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How to make particles conform to 2D shapes?

I have an idea for something I’d like to try with particle systems but haven’t a clue where to begin. Imagine a 2D sidescroller view first of all…

Say there is a particle system that emits a bunch of particles. I want those particles to be emitted into my scene from their source, and then I want to use those particles to cling to the edges of certain objects. The effect might be thought of as similar to what would happen if you poured icing sugar over your finger if it was wet. The icing sugar particles would cling to the wet surface until the surface becomes saturated, and the rest of the icing sugar continues to fall (the particles continue to do what they normally do). So if you’re following so far, what I would like to be able to do is define ‘wet areas’ that particles would cling to.

Secondly though, I would like to be able to use particles within my scene that have an infinite lifetime so that it is always the same bunch of particles that are clinging to defined areas in the scene. The idea is that the particles would float around the scene with no real agenda, but would sometimes be attracted in the direction of certain game objects and then cling to the outer edges of those objects. This gets more complicated because the ‘objects’ I speak of have to represent detailed shapes. For example, I might want to have an ‘object’ in my scene that represents the letter A, and have my particles cling to the shape of the letter A. The actual object would be invisible to the player - the important thing is the form that particles take which in this case would look like particles making a letter A in the level. I hope I’ve explained this well enough, but that last part is what I’m trying to do in a nutshell; I want my particles to take forms that I define. Maybe an even simpler way to imagine it would be covering a mesh in particles instead of a material. (I am interested in how I would do that too actually)

Now, I read the documentation on vector fields with GPU emitters which sounded somewhat promising but I’ve never used Maya and I’ve never done any animation or 3D modelling so I may be a little out of my depth there. The vector field explanation did give me an idea though. It described a vector field as a volume that had a sort of ‘wind’ inside it that would blow and influence the direction particles would flow in. Effectively that’s what I want to do, except I want the particles to make a shape, and then remain in that shape until I want them to make another shape somewhere else. Would it be possible to have black and white images (for a example a white background with a black letter A), and use this image like a 2D volume in the scene that tells particles to ‘cling’ to the black parts of the image but ignore the white parts, effectively giving you particles that make the shape of a letter A in your scene? There’s also the catch that this image would need to be invisible, so effectively it’s the data from the image I’d want to use like a volume where I say “if the particle occupies the same space (or is aligned with) the black part of the image, stop moving”.

Please just say anything to help me figure out how I’m going to achieve this level of control over particles!