Procedural animated texture generation/Nether portals

# assume time is a value from 0 to 1; a value of 1 results in the same
# image as a time value of 0 (excluding the random shimmering effect)
def setup_portal_sprite (time: float, output: Image):
	random = Random(100)
	wide, tall = output.size
	for x in range(wide):
		for y in range(tall):
			n = 0.0
			for dir in range(2):
				# All in this loop is done twice to create two spirals,
				# one of which offset to the topright
				spiral_x = (x - dir * (wide // 2)) / wide * 2.0
				spiral_y = (y - dir * (tall // 2)) / tall * 2.0
				
				spiral_x += 2 if spiral_x < -1 else -2 if spiral_x >= 1 else 0
				spiral_y += 2 if spiral_y < -1 else -2 if spiral_y >= 1 else 0

				mag = spiral_x ** 2.0 + spiral_y ** 2.0
				out_spiral = atan2(spiral_y, spiral_x)

				# Mag is used here to make the arms of the spiral constrict
				# the further from the centre of the spiral they are
				out_spiral += ((time * pi * 2) - (mag * 10) + (dir * 2)) * (dir * 2 - 1)
				
				# `(i * 2 - 1)` reverses the direction of the spiral if is 0
				# `* 0.5 + 0.5` brings the spiral into the range of 0 to 1 rather than -1 to 1
				out_spiral = sin(out_spiral) * 0.5 + 0.5
				
				# make the intensity of the spiral's arms diminish with distance from the centre
				out_spiral /= mag + 1
				
				# divide by two so that the final range is 0 to 1 instead of 0 to 2,
				# as we're generating two spirals
				n += out_spiral / 2

			n += random.range(0.0, 0.1) # make the spirals shimmer slightly at random
			
			r = int(n ** 2 * 200 + 55)
			g = int(n ** 4 * 255)
			b = int(n * 100 + 155)
			output.set_pixel(x, y, r, g, b, b) # blue is used for transparency