## 1 Introduction
### Physical Features  
The external form of a building comprises various physical characteristics, which directly impact the visual impact assessment. Previous studies have shown that the color, materials, and shape of a building affect people's assessment of its exterior form [47].  
Many scholars have studied the influence of colors on people's visual impact assessment, as colors are known to have a certain influence [48; 49]. [PERSON], [PERSON] [50] found that colors on the building facade have an influence on people's visual impact assessment. A variety of facade colors is a crucial element in architectural design; it is also a prominent feature that the public could consider important.  
Building materials represent another important physical feature [51]. [PERSON] [52] and [PERSON], [PERSON] [53] discovered that changing the building materials influenced people's visual impact assessment. [PERSON], [PERSON] [54] argued that building materials serve as the physical feature by which buildings can be distinguished. [PERSON] and [PERSON] [47] explored the visual impact assessment of a library by some of the students of Mahatma Gandhi University and discovered that the building materials had an influence on the student's assessment. Therefore, the type of building material is an important attribute that affects people's visual impact assessment.  
According to studies, people's preference for a building is related to the richness of its contour [55; 56]. [PERSON] and [PERSON] [57] not only described a method for defining the shape of the building facade but also clarified the geometric differences between building facades. [PERSON] [58] pointed out that the contour complexity of residential buildings can influence people's visual impact assessment. A building's scale and proportion are influential physical attributes that affect residents' assessment of the external space of the building [59]. According to [PERSON] and [PERSON] (2012), the scale and proportion of a building are the determining factors in its visual appeal. [PERSON] and [PERSON] [60] found that the volume of buildings on the street affects people's visual impact assessment. Hence, the contour and form of a building significantly influence people's visual impact assessment of its external space.  
Decorations and details on the exterior of buildings are important physical characteristics when evaluating structures [61]. [PERSON], [PERSON] [62] and [PERSON] and [PERSON] (2012) noted that decorations and details are key physical features that affect people's visual perception of buildings. In addition, the exterior architectural decoration can also reflect the history of a city [61; 62]. Therefore, the exterior details and decoration of buildings have a strong influence on people's visual impact assessment.  
According to earlier research, permeability is recognized as a factor influencing people's visual impact assessment of the exterior shape of buildings [63]. [PERSON] [64] found that facades with varying permeability have a certain influence on how people assess the visual impact of residential buildings; compared with men, women were found to prefer facades with higher permeability. [PERSON], [PERSON] [65] concluded that permeability influences people's preference for the appearance of a building facade. Since it can positively influence visual impact assessment to some degree, it can be employed not only as a criterion for aesthetic evaluation but also as a design strategy for architecture. [PERSON] and [PERSON] [66] conducted an experiment on building permeability and discovered that respondents usually expected high permeability in the building facade in order to see the scenery outside.  
Based on the related research, in this study, we selected five physical features as research dimensions to explore the external form of subway station entrance pavilions: the number of exterior colors, the decorative material, the exterior contour of the pavilion, the cultural characteristics, and the permeability of the facade.