|
||||
|
||||
|
|
|
|
|
|
The Xpresso Expression To see the Xpresso Expression, open the Object Manager and locate the Null Object. Double-click its Xpresso Tag: None of the yellow Nodes are active. They are of two types: Value Indicators and Remarks. Result Value Indicators display the value currently being produced by a Node or being delivered to a Node, depending on the Port to which the Indicator is Linked. A Spy Indicator simply displays the value being passed between the two nodes to which it is Linked and does not alter that value. Indicators are useful when debugging. (Result and Spy Nodes do not show correct values while the animation is running.) Remark Nodes are there to add helpful information about what is going on. Additional remarks are sometimes included in the Node's Attributes (in the Attribute Manager) revealed after pressing the Basic button. The Generator is simply a Null Object and is represented by its Node and this was done by simply dragging the name of the Generator from the Object Manager on to the Xpresso Editor window. The Node has an Output Port which will produce the Global Y-coordinate of the Generator as a result of it being animated to move gradually up and down as described earlier - our Simple Sequence of Numbers. Clever, huh? Newton's Formula This Simple Sequence of Numbers (representing Time - T - as it marches steadily on) is then fed to the Formula Node. This very useful node allows you to create simple formulas - a description of it is in the manual on page 949. Newton's formula has been rearranged slightly to suit the Node and it is specified in the Node's Attributes. It is $2*$3/$1, which means in ordinary English "multiply the value arriving at Input Port 2 by the value arriving at Input Port 3, then divide by the value at Input Port 1". Because Input Ports 2 and 3 are both provided with the current number in our Simple Sequence of Numbers (and which is really T in Newton's formula), we now have T x T. Dividing that by the constant arriving at Port 1 completes the formula as T x T x K, where in this demonstration K = 1/3, chosen simply to get small enough numbers for the Ball's Y-coordinates to keep the Ball inside the Viewport. Negate Node For this demonstration it seemed sensible to have the Ball dropping down rather than up (gravity being what it is), so we want all the Y-coordinates to be negative. That is what the Negate Node does. We now have our Suitable Sequence of Numbers. (Although, come to think of it, it would have been simpler to have the Generator Object move down rather than up, so generating negative numbers in the Simple Sequence of Numbers and therefore avoiding the need for negation.)
Movie Make a movie of the Scene to see the full effects. |
||
|
|
|
 |
||||
 |
||||
|
|
|
|
|
and the 
If some of the Nodes are positioned outside the window, make them all fit in by choosing Xpresso Editor window > Menu Bar > View > Show All.
Finally, here is a graph showing how the distance of the of the Ball (D) from the starting position varies over time for one movement, downwards.) Note how the Distance (on the vertical axis of the graph) increases at an increasing rate as time increases (horizontal axis). This is a non-linear graph because it is not straight. Its curve is due to the fact that T is multiplied by itself in Newton's formula (T is squared, also called raised to the power of 2). Squared terms (or cubed ones) always produce curves. Is that why bananas are curved?