How do I create a blinking effect with SKEmitterNode?

Question

I have used the particle emitter to create a background with stars. It looks ok, but I would like them to blink, or flicker. The closest I get is when I change the birthrate and lifetime variables so that particles disappear and appear at different places. I would like the particles to remain in the same place and fade in and out, randomly, though. Any ideas on how to do this? This is what I've got so far:

Answer 1

I don't think you can do much directly in the editor. If you're comfortable working with code for adjusting the emitter, you have a couple of possibilities: setting a particle action to animate color or alpha or scale or texture, or a custom shader to do whatever sort of animation. (I'm assuming based on your picture with a basically infinite lifetime that you don't want things to move or disappear. That may rule out keyframing, but perhaps having the keyframe sequence set to repeat mode with the frames spaced by really tiny values would work.)

Another possibility since positions are static would be to just make some fixed sprites scattered around at random and have them run actions to animate them. We've used this approach before with ~100 animated sprites against a backdrop that has a bunch of dimmer stars, and it looked pretty good. Something along these lines:

let twinklePeriod = 8.0
let twinkleDuration = 0.5
let bright = CGFloat(0.3)
let dim = CGFloat(0.1)
let brighten = SKAction.fadeAlpha(to: bright, duration: 0.5 * twinkleDuration)
brighten.timingMode = .easeIn
let fade = SKAction.fadeAlpha(to: dim, duration: 0.5 * twinkleDuration)
fade.timingMode = .easeOut
let twinkle = SKAction.repeatForever(.sequence([brighten, fade, .wait(forDuration: twinklePeriod - twinkleDuration)]))
for _ in 0 ..< 100 {
  let star = SKSpriteNode(imageNamed: "star")
  star.position = CGPoint(x: .random(in: minX ... maxX), y: .random(in: minY ... maxY))
  star.alpha = dim
  star.speed = .random(in: 0.5 ... 1.5)
  star.run(.sequence([.wait(forDuration: .random(in: 0 ... twinklePeriod)), twinkle]))
  addChild(star)
}

That's cut-and-pasted from various bits and simplified some, so there may be typos, but it should give the idea. If you keep the emitter, you can try something like the twinkle above as the particle action. I don't see how you can change the relative periods of particles though like you could with separate sprites, and the only offsets would come from differences in the birth time of the particles.