openness (less than 10%, 10–20%), this study supports a more context-sensitive strategy that maintained cultural identity while improving the quality of the space. Enhancing the historic alleys’ spatial experience could also boost local business and heritage tourism, producing long-term social and economic advantages that support sustainable urban development [55,56].

Methodologically, this study advances the analytical framework for measuring and interpreting spatial openness in complex urban heritage contexts. By integrating the SD method with a panoramic spatial openness framework, and combining GIS-based viewshed analysis, D/H ratios, BD, and SCI, this research expands the methodological toolkit for quantifying openness in an irregular, historically layered alley. This hybrid approach bridges the gap between purely quantitative urban morphology and subjective perceptual experience, offering a more comprehensive means to study spatial perception in heritage conservation. The refined methodology served as a reference for future research seeking to combine objective spatial metrics with human-scale perceptual evaluation.

6. Conclusions

In summary, this study demonstrates that spatial openness plays a crucial role in shaping the visual perception of historic alleys. It affects key factors such as natural lighting, sky visibility, and elevation angles. By combining a panoramic openness framework with GIS-based viewshed analysis, D/H ratio calculations, and SD evaluation, this study proposes a comprehensive method for measuring openness in an irregular and historically layered alley. The design strategies derived from this framework provide practical guidance for conservation and renewal, supporting cultural heritage preservation while enhancing visual comfort and environmental quality.

Several limitations should be noted. First, the SD method relies on photographic evaluations, and perception ratings are sensitive to image quality. Second, the GIS-based line-of-sight analysis is based on simplified urban models, which are less precise than advanced 3D scanning methods such as LiDAR [57]. Future studies could use immersive virtual reality or high-resolution photogrammetry to replicate dynamic perceptions in real time, providing more in-depth understanding of the connection between openness and user experience. It should also be acknowledged that the Kendall’s τ correlation were obtained from a limited number of alleyways. Therefore, our results are exploratory rather than conclusive. Larger samples and additional case studies are required to validate the findings. Extending the framework to different cultural contexts and larger urban scales will test its applicability and provide further evidence for the sustainable conservation of historic urban environments.