Procedural animated texture generation/Nether portals

This page describes content that has been removed and was only present in earlier versions of Minecraft.

Procedurally generated textures such as this one were removed and replaced by pre-rendered animations in Java Edition 1.5 and Xbox 360 Edition TU12.

Due to the large number of Atan2 and Sine operations, the nether portal frames are not generated in real time; instead, 32 frames of animation (at a resolution of 16×16) are generated once at startup and stored into an internal animation strip. The random shimmer is the same every time, as the game always uses a random number generator with a seed of 100.

To generate one frame of the nether portal animation:

# 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

Due to internally using a looping animation already, the nether portal is the only one of the procedural textures that is ported 1:1 in the 1.5 resource pack changes. Unlike the water animations, the alpha channel was not made uniform in the 1.15 texture update.

The dir value shifting between 1.0 and 0.0 to show its effect on a single spiral
The crude way the spirals are tiled (seen on lines 15 and 16 of the above script)
Ditto, but zoomed out to show the full extent
The two spirals joined, shown with and without the tiling effect
Ditto, but with the final shimmer and colors applied