## 4 Results
### 4.2.2 User-centered design
#### 4.3.3 Four-Dimensional Browser Features and Visualizations  
The other frontend application is the 4D browser. It is developed as a desktop- and browser-based virtual research environment, targeting primarily (but not only) art, architectural, and urban history scholars. The basic idea originates from shortcomings in conventional media repositories, where data can only be queried by their metadata [309]. To this end, the 4D browser extends conventional search interfaces (e.g., search bars, faceted search) by a 3D viewport that displays spatially oriented photographs within a 3D city model [318]. Since the 3D models only display buildings, (historical) maps have been incorporated as the texture of the terrain model, showing additional clues regarding the building situation, street names, and other infrastructure that supports orientation within a city. A tripartite time slider enables users to select a time range to filter images, choose a point in time that should be represented by the city model, and toggle between (historical) maps. With this spatial approach, users are able to find images without precise knowledge of the metadata by spatially browsing around the building of interest. Since photographs are linked to the buildings they depict, interaction with the respective 3D objects can additionally filter the search results. Similar images can be browsed by easily navigating to neighboring camera positions. The 3D approach also makes it possible see the photographer's perspective and to understand their situation while taking the photo [319]. With the time slider, it is possible to detect changes over time in the architecture, cityscape, and distribution of images. Additional features aim to support scholars in answering their research questions (e.g., Which positions did photographers prefer when taking photos of a given building? Which areas or parts of buildings were never or hardly ever photographed?). Various visualizations extract the spatial information of the images to analyze not only their distribution, but also the acquisition habits of the photographers and the coverage of building parts [320; 321]. A conventional heat map is used to identify aggregations of camera positions (Figure 25a), a heat map of buildings shows which parts have been photographed more frequently (Figure 25b), and popular angles of photographs are visualized by cluster-based radial fans (Figure 25c). Current work focuses on the addition and browsing of text documents, as well as annotation of architectural elements in texts, images, and 3D models to make connections across different types of media in order to further improve art and architectural history research [86]. Next to browsing and data exploration, the 4D browser also serves as an authoring and content management tool. Authorized users are able to upload images and 3D models, manually spatialize images, update metadata (including the date the photograph was taken as well as construction and destruction dates of buildings), and add custom POIs. All additions and changes made in the 4D browser also directly affect what can be seen in the mobile 4D City application. It is planned for the future that users are able to upload their  
Figure 25: Different visualizations of distributions of images. Red indicates higher concentrations of positions, coverage, or angles. own content to feed the system on the one hand (i.e., crowdsourcing), but also to create and analyze custom collections according to their specific research questions on the other hand. On a technical level, the 4D browser utilizes the same browser-based technologies and frameworks (i.e., Angular, three.js) as the 4D City application, since the same data provided by the backend need to be processed. However, since it is for desktop usage, the application is not limited by the lower computation power and bandwidth of a mobile device. It can load and display more data at once, covering a bigger area.