For our investigation we have restricted the world to a stage

= Setting for the research: a systems perspective

In the introducti= on we differentiated between public (measurable) and private (introspective) information, and looked at 4 types of information: action, spatial info, ti= ming info, and planning. For actio= n and perception we also looked at 3 layers of private information – using = an analogy of peeling an onion.

In the second and= third chapters we introduced the stage and the skeleton that represent the interf= ace between public and private information, and that form the basis for action = in the world.

In this chapter we explore the interface between public and private information from a systems perspective. To continue with= the pictures of the introduction, we shall discuss action, visual perception, planning, and timing. We shal= l also discuss briefly the nature of our knowledge about these systems, differentiating between public (measurable) knowledge and private (introspective) knowledge.

= Systems and interfaces

Systems can be se= en as black boxes that have boundaries. What happens inside the system might be not open to inspection or measurement (the black box), so that we can only inspect the interfaces, i.= e. what flows across the boundaries into and out of the system (the inputs and outputs). Other systems might= be more open to inspection.

We should be able= to utilize this concept to help with our analysis and modeling. The public, physical world can be = seen as one system, and the mind, brain, and nervous system of the actor can be = seen as another system. The ‘inside’ or private part of an actor is a black-box system. The physical world we know somethi= ng about, but not everything, so it is neither a black system nor a transparent system, but something in between.

The two systems i= nteract through the bones of the skeleton and though the optics associated with perceiving = the bones of the skeleton. Let us= start with the public system, ‘the world’.

= ‘The world’ and its interfaces

There are two sub-systems. The first, manag= ing the output from the system, is the optical sub-system that controls the opt= ical information that flows to the audience and to any other viewers of what is happening on the stage. All v= isual information provided by this subsystem is two dimensional and depends on the location or perspective of the viewer.&nbs= p; The visual information reflects the present location of the bones, i= .e. there is no significant delay and no memory of previous states.<= /span>

This model can be implemented in a client - server arrangement (e.g. browser = 211; SVG (http) - server), where the server represents the world.

The second sub-sy= stem consists of the collection of connected bones representing each of the acto= rs on the stage. This is a mecha= nical sub-system, in that each of the bones is moved about its relevant joints de= pending on the forces applied by muscles. We can observe this movement in kinesthetic studies, where little lights or markers are attached to various points on the body and limbs, and the movem= ent of these lights or markers is tracked.&nbs= p; The muscles in turn are stimulated to contract or expand by nerve impulses. Tracking the indivi= dual muscles, or the electrical stimulation applied to these muscles is more challenging, but some progress is being made with modern technology. To simplify this story, we shall f= ocus on the information supplied by the neural connections through the muscles to the bones on how much to move the bone relative to its previous position by rotating it about the appropriate joint.&n= bsp; We can infer this information by observing its effects, i.e. by investigating the changes in angles about the joints – such as through the kinesthetic studies mentioned above.&n= bsp; This neural information on relative motion then becomes the input to= the system – and the sub-system. We assume that each bone is moved individually, relative to bones th= at are connected to it at the joints. We assume that the neural information supplied to each bone specifies only incremental relative motion for a very short period of time. (No history, no information about t= he future.) The motion, of course, is limited = by the constraints of the joint and by other physical constraints such as not penetrating the floor of the stage.)

This model can be implemented in a browser – http-server arrangement, (i.e= . client - server), where the browser representing the brain requests a form listing= the bones and enters the requested change in joint angles. The information on this form is th= en uploaded to the server representing the world, and the bone positions are modified as per request (assuming the changes do not violate constraints).<= span style=3D'mso-spacerun:yes'> These modified bone positions are = then shared with the optical subsystem where they can be inspected.

Note 1: To further simplify the mechanical model, we have split off control over the hands into a separate subsystem t= hat is not included in this model. Only the rough orientation of the hands relative to the wrists is included.=

Note 2: The visual subsystem assumes that = the location of each bone is known relative to the stage, other bones, and other objects such as other actors. We assume that the bone subsystem only receives information for incremental mo= tion of each bone relative to the ‘preceding’ bone and relative to t= he rotary motion allowed for the connecting joint. The absolute position of each bone relative to the stage therefore has to be calculated by the mechanics of the world rather than by the information coming across the interface. Using introspection we would infer= that most of us know (can feel - proprioception?) how the bones are angled, but = do not know without looking just where they are.

= The problem of time for ‘the world’ interfaces

In both subsystems discussed above we assume that there is no memory of the past and no plan f= or the future, but only the present. The optical subsystem bears a resemblance to video, where time is handled as a stepwise sequence of frames.&= nbsp; We can use the same approach for the mechanical subsystem model of t= he world. This allows us to synchronize optics and mechanics by assuming that we go from state to state= in the bone positions which corresponding to a frame to frame sequence in the optical domain. In other word= s, the bone positions remain constant within the frame and are reflected in the views. At the same time, position-change information is received for each bone, which then results in modified bone positions for the next frame. In this model, perception can lag = by as much as the duration of a frame.