Particles alpha decay

rrraptor

I wonder, how to calculate alpha decay for particles to make them lose alpha over time... When life time = 0, alpha = 1, when life time = ttl, alpha = 0 (or wise versa).
From wiki:

factor applied to speedX and speedY, speedAngular, speedGrowth and alpha at the beginning of each frame

So I did a quick test:

local frames = 0 -- frames counter local ttl = 100 -- how many frames particle will be alive local alpha = 1 -- initial alpha local decay = 0.955-- magic number local obj = Pixel.new(0, alpha, 100,100) -- fake "particle" stage:addChild(obj)   local realParticles = Particles.new() stage:addChild(realParticles) realParticles:addParticles{{ ttl = ttl, decayAlpha = decay, alpha = alpha,   x = 150, y = 50, size = 100, color = 0, }} function update() frames += 1 if (frames > ttl) then print("Fake alpha:", alpha)   local _,a=realParticles:getParticleColor(1) print("Real alpha:", a) -- fake is pretty close to real stage:removeEventListener(Event.ENTER_FRAME, update) end alpha *= decay obj:setAlpha(alpha) end   stage:addEventListener(Event.ENTER_FRAME, update)

And seems like this is exactly how it works, but real code for alpha decay is

alpha=alpha*pow(decay_[i * 4 + 1],nframes);

and I have no idea how to calc that magic alpha decay number >_< Any help?
Also, since it gets multiplied by a factor, you cant set initial alpha to 0, because
0*anyNumber = 0
Maybe it would have been better to replace multiplication with addition/substraction?
E.g. decay = 1/ttl alpha -= decay will decrease alpha exactly from 1 to 0 (or alpha += decay for 0 to 1).