# GazeRayHand: Combining Gaze Ray and Hand Interaction for Distant Object Manipulation
## 2 Related Work
### Object Manipulation in VR  
A simple but natural way to accomplish object manipulation is to directly grasp and manipulate an object with a Simple Virtual Hand (SVH) [1]. This method supports 6-DOF transformation (i.e., three for translation and three for rotation), similar to object manipulation in the physical world. However, this approach has two main limitations: (1) limited precision due to handling 6 DOFs simultaneously and (2) difficulty in manipulating out-of-reach objects.  
To address the first issue of precise manipulation, researchers separated 6-DOF manipulation into multiple manipulations of one or two DOFs [18, 20, 21]. For example, [PERSON] et al. [22] employed three geometrical constraints (plane, ray, and point) to guide the object manipulation. These constraints allow users to manipulate restricted DOF at once: a plane only supports 2D translation on the plane, a ray supports 1D translation (i.e., linear movement along the ray) and 1D rotation (rotation around the ray), and a point supports 2D rotation and 1D translation anchored by the point. Another solution for precise manipulation is scaling the movement of the virtual object based on the speed of hand movements [23; 24; 25]. For example, [PERSON] et al. [23] introduced PRISM (Precise and Rapid Interaction through Scaled Manipulation) where a user could perform fine-grained object manipulation with a scaled-down 6-DOF transformation by reducing the amount of manipulation according to the speed of hand movement.  
To overcome the challenges of manipulating distant objects, many researchers have explored methods for extending the user's reach [2; 4; 24]. For instance, [PERSON] et al. [2] proposed Go-Go interaction, which allows the user to lengthen a virtual arm to reach distant objects. When stretching user's arm beyond a predefined distance, the virtual arm grows non-linearly proportional to reach, enabling the grasping of distant objects. Similarly, HOMER (Hand-centered Object Manipulation Extending Ray-casting) [4] lengthens the user's arm after a user selects a distant object by a ray-casting method first. Once the object is selected, the virtual arm grows to place the hand at the selected object, allowing the user to manipulate it directly as if the user's body was in front of the object.  
Another approach for distant object manipulation is providing a miniature copy of the environment and synchronizing the user's manipulation of the miniature to the full-scale objects [26; 27; 28]. For instance, [PERSON] et al. [26] proposed the World-In-Miniature (WIM) interface. The miniature is identical to the virtual space where a user is in but scaled-down and positioned nearby the user. This allows the user to manipulate full-size, distant objects by interacting with miniature objects as they are synchronized with each other. However, reducing the size of virtual objects could increase the difficulty of fine-grained manipulation because a small geometrical error in the miniature results in a larger geometrical error in the full-sized objects. Additionally, since the miniature is always in front of a user, it could sometimes occlude the full-size VR environment. As a solution, Scaled-World Grab [27] was introduced. This method automatically scales down a virtual world when a user selects an object and scales it back up when the user releases the object.  
Additionally, [PERSON] et al. [20] presented Unimanual Metaphor with Scaled Replica (UMSR), a method for precise and distant object manipulation. This method places a scaled replica of the distant object within the user's near-field and allows the user to manipulate it. For precise manipulation, this method supports restricted DOF manipulation when selecting the face, edge, or vertex of the bounding box of a replica.  
Nevertheless, performing hand manipulation for more than a few minutes leads to arm fatigue (a.k.a. the gorilla arm effect [29]). Hence, recent studies have explored interfaces that combine hand and gaze interactions for distant object manipulation.