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Grabbing the Air
The "grabbing the air" technique, also first discussed in 1995, uses the metaphor of literally grabbing the world around you (usually empty space), and pulling yourself through it using hand gestures. This is similar to pulling yourself along a rope, except that the "rope" exists everywhere, and can take you in any direction. The grabbing the air technique has many potential uses in video games including climbing buildings or mountains, swimming, and flying.
To implement the one-handed version of this technique (the two-handed version can get complex if rotation and world scaling is also supported), when the initial button press is detected, we simply obtain the position of the hand in the world coordinate system.
Then, every frame until the button is released, get a new hand position, subtract it from the old one, and move the objects in the world by this amount.
Alternately, you can leave the world fixed, and translate the viewpoint by the opposite vector. Before exiting the callback, be sure to update the "old" hand position for use on the next frame.
Note it is tempting to implement this technique simply by attaching the world to the hand, but this will have the undesirable effect of also rotating the world when the hand rotates, which can be quite disorienting. You can also do simple constrained motion simply by ignoring one or more of the components of the hand position (e.g. only consider x and z to move at a constant height).
3D selection is the process of accessing one or more objects in a 3D virtual world. Note that selection and manipulation are intimately related, and that several of the techniques described here can also be used for manipulation.
There are several common issues for the implementation of selection techniques. One of the most basic is how to indicate that the selection event should take place (e.g. you are touching the desired object, now you want to pick it up). This is usually done via a button press, gesture, or voice command, but it might also be done automatically if the system can infer the user's intent.
One also has to have efficient algorithms for object intersections for many of these techniques. We'll discuss a couple of possibilities. The feedback given to the user regarding which object is about to be selected is also very important. Many of the techniques require an avatar (virtual representation) for the user's hand.
Finally, consider keeping a list of objects that are "selectable", so that a selection technique does not have to test every object in the world, increasing efficiency. Four common selection techniques include the virtual hand, ray-casting, occlusion, and arm extension.
The Virtual Hand
The most common selection technique is the simple virtual hand, which does "real-world" selection via direct "touching" of virtual objects. This technique is also one of the oldest and dates back to the late 1980s. In the absence of haptic feedback, this direction manipulation is done by intersecting the virtual hand (which is at the same location as the physical hand) with a virtual object.
The virtual hand has great potential in many different video game genres. Examples include selection of sports equipment, direct selection of guns, ammo, and health packs in first person shooter games, as a hand of "God" in real-time strategy games, and interfacing with puzzles in action/adventure games.
Implementing this technique is simple, provided you have a good intersection/collision algorithm. Often, intersections are only performed with axis-aligned bounding boxes or bounding spheres rather than with the actual geometry of the objects.
Another common selection technique is ray-casting. This technique was first discussed in 1995 and uses the metaphor of a laser pointer -- an infinite ray extending from the virtual hand. The first object intersected along the ray is eligible for selection.
This technique is efficient, based on experimental results, and only requires the user to vary 2 degrees of freedom (pitch and yaw of the wrist) rather than the 3 DOFs required by the simple virtual hand and other location-based techniques. Ideally, ray-casting could be used when ever special powers are required in the game or when the player has the ability to select objects at a distance.
There are many ways to implement ray-casting. A brute-force approach would calculate the parametric equation of the ray, based on the hand's position and orientation. First, as in the pointing technique for travel, find the world coordinate system equivalent of the vector (0,0,-1). This is the direction of the ray. If the hand's position is represented by (), and the direction vector is (), then the parametric equations are given by
Only intersections with should be considered, since we do not want to count intersections "behind" the hand. It is important to determine whether the actual geometry has been intersected, so first testing the intersection with the bounding box will result in many cases being trivially rejected.
Another method might be more efficient. In this method, instead of looking at the hand orientation in the world coordinate system, we consider the selectable objects to be in the hand's coordinate system, by transforming their vertices or their bounding boxes. This might seem quite inefficient, because there is only one hand, while there are many polygons in the world. However, we assume we have limited the objects by using a selectable objects list. Thus, the intersection test we will describe is much more efficient.
Once we have transformed the vertices or bounding boxes, we drop the z coordinate of each vertex. This maps the 3D polygon onto a 2D plane (the xy plane in the hand coordinate system).
Since the ray is (0,0,-1) in this coordinate system, we can see that in this 2D plane, the ray will intersect the polygon if and only if the point (0,0) is in the polygon. We can easily determine this with an algorithm that counts the number of times the edges of the 2D polygon cross the positive x-axis. If there are an odd number of crossings, the origin is inside, if even, the origin is outside.