rainbow_rain.py

#!/usr/bin/env python

# Nicolas Seriot, 2018-11-14
# According to...
# Sample output: ...

import cairo
import random
import math

def rainbow(pct, offset = 0.0):
    r = math.sin(2 * math.pi * (offset + pct) + 4) * 0.5 + 0.5
    g = math.sin(2 * math.pi * (offset + pct) + 2) * 0.5 + 0.5
    b = math.sin(2 * math.pi * (offset + pct) + 0) * 0.5 + 0.5    
    return (r, g, b)

WIDTH = 800
HEIGHT = 600

surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, WIDTH, HEIGHT)
c = cairo.Context(surface)
c.set_source_rgb(0.0, 0.0, 0.0)
c.paint()

for col in xrange(WIDTH):

    color = rainbow(1.0 * col / WIDTH, offset = 0.5)
    
    y = random.randint(0, HEIGHT)
    
    alpha = random.uniform(0.4, 0.9)
    
    g = cairo.LinearGradient(0, 0, 1, y)
    g.add_color_stop_rgba(0, 0, 0, 0, 0.0) 
    g.add_color_stop_rgba(1, color[0], color[1], color[2], 1.0)
    
    c.rectangle(col, 0, 1, y)
    c.set_source(g)
    c.fill()
    
surface.write_to_png("rainbow_rain.png")