-- Lesson 4: Doing It Again

LAYERS          = 3
STARS_PER_LAYER = 40
ACROSS_STEP     = 37
DOWN_STEP       = 61
LAYER_SHIFT     = 13
LAYER_SPEED     = 0.5
LAYER_BRIGHT    = 55

width  = overlayGetWidth()
height = overlayGetHeight()

drift = 0


function onOverlayUpdate()
    drift = drift + 1
    overlayClear()

    for layer = 1, LAYERS do
        for star = 1, STARS_PER_LAYER do
            starX = (star * ACROSS_STEP + layer * LAYER_SHIFT) % width
            starY = (star * DOWN_STEP + math.floor(drift * layer * LAYER_SPEED)) % height
            shade = math.random(60, 90) + layer * LAYER_BRIGHT
            colorForeground(shade, shade, shade)
            overlayPlot(starX, starY)
        end
    end

    return OVERLAY_UPDATED
end