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PA Particle Field is a GPU Based alternative particle system, built from scratch specifically designed for looping ambient effects (though it can do so much more).
Built to reduce drawcalls, fillrate and CPU overhead (especially on mobile devices and VR), each field supports up to 16250 particles with just 1 drawcall and can be fully customized.

Features:
New Turbulence modes
Mesh Particles
WorldSpace and LocalSpace simulation
Streched billboards
Custom facing direction
Multiple sprites in a single field (using a spritesheet)
Spritesheet animation
Per particle color
Custom shader support with helpful macros

Also Exclusion Zones allow you to exclude particles from areas and assign priority between them, useful for bus shelters in the rain etc. Each field can handle 3 Exclusion Zones at once and are assigned and handled automatically.

The package comes with 7 demo scenes:
Snow, Caustics, Rain, Stardust, Bokeh, Clouds, Meshes, Animated Meshes, Fish plus a custom vertex program demo and a comparison scene showing PA Particle Field handling 195000 particles with ease.

PA Particle Field manages its efficiency by getting rid of particle birth and death. Instead it recycles particles using a bounding box, when a particle reaches the edge of the box it reappears on the opposite side.

When simulating in worldspace (the default mode) the field can be moved without moving the particles it controls, giving the effect of simply revealing the particles at the new position, creating the illusion of an infinite particle field.