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\section{3D Bookmarks and Navigation Aids}

Devising an ergonomic technique for browsing 3D environments through a 2D interface is difficult.
Controlling the viewpoint in 3D (6 DOFs) with 2D devices is not only inherently challenging but also strongly task-dependent. In their review,~\citep{interaction-3d-environment} distinguish between several types of camera movements: general movements for exploration (e.g., navigation with no explicit target), targeted movements (e.g., searching and/or examining a model in detail), specified trajectory (e.g., a cinematographic camera path), etc.
For each type of movement, specialized 3D interaction techniques can be designed.
In most cases, rotating, panning, and zooming movements are required, and users are consequently forced to switch back and forth among several navigation modes, leading to interactions that are too complicated overall for a layperson.
Navigation aids and smart widgets are required and subject to research efforts both in 3D companies (see \url{sketchfab.com}, \url{cl3ver.com} among others) and in academia, as reported below.

Translating and rotating the camera can be simply specified by a \textit{lookat} point.
This is often known as point-of-interest movement (or \textit{go-to}, \textit{fly-to} interactions)~\citep{controlled-movement-virtual-3d}.
Given such a point, the camera automatically animates from its current position to a new position that looks at the specified point.
One key issue of these techniques is to correctly orient the camera at destination.
In Unicam \citep{two-pointer-input}, the so-called click-to-focus strategy automatically chooses the destination viewpoint depending on 3D orientations around the contact point.
The recent Drag'n Go interaction \citep{drag-n-go} also hits a destination point while offering control on speed and position along the camera path.
This 3D interaction is designed in the screen space (it is typically a mouse-based camera control), where cursor's movements are mapped to camera movements following the same direction as the on-screen optical-flow.

Some 3D browsers provide a viewpoint menu offering a choice of viewpoints \citep{visual-perception-3d,showmotion}.
Authors of 3D scenes can place several viewpoints (typically for each POI) in order to allow easy navigation for users, who can then easily navigate from viewpoint to viewpoint just by selecting a menu item.
Such viewpoints can be either static, or dynamically adapted:~\citep{dual-mode-ui} report that users clearly prefer navigating in 3D using a menu with animated viewpoints than with static ones.

Early 3D VRML environments \citep{browsing-3d-bookmarks} offer 3D bookmarks with animated transitions between bookmarked views.
These transitions prevent disorientation since users see how they got there.
Hyperlinks can also ease rapid movements between distant viewpoints and naturally support non-linear and non-continuous access to 3D content.
Navigating with 3D hyperlinks is potentially faster, but is likely to cause disorientation, as shown by the work of~\citep{ve-hyperlinks}.
\citep{linking-behavior-ve} examine explicit landmark links as well as implicit avatar-chosen links in Second Life.
These authors point out that linking is appreciated by users and that easing linking would likely result in a richer user experience.
\citep{dual-mode-ui} developed the Dual-Mode User Interface (DMUI) that coordinates and links hypertext to 3D graphics in order to access information in a 3D space.

The use of in-scene 3D navigation widgets can also facilitate 3D navigation tasks.
\citep{navigation-aid-multi-floor} propose and evaluate 2D and 3D maps as navigation aids for complex virtual buildings and find that the 2D navigation aid outperforms the 3D one for searching tasks.
The ViewCube widget \citep{viewcube} serves as a proxy for the 3D scene and offers viewpoint switching between 26 views while clearly indicating associated 3D orientations.
Interactive 3D arrows that point to objects of interest have also been proposed as navigation aids by Chittaro and Burigat~\citep{location-pointing-navigation-aid,location-pointing-effect}: when clicked, the arrows transfer the viewpoint to the destination through a simulated walk or a faster flight.
