jpeg.js

/**
 * https://en.wikipedia.org/wiki/YCbCr#JPEG_conversion
 *
 * JPEG conversion without head/footroom
 *
 * @module  color-space/jpeg
 */
import rgb from './rgb.js';

/** @type {Partial<import('./index.js').ColorSpace>} */
var jpeg = {
	name: 'jpeg',
	min: [0, 0, 0],
	max: [255, 255, 255],
	channel: ['Y','Cb','Cr'],
	alias: ['JPEG']
};

export default /** @type {import('./index.js').ColorSpace} */ (jpeg);


/**
 * JPEG to RGB
 * transform through analog form
 *
 * @param {Array<number>} arr RGB values
 *
 * @return {Array<number>} JPEG values
 */
jpeg.rgb = function (arr) {
	var y = arr[0], cb = arr[1], cr = arr[2];

	return [
		y + 1.402 * (cr - 128),
		y - 0.34414 * (cb - 128) - 0.71414 * (cr - 128),
		y + 1.772 * (cb - 128)
	]
};


/**
 * RGB to JPEG
 * transform through analog form
 *
 * @param {Array<number>} arr JPEG values
 *
 * @return {Array<number>} RGB values
 */
rgb.jpeg = function(arr) {
	var r = arr[0], g = arr[1], b = arr[2];

	return [
		0.299 * r + 0.587 * g + 0.114 * b,
		128 - 0.168736 * r  - 0.331264 * g + 0.5 * b,
		128 + 0.5 * r - 0.418688 * g - 0.081312 * b
	]
};