Regarding interactive lenses for visualization, extensive surveys in the literature [
4] have highlighted their elegance, flexibility, and simplicity when applied to several fields and case studies. This work also offers a robust conceptual model for designing and implementing interactive lenses by formalizing, in particular, a selection
(the region affected by the lens) and a lens function
(how the visualization is altered). There are, in fact, several past projects and studies developed on top of these building blocks to create interactive lenses for 2D and 3D interaction models. Past works, such as the “Temporal Magic Lens” [
6], introduced a 2D lens technique that considers the time dimension to locally alter video streams for analysis by combining temporal and spatial aspects of a video-based query and offering a unified presentation to the user. Magic lens techniques have also been applied in the literature in Augmented Reality (AR), for instance, to inspect and discover portions of physical objects using a hand-held flexible sheet [
7]. Within the field of cultural heritage, previous research already explored expressive and accessible interactive lens models in order to discover the past (or the future) in a specific location of a physical or virtual space. Regarding approaches for casual visitors and non-professional users, the “revealing flashlight” [
8]—on display during the Keys to Rome exhibition (
http://keys2rome.eu/-accessed on 29 April 2021) at the Trajan Markets in 2015—represents an excellent example of a spatial augmented reality technique that allows one to locally reveal the details of physical museum artifacts from a specific point of view. The projection mapping in this case can be exploited by users to locally decipher inscriptions on eroded stones or to interactively discover the geometric details and meta-information of cultural artifacts, with proven effectiveness, ease of use, and ease of learning.
Regarding the interactive exploration of cartographic heritage for public exhibits, a previous work [
9] exploited the interactive lens approach with temporal alteration to visually communicate old 2D maps to visitors in an efficient manner. Such datasets present dense and precious information that is very difficult to retrieve and study without special techniques, thus offering visitors a very simple, immediate, and efficient interface. “TangibleRings” [
10] is another example of a 2D interactive lens concept applied to tangible interfaces, offering visitors an intuitive and haptic interaction to discover different information layers. Within immersive virtual reality (VR) analytics, through consumer-level head-mounted displays (HMDs), recent works investigated applications of interactive lenses that exploited a virtual 3D space [
11]. Due to the “see-through” concept offered by the interaction model, different lenses naturally lend themselves to the combination of their individual effects and integrations with six-DoF controllers (hand-held controllers with six degrees of freedom) to naturally control the final effect. Regarding virtual representations of archaeological sites or artifacts, previous research investigated volumetric techniques that allow professionals to locally “carve” space and time in multi-temporal (4D) virtual environments [
12] that are supported by formal languages [
13] to keep track of virtual reconstruction processes. These techniques fully and interactively operate in a virtual 3D context, thus enabling the adoption of advanced 3D interactions and 3D user interfaces when designing these tools. These interactive techniques are generally employed within ad hoc, desktop-based applications and tools; what about the application of the 3D lens concept to web-based visualization? We have witnessed large advancements regarding the presentation and dissemination of interactive 3D content on desktop and mobile web browsers (Google Chrome, Mozilla Firefox, etc.). A prominent example of interactive 3D presentation on the web is offered by the SketchFab platform (
https://sketchfab.com/-accessed on 29 April 2021) to present and inspect 3D models through a normal web page. Nowadays, web technologies offer improved hardware integration; thus, a web page (once it has obtained a user’s permissions) is able to access the built-in microphones and cameras, GPS, compass, and much more. Such integrations—especially on mobile web browsers—are very appealing for web applications that target cultural heritage because they do not force the user to install any additional software on their device. Past research projects have already exploited these technologies to present large interpreted or reconstructed 3D landscapes [
14], browser-based, city-scale 4D visualization based on historical images [
15], augmented museum collections using device orientation [
16], or gamified 3D experiences [
17,
18]. Regarding immersive VR, since the introduction of the first open specification for WebVR [
19], it became possible to consume immersive experiences by using common web browsers and consumer-level HMDs (including cardboard). The API recently evolved into WebXR (
https://immersiveweb.dev/-accessed on 29 April 2021) [
20]; the new open specification has the goal of unifying the worlds of VR and AR, in combination with a wide range of user inputs (e.g., voice, gestures), thus offering users options for interacting with virtual environments on the web [
21]. Immersive computing raises additional challenges and strict requirements for low-latency communication to deliver a consistent and smooth experience. These must be taken into strong consideration, especially when designing interactive techniques for web-based tools or applications dealing with the exploration of the past.