Transparency can be provided as an Age-Effect. This is specified in the Alpha Channel of the Age Page. To see this, open the disclosure triangle to the right of the word "Colour" in the Colour Gradient (and uncheck the Display Result button). It is important to note that the Colour Gradient Field (the long slot in which the gradient is displayed) is shared by the Alpha Channel. Check the Edit Alpha button to see the Alpha's Gradient (which is entirely pure white at the moment so that transparency has no effect.) Where the Alpha Gradient is white the puff will be opaque; where it is black it will be totally transparent; greys will give semi-transparency. Change the Gradient to the picture above. The grey then black at the right end will make the puffs gradually become transparent at the end of their life. Transparency is important when modelling smoke or steam, or high temperature flames which are transparent near their point of creation. Without gradual transparency, puffs disappear suddenly which is not natural. (The Globals Page Colour Gradient also offers an Alpha Channel but I have not found much change in the result if this is experimented with.)
Save this version of the file as simple-2.c4d
Points to Note
The Preview Window shows what a single representative puff would look like as it ages from its birth to its death. If the Life of particles in the Emitter is specified as a number of frames less than the number of frames in the animation, then the complete effect of Ageing will be shown in the Preview Window. If on the other hand the Life of the particles is greater, the Preview Window will still show the complete effect of Ageing, but the animation will not (because death occurs after the animation has completed). The Preview Window "knows" nothing about the number of frames in the animation.
An Alpha Channel remains in effect even when the Edit Alpha button is not checked. If you want to inactivate an Alpha channel (therefore preventing any transparency effects), you must make it all pure white.
PyroCluster effects are most effective when animated. Whenever animating, specify your parameters so that the Preview Window of your project shows the puffs disappearing gradually due to increasing transparency, and have disappeared completely before they die, otherwise they will disappear suddenly in a very unnatural way. This is difficult in an animation of only a few frames, eg. 90, because the gradually increasing transparency would occupy too much of the animation and therefore hide much of the flame, fire or whatever. Also, it is best to prevent the beginning of the animation showing puffs bursting into life instantaneously, as they do, which is unnatural. You could hide young puffs behind an object (a volcano, a chimney, etc.)
Still keeping with simple-2.c4d, open the Rendering Settings (the right hand of these two icons) and adjust as necessary (Save Page - Path; Format - QuickTime Movie; Options - Sorensen Video 3; All Frames.) Now create the movie by double clicking the left hand icon. The render will take about 20 seconds. Run the movie and observe the various effects discussed above. Save the project again.
A PRETTIER EXAMPLE
The above project was designed to demonstrate some essential features of PyroCluster, but a more practical example of a flame would be, for example, burning crude oil at a refinery. The file in your downloaded folder called fire.c4d shows this in a simplified scene. The project builds on the techniques described above but several more advanced features have been included to give greater realism, as follows:
The Emitter Object: Particle Page has had some changes. In particular the Birthrate has been increased; the Speed also, and with Variation; Lifetime has also got Variation; Rotation has been introduced (with Variation) and to allow this to work, the PyroCluster: Shapes Page has had the Shape changed to Cylindrical and Auto Rotation is checked (Pages 35 and 36 of the PyroCluster Manual and Page 395 of the Release 7 main Reference Manual); Wind has been introduced; a camera ("Main Camera") has been created to allow the best viewpoint; a Sun Object is used to light the scene, along with a fill light, and Cast Shadows has been enabled in the PyroCluster Material: Shadows Page (enabling other shadows would increase render times by a huge amount).
The movie of the project takes 11 1/2 hours to render on a G4/400, (320 X 384 pixels, 210 frames, 30 fps. The extra effects can be clearly seen. The movie runs faster than realism requires, given that it is a static fire of relatively non-volatile crude oil rather than a flamethrower, but the high speed shows the various effects well.
If you have made it this far you will now have a headache, so wrap a wet towel on it and lie down in a dark room.