The Virtuality Continuum was presented by Miligram and Kishino's "A Taxonomy Of Mixed Reality Visual Displays" [22]. Within their continuum, Real Environment (consisting solely of real objects) and Virtual Environment (consisting solely of virtual objects) sit at opposite of the spectrum, with Augmented Reality and Augmented Virtuality existing between them. They provide a definition of a Mixed Reality environment as one where the real and virtual world are presented within one display [22], such as within an Extended Reality headset. Skarbez et al. has since revised Miligram and Kishino's original continuum, renaming Virtual Environment to External Virtual Environment and adding "Matrix-like" Virtual Environment to the far right. The reasoning for this is due to a virtual environment being unable to be fully immersive [30]. Additionally, Bekele et al. [2] define Augmented Reality (AR) as the superimposing of virtual information on the view of the real-world, Virtual Reality (VR) as a solely computer generated environment with no means to see or interact with the real-world, and Mixed Reality (MR) as a blend of the real and virtual environment.

XR systems have also often been framed as Hypertextual - both in terms of game technologies and understanding play and interactions as links [1, 19, 23], but also in terms of links between space and reality. The composition of items in a virtual environment (such as a virtual museum) form a spatial hypertext of the subject matter of that exhibition using the spatial links of proximity and geometry [3], and MR locative systems similarly link items from one reality to places in another creating links of not only physical traversal but also across reality [16, 17]. Consequently it is important to understand XR experiences as Hypertext with links that are navigational, spatial, and across reality.

Authoring tools can be defined as a tool which allows the author to create a solution within a particular domain [5, 14] without a need for technical skills, such as programming [20], and can provide a WYSIWYG environment, allowing for real-time feedback and decreasing the number of iterations required for development [10]. Authoring tools are a fundamental part of the Hypertext research landscape [4, 6, 11, 17, 25] in that how we support the creation of Hypertext shapes the Hypertext itself [15].

The use of authoring tools can allow cultural heritage institutions to create innovative and interactive exhibits more efficiently at lower costs, allowing them to attract new visitors and stay relevant [8]. Various cultural heritage authoring tools exist within the literature. One such tool is the CHESS system, whose aim is to enrich a museum visit by using personalized interactive storytelling [32]. CHESS caters to visitors who consume CHESS experiences and provides authors with the CHESS Authoring Tool (CAT), where they can design CHESS experiences for visitors. Another example which uses virtual reality is the VIRTUE system (VIRTUal Exhibition hall) [13]. VIRTUE is a customisable, virtual reality system that allows curators to easily create multi-modal, virtual 3D exhibitions for visitors to navigate and enjoy. The authors note that the VIRTUE offers unlimited exhibition space and support for any configuration of exhibition. Furthermore, other tools are often used in a cultural heritage context, such as the explicitly Hypertextual StoryPlaces [17, 24]. While some authoring tools do work with XR [6], this is normally as part of a desktop tool rather than authorship within XR itself.

The development of a cultural heritage exhibition requires collaboration, either internally within the institution or with elements outsourced to external partners [18, 21, 31]. This collaboration could take place in-person, or remotely over platforms such as Google Meet, Zoom, or Microsoft Teams. Schäfer et al. [29] provide a taxonomy for remote collaboration, consisting of Environment, Avatars, and Interaction. Additionally, they categorise remote collaboration systems uses as Meeting (users share a common workspace), Design (combines systems such as product design or architectural design), and Remote Expert (includes a local and a remote user, with the remote user often being an expert). Furthermore, Druta et al.  [12] notes that tasks are accomplished better and faster when a visual workspace is shared between participants, as well as mixed reality showing to be useful for tasks involving complex design instructions, whilst also decreasing time and mental effort.

CATCH-XR is a collaborative mixed and virtual reality authoring tool that enables multiple users to meet remotely in a recreation of the host's real-world space. It can be used to experiment and create visual prototypes for physical exhibitions for a real-world space (e.g. galleries, museums); to create a prototype or a complete virtual exhibition for a real-world or digital space; or to create a prototype or a complete mixed reality exhibition for a real-world space.

Within CATCH-XR, users can:

Curate (see Section 3.1) their exhibition, by placing and editing various Display, Signage, and Artefact objects into their real-world space using mixed reality, or the virtually recreated space.\

Create Custom Walls (see section 3.2) to introduce virtual walls into existing spaces which can also act as surfaces for Displays or Signages\

Place Annotations (see Section 3.3) in the form of digital post-it notes with an attached voice recordings, allowing users add notes, comments or feedback for themselves or others\

Access to a Sketchbook (see Section 3.4), which draws from an external image repository, enabling users to reference assets, such as sketches or blueprints without needing to remove the headset\

A variety of other authoring features (see Section 3.4), such as saving and loading, exhibition realignment, lighting editing, and viewing the exhibition without authoring elements or other users present\

Using the Meta headsets room scanning functionality, the Cultural Heritage Institution (CHI) can capture their real world exhibition space and share an automatically generated simplified version of this with the Cultural Heritage Developer (CHD). This allows both parties to effectively "meet" and build in the same location remotely. From a networking perspective, the CHI functions as the host, and the CHD functions as the client. Participants will see rudimentary avatars of each other and can use the built in microphone to communicate within CATCH-XR. Once the virtual space is generated, it can be used in situ (at the original world space) as a Mixed Reality experience, or anywhere as a purely Virtual Reality Experience. CATCH-XR was developed using Unity 6000.0.58f2 and Meta Quest 3 and Pro.

Within the Create sub-menu, users can select from a variety of Displays, Signage, and their uploaded Artefacts objects to build their exhibition as desired (see Figure 1). Users point to their desired location and see a preview of the desired object in their mixed reality or virtual reality space before placing. The object responds and adjusts according to the scanned surroundings, such as portraits rotating to lie flat against the walls, or not allowing floor displays to be placed on walls. Users can also rotate and scale their chosen object, with the exclusion of 2D Artefacts (images and video). Within the Edit sub-menu, users can select an existing object to edit it. The user will see the desired object highlighted orange and, once selected, will be highlighted green for that user and red for others to indicate editing.

3.1.1 Displays. Within CATCH-XR, Displays are used to house Artefacts. Floor based displays, such as display cases and pedestals, are used to house 3D object Artefacts. Wall based displays can be used to house image or video Artefacts.

3.1.2 Signage. Signages are used as containers of information, and there are versions that can be placed on the floors or on the walls. Signages can link to Displays or Artefacts. For floor-based displays, the text on the Signage will find the description of the Display. For wall-based displays with an Artefact attached or an Artefact object, the Signage will automatically find the information for that Artefact. The information is downloaded from an external repository and is cached within CATCH-XR for quick retrieval (Section 3.1.3 for further details).

3.1.3 Artefacts.

To allow assets to be imported into a CATCH-XR exhibition, CATCH-XR connects to an external repository which contains user uploaded content in the form of text for descriptions and titles, links to a download location, images as JPGs, videos as Vimeo links, and 3D content as Unity asset bundles. In this case, CATCH-XR is built to connect with Noho's Cross-Culture Platform 1 to access user uploaded content, as seen in Figure 2. While this work uses the Cross Culture Platform, the integration is not inherently platform-dependent and could be adapted to alternative open platforms. Within the Cross Culture Platform, entries can be further grouped into collections, allowing similar entries to be grouped together for ease of use. By using the Cross Culture Platform, simple content (e.g. images and video) can be easily uploaded by the CHI, while more complex content, (e.g. 3D objects), should be given to the CHD to be processed 2 and uploaded. Within CATCH-XR, users can view and select collections, see the available entries for these collections, select an entry to add to the exhibition, and see it previewed before placing it.

3.1.4 Editing of Displays, Signage, and Artefacts.

Once a Display, Signage or Artefact has been selected for editing (see Figure 3), or a new one placed, the user is presented the following options:

Finish, Delete, Cancel Changes — complete, remove, or revert edits (with local undo available)\

Move, Scale, Rotate; X/Z Scaling (Displays and Signage only); Match to Another Display/Signage/Artefact — transformation and scaling operations\

Font, Colour, Alignment (Signage only); Link Signage (Signage only) — signage formatting and linking\

Scale Audio Distance (Displays and Artefacts only) — adjust audio range\

Hide Display Method (Displays only); Hide Signage Method (Signage only) — visibility controls\

When editing, the object highlights, and changes are seen in real time by both host and client, consistent with placing a Display, Signage or Artefact object. The object also reacts and adjusts contextually to the scanned environment.

To further develop the desired exhibition, users can also create their own virtual walls within their environment. These walls can function as additional display surfaces for wall-based Displays on the vertical faces and floor-based Displays on horizontal faces. They may also be used to create new partitions or obscure architecture within the existing space. This feature is particularly well suited to mixed reality, as it allows for the same physical space to be extensively customised according to the needs of the users.

3.2.1 Custom Wall Creation.

Within the Create sub-menu, users can create their custom wall by selecting a series of points in the environment (as illustrated in Figure 4), by selecting the initial starting point of the wall by aiming at their real-world or replicated floor, then specifying the width of the wall by aiming at another point on the floor, then specifying the depth of the wall, by aiming at point at the floor perpendicular to the line created by points one and two, then setting the height of the wall, either by snap to a ceiling socket or rotating the controller. Whilst building the wall, the user will see a preview of the wall.

Within the Edit sub-menu, users can edit already existing Custom Walls. When a wall is selected to be edited, the user is presented with the following options (as illustrated in Figure 4):

Finish, Delete, Cancel Changes — complete, remove, or revert edits (with local undo available)\

Left Side, Right Side, Depth Side — reposition corresponding points\

Height — adjust height via controller rotation or socket snapping\

As with editing Display, Signage or Artefact elements (see Section 3.1), changes to walls will be seen in real-time between client and host, and the highlight will change to the appropriate colours.

To assist with the exhibition prototyping process, users can place digital post-it notes with attached audio recordings. These allow users to leave comments such as requests, fixes, and reminders to themselves or others entirely within the application, eliminating the need to remove the headset and record external notes. This positions Annotations as key feature for enabling remote collaboration by allowing both synchronous and asynchronous collaboration in a remote mixed / virtual reality environment.

Within the Create sub-menu, users will be given a preview of the annotation (see Figure 5). Users can move their controller to choose a location for the annotation. The annotation can also be rotated and offset from the controller. Upon confirming the annotation, the user will automatically be able to edit the annotation.

Within the Edit sub-menu, users can edit existing annotations. When an annotation is selected, the user is presented with the following options (as illustrated in Figure 3.3):

Finish, Delete, Cancel Changes — complete, remove, or revert edits (with local undo available)\

Move and Rotate — reposition or rotate the annotation\

Rerecord — Rerecord the audio annotation\

Set Colour to Yellow/Green/Orange/Red/Blue — assign a predefined colour\

As with the previous features, the changes made to the Annotations are reflected across the server in real-time, with the appropriate colour highlighting.

CATCH-XR also features the following additional authoring features:

Sketchbook - Allows users to view external documents in the forms of images without removing the headset\




Lighting - The user can rotate the scene's directional light to allow them to edit the overall lighting of the scene\

Realign Exhibition - Due to the use of mixed reality, there is a possibility that the exhibition may be come misaligned from it's intended location. To address this, the entire exhibition can be moved and rotated as a single unit, avoiding the need to manually relocate each object\

Saving and Loading - To allow users to work on an exhibition over a long period of time, the exhibition can be saved as a JSON file on an external server, which can be later loaded back in when needed\

Preview Exhibition - Hides authoring elements, such as annotations or Signage links, to allow the user to preview the exhibition as if it were complete.\

Hide All Collaborators - Allows users to view their exhibition without seeing collaborators\

By integrating the features described in this section, CATCH-XR provides a fully immersive WYSIWYG authoring environment in which users author within extended reality itself, rather than through an external or abstracted interface. Users can place and visualise provided or user-generated content directly within a reconstructed real-world space, temporarily modify the spatial environment, and persist their work through saving and loading, all inside the same XR workspace. Crucially, these tools are designed to operate within a shared spatial context, supporting geographically distributed collaborators who co-author in real time while viewing changes as they occur. This XR-native approach to authoring reduces abstraction, grounds design decisions in embodied spatial experience, and positions CATCH-XR as a distinctive collaborative authoring system that leverages the affordances of mixed and virtual reality to enhance remote exhibition design and prototyping.