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WALKTHROUGH NOs. E4 & E5 Parabolic Reflectors
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OBJECTIVE: to write a program in Cinema 4d R8 in Xpresso and Thinking Particles to demonstrate the operation of a parabolic reflector in two ways - as a reflector emitting parallel beams of objects or light (as in a searchlight); and as a reflector receiving objects, light or sound (as in a digital TV dish or a birdwatcher's "telemicrophone"). The first step in any computer programming task is to be clear about the objective that is to be achieved, (see above), followed by a statement of the logic that the program is to follow to achieve that objective. Skipping these steps leads to muddle, frustration and waste of time. LOGIC 1. Create a parabolic Reflector as an Object 2. Using Thinking Particles, place a particle emitter at the appropriate (accurate) position. 3. Animate the particles, aiming them at the reflector, so demonstrating the two methods of operation. Much of this Walkthrough is common to both operations. Let's start with the searchlight (E4). E4 SEARCHLIGHT Creation of the Scene Download the above Project folder and load the file in it called "ParabolaE4.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 stream from a point (the Focus), inwards towards the reflector, are reflected by it and emerge towards the left as a stream with each particle parallel to every other, thus demonstrating the essential feature of the operation of a parabolic reflector when acting as a "searchlight". This Walkthrough assumes that you know the basics of creating Objects and how to set up a simple animation. Therefore examine the Object Manager and the Timeline to familiarise yourself. The Reflector
The formula for a parabola has a second term "+ K*t" where K is a constant of a suitable value. but I found that making K = 0 made the movement of the particles more accurate.) Tmin was set to 0 so that only half the curve was present and the Samples set to 40. The Attributes of the Lathe NURBS were then specified as shown on the right - Subdivisions at 80 and Flip Normals checked. It is very important to Flip the Normals or the particles will not bounce off the Reflector. This is because, with an object like our Reflector with only one surface level (unlike a Cube for instance which has a front and a back even if it is thin), the Normals for each polygon point by default away from the convex (bulging) surface and need to point out from the concave (inside) surface for Xpresso to detect that particles have reached it. Finally, the Lathe NURBS was made Editable. It is important that that is done after the Formula Object is made a Child of it (otherwise the particles will not bounce off it). It should be noted that Objects in Thinking Particles (and in Dynamics) do not collide - it is polygons that collide, Objects being made of polygons. Therefore, with our Reflector, it is important to make its curve as accurate as practicable, which means the more polygons the better. 400 Samples for the Formula Spline and 400 Subdivisions for the Lathe NURBS result in all the particles leaving the Reflector accurately parallel to each other but produce a Project File of about 8 Mb ! The parameters in this Project are a compromise, resulting in some of the particles wandering a little. |
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The reflector was created by first creating a spline with the correct shape. The Spline Primitive Formula was used (Cinema Main Menu Bar > Objects > Spline Primitive > Formula ). This has a Sine curve by default and this was changed to a parabolic shape by entering the parameters shown here on the right. You can experiment with other constants to get different shapes
The Formula Object was then made Editable and made the Child of a Lathe NURBS (Cinema Main Menu Bar > Objects > NURBS > Lathe NURBS)