So as far as I can tell there are actually no features like this, but figured I'd ask anyway. Is there a way to:
a) Make particles fade in from start? Normally particles fade out of existence, but I'm trying to achieve a reverse effect where individual particles fade in from invisible when they're spawned. Particles have an option to set initial opacity and an opacity randomizer, but no gradual opacity change (other than fading out as part of their destroy conditions).
b) Offset particles from origin relative to their angle of travel? Particles have an offset option, but this is determined separately from their angle. I was looking to create an effect where particles will fan out in a cone, but instead of all originating at one spot, have a slight offset to their spawn point. Unfortunately, with the current setup particles that are offset are also given a random angle after they spawn, meaning that a particle can travel in a different direction than its angle relative to the origin of the object. In a way, this effect could actually be faked by the feature described in point (a).
Thus far I've faked both of these effects by placing a gradient (basically a sprite of a brush stroke with hardness set to 0) on top of the particle object, so particles look like they fade in as they move out from behind the object. However, this only works because the gradient blends in with the background, but the background is likely to be dynamic and change, thus revealing the object. I've attempted to play around with blend modes, but so far no luck as these tend to conflict with the blend modes of the particles.