If you're using Foam or Particular, you can create paths for the particles to travel. Foam uses flow maps (see the help file I linked) and Particular uses motion paths (see your Particular documentation).
You can use Bezier Warp, Mesh Warp, Liquify and/or any recipe of other distortion tools to direct the flow of something as well. (Bezier Warp technique demonstrated in this tutorial)
Thank you very much I will try that.
In the olden days, before Foam's ability to follow an alpha map, we created the liquid element (shot it on video or rendered a simulation) and then masked the movie into the pipes. As long as all you need is a flat image, that works and is easy to create and modify. Foam is great fun but a bit unpredictable until you've spent a long time playing with it. ONe thing you must know about the alpha map, keep the edges smooth, a gradual fade from white to black. The reasons are obscure but the temptation is to assume a hard line represents an impassable boundary. Foam's mathematic engine sees the alpha as a wall with steepness represented by the grayscale. Particles will not climb a wall of gray (because they interact with the gray pixels) but they can actually get catapulted easily over a the single-pixel transition from black to white, they just don't see it.
Thanks bogiesan, you are the only one who talked about Alpha paths . . . but, how do I make an Alpha path in AE -- with grey scale sides -- say, in the shape of a snake (or vein)? Or do you know of a better AE tool to create blood flows through tubes? Neil
Foam is intended to use greyscale images. Alpha map is just another term for it. Bogiesan and the rest of us are all talking about the same thing.
There is actually quite a bit of information here. You should read up on it. It should explain a lot.
How do I make an Alpha path in AE -- with grey scale sides -- say, in the shape of a snake (or vein)? Or do you know of a better AE tool to create blood flows through tubes?
Foam is a pretty good tool for creating blood flow through tubes if you're wanting to see the individual blood cells.
There are several ways to make the shape.
One would be to use Vegas on a mask and set the number to 1 and set the mid point opacity to half and the color to white or black.
Another would be to create a black to white gradient ramp and use the Puppet Tool on it.