starfield_effect.lua

local function makeGenerator()

local kernel = {
    category = "generator",
    name = "stars",
    isTimeDependent = false
  }

  kernel.vertexData = {
    {
      name = 'resolutionX',
      default = display.pixelWidth,
      index = 0
    },
    {
      name = 'resolutionY',
      default = display.pixelHeight,
      index = 1
    },
    {
      name = 'seed',
      default = 234203212011.0,
      index = 2
    }
  }

  kernel.fragment =
  [[
  #ifdef GL_ES
  precision P_NORMAL float;
  #endif

  // based off http://glslsandbox.com/e#32816.0

  float rand(vec2 co)
  {
      return fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 437.585453);
  }

  vec4 FragmentKernel( P_UV vec2 texCoord ){

    //Play with prob and resolution to tweak effect depending on your tile size
    float prob = 0.998;
    vec2 resolution = vec2(15);

    //if you want 'twinkle' effect, turn on isTimeDependent  for kernel, and set
    //seed to be CoronaTotalTime
    float seed = CoronaVertexUserData.z;
    
    float color = 0.0;

    if (rand(texCoord.xy / resolution.xy) > prob)
    {
      float r = rand(texCoord.xy);
      color = r * (0.85 * sin(seed * (r * 5.0) + 720.0 * r) + 0.95);
    }

    P_COLOR vec4 ret = vec4(vec3(color), 1.0);
    return CoronaColorScale(ret);
  }
  ]]

  graphics.defineEffect(kernel)
end

makeGenerator()

local tile = display.newRect(
  display.contentCenterX, display.contentCenterY,
  display.pixelWidth, display.contentHeight
)

tile.fill.effect = "generator.custom.stars"
tile.fill.effect.resolutionX = display.pixelWidth
tile.fill.effect.resolutionY = display.pixelHeight

-- tile.fill.effect = "generator.sunbeams"
-- tile.fill.effect.posX = 0.5
-- tile.fill.effect.posY = 0.5
-- tile.fill.effect.aspectRatio = ( tile.width / tile.height )


local function tap( )
  print("tap")
  tile.fill.effect.seed = math.random( 10000000 )

end
Runtime:addEventListener("tap", tap)