I am trying to something I assume is quite basic. Have an emitter display is animation within the confines of a simple alpha shape (a very simple logo). I followed Elvis's 'charmed' orbing tutorial and disintegrating logo tutorial for learning how to do this. Using the 'orbing cover' emitter with 'get transparency from layer' & increasing the 'number' property I was able to get a vague shape of my logo. However, it is only vague. The emitter decidedly spills beyond the confines of the shape of my logo. Why is this? Worse, trying this same technique with some other emitters (an example would be the emitters from Oct.'06) the emitters seem to almost ignore the logo alpha altogether. To be sure, the alpha is doing something with these emitters, their behavior is affected by the alpha, but the result is certainly not that they only appear within the confines of my alpha.
What is the property of an emitter that allows it to animate beyond the confines of an alpha? Is there some 'randomness' property? (if so I haven't spotted it). I have tried using 'mask emitter with image' which seems to work no better.
The biggest factor that will affect this is the size of the particles. The bigger the particle, the more it will break the illusion, since particles born at the very edge pixel of an opaque part of the background image will be born hanging over the edge of your logo.
Motion Randomness too will affect it, especially if Frames to Preload is set to something high, since then the particles have already been born and are going off in all directions.