Configurable Minecraft nether portal atlas generation script for Node.js. Made possible by coyo_t's work and the Minecraft wiki (original code at http://catsofwar.com/mc_proctex/mc.html).

configurablePortal.js

const { createCanvas, ImageData } = require("@napi-rs/canvas");
const { writeFileSync } = require("fs");

/**
 * @typedef {Object} PortalOptions
 * @property {Number} width
 * @property {Number} height
 * @property {Number} [frameCount]
 * @property {Number} [spiralCount]
 * @property {Number} [noise]
 */

const clamp = (x, mini, maxi) => Math.max(mini, Math.min(x, maxi));

function toRGBA(input) {
	const r = Math.floor(input ** 2 * 200 + 55);
	const g = Math.floor(input ** 4 * 255);
	const b = Math.floor(input * 100 + 155);

	// alpha channel is the same as blue
	return [clamp(r, 0, 255), clamp(g, 0, 255), clamp(b, 0, 255), clamp(b, 0, 255)];
}

/**
 * Generates a nether portal frame at the given index and frametime
 * @author Evorp, coyo_t, LethalChicken?
 * @param {Number} index - Frame index
 * @param {PortalOptions} options - Options for frame
 * @returns {ImageData} Generated frame data
 */
function generatePortalFrame(index, options) {
	const pixels = new ImageData(options.width, options.height);

	// going pixel by pixel
	for (let posX = 0; posX < options.width; ++posX) {
		for (let posY = 0; posY < options.height; ++posY) {
			// color value for the pixel as added sines
			let output = 0;

			// there's two main spirals so we iterate over both per-pixel
			// there's the one in the middle and one offset to the top right
			for (let spiralCount = 0; spiralCount < 2; ++spiralCount) {
				// get offset for each spiral
				let spiralX = ((posX - spiralCount * (options.width / 2)) / options.width) * 2;
				let spiralY = ((posY - spiralCount * (options.height / 2)) / options.height) * 2;

				// make sure the offset spiral appears on every corner
				spiralX += ((spiralX < -1) - (spiralX >= 1)) * 2;
				spiralY += ((spiralY < -1) - (spiralY >= 1)) * 2;

				const distance = spiralX ** 2 + spiralY ** 2;

				// https://en.wikipedia.org/wiki/Atan2#/media/File:Atan2definition.svg
				let spiral = Math.atan2(spiralY, spiralX);

				spiral +=
					// percentage of frames done
					((index / options.frameCount) *
						// convert percentage into radians (2pi = 360˚)
						Math.PI *
						2 -
						// creates the alternating stripes
						distance * options.spiralCount +
						// make the center spiral a bit bigger
						spiralCount * 2) *
					// mirror direction of corner spiral
					(spiralCount * 2 - 1);

				// converts the radians into ratio, and clamps to [0, 1]
				spiral = Math.sin(spiral) * 0.5 + 0.5;

				// decrease color intensity depending on distance from center
				spiral /= distance + 1;

				// tone down the colors a bit more before adding the value
				output += spiral * 0.5;
			}

			// add a tiny bit of noise so the added sines aren't too smooth
			output += Math.random() * options.noise;

			// convert sines to rgba
			const converted = toRGBA(output);

			// multiply by length of [r, g, b, a] array
			const pxi = (posY * options.width + posX) * 4;
			pixels.data[pxi] = converted[0];
			pixels.data[pxi + 1] = converted[1];
			pixels.data[pxi + 2] = converted[2];
			pixels.data[pxi + 3] = converted[3];
		}
	}

	return pixels;
}

/**
 * Generates a nether portal texture with given dimensions
 * @author Evorp
 * @param {PortalOptions} options - Generation options
 * @returns {Buffer} PNG buffer of the compiled portal atlas
 */
function generatePortalTexture(options = {}) {
	if (!options.width || !options.height)
		throw new SyntaxError("You need to specify width and height properties!");
	options.frameCount ||= 32;
	options.noise ??= 0.1;
	options.spiralCount ??= 10;

	const canvas = createCanvas(options.width, options.height * options.frameCount);
	const ctx = canvas.getContext("2d");

	for (let frameIndex = 0; frameIndex < options.frameCount; ++frameIndex)
		ctx.putImageData(generatePortalFrame(frameIndex, options), 0, frameIndex * options.height);

	return canvas.toBuffer("image/png");
}

// sample usage
writeFileSync(
	`./nether_portal.png`,
	generatePortalTexture({ width: 16, height: 16, noise: 0.1, frameCount: 32, spiralCount: 10 })
);