The Xpresso Expression
To see the Xpresso Expression, open the Object Manager and locate the Null Object. Double-click its Xpresso Tag: and the Xpresso Editor window will open showing various Nodes Linked together. If you need to move the whole collection of Nodes within the window to a more convenient position, click and drag the Move Icon at the top right corner of the Xpresso Editor window: 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.
The PStorm Node is the central one. This node controls a Particle Emitter (which is a different Particle Emitter from the conventional one that takes the form of an Object in the Object Manager.) It can have many Ports and Parameters. In this case the relevant Input Ports are Emitter Position which controls the position of the Emitter (its position in X, Y and Z coordinates); and Emitter Alignment which controls the direction in which the emitter sends its Particles. It has only one Output Port activated in this Expression: Particle Birth, which produces relevant information about which particular particles are currently being born (particles can be designated to different Groups although not in this animation). In this case it sends that information to a PShape Node which controls the details (shape, size etc.) that those particles will have when emitted. To make the particles appear as the Sphere, the Sphere must be the child of a Particle Geometry Object (Cinema Main Menu > Plugins > Thinking Particles > Particle Geometry).
Emitter Position Node
This is a Null Object and its two Output Ports feed the PStorm Node to control the PStorm Node's emitter position and direction. If you drag or rotate this Null Object in the Viewport, the emitter will follow, (although it will not move to its new position or rotation until the animation starts). This Null Object is also used to give a home to the Xpresso Expression Tag. An Xpresso Expression Tag can be attached to any Object in the Object Manager - it doesn't matter which one - but a Null Object is usually most convenient.
This node controls how the particles emitted by the PStorm node will collide with the Object specified in its Attributes. After selecting the PDeflector Node, the name of the object (in this case the Reflector) was dragged into the Attribute Manager : Parameter button, and on to the Object Field. It has many Parameters and page 47 of the Thinking Particles manual describes their characteristics. The important ones in this project are Deflector Type (set to Object) and Bounce (set at 30). The purpose of the PPass Node is to pass information to the Deflector Node about the particles being emitted and their positions frame-by-frame.
You should study the above Walkthrough (E4) before going further.
Creation of the Scene
Load the other file called "ParabolaE5.c4d" into Cinema. The file is already complete. Choose Front View of the Viewport and run the animation to get an idea of how it works. You will see that the particles move in a wide parallel stream from the left towards the Reflector, are reflected by it and converge to a point (the Focus), thus demonstrating the essential feature of the operation of a parabolic reflector when acting as a "telemicrophone". To demonstrate that the particles are indeed converging on the Focus, a small sphere has been placed there and the particles invited to collide with it, and if they do so within a certain accuracy of position, to die immediately.
This is the same in principle as the Reflector, with the following changes. It is flatter, to position the Focus outside it and therefore make convergence more dramatic. This was done in the Formula Object whose parameters are slightly different. The number of Samples remains at 40. The Lathe NURBS was created with 80 Subdivisions as before.
The Xpresso Expression
This is identical to the one for the Reflector but with the addition of the top four nodes. Their purpose is to kill any particles that reach the Focus. Also, some of the Parameters of the PStorm Node are different.
The Field of View Parameters - X Fov and Y Fov - (select the PStorm Node and look at the Attribute Manager : Parameter button) are set to 0.1 (which is a very small angle) to make all the particles move parallel to each other. Also, the X- and Y-Size of the emitter has been increased to 300 each to provide a broad beam of particles.
PPass and the Top PDeflector Nodes
The PPass Node passes the details of all the particles to the PDeflector Node which produces a signal at its Event Output Port whenever a particle collides with the Focus. The PDeflector Node has identical Parameters as the other PDeflector Node except for the Object specified as the collision object (the Focus). With this PDeflector Node another difference is that it has an Event Output Port which will produce a signal whenever a particle collides with it, passing this to the Particle Input Port of the PDie Node, switching it on at that moment. The PDie Node is also provided by the PPass Node with details of the particles so that it can kill the correct one.
Make a movie of the Scene to see the full effects.