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/**
* @preserve
* Copyright 2015-2018 Igor Bezkrovnyi
* All rights reserved. (MIT Licensed)
*
* pngQuant.ts - part of Image Quantization Library
*/
import { AbstractDistanceCalculator } from './distanceCalculator';
/**
* TODO: check quality of this distance equation
* TODO: ask author for usage rights
* taken from:
* {@link http://stackoverflow.com/questions/4754506/color-similarity-distance-in-rgba-color-space/8796867#8796867}
* {@link https://github.com/pornel/pngquant/blob/cc39b47799a7ff2ef17b529f9415ff6e6b213b8f/lib/pam.h#L148}
*/
export class PNGQuant extends AbstractDistanceCalculator {
/**
* Author's comments
* px_b.rgb = px.rgb + 0*(1-px.a) // blend px on black
* px_b.a = px.a + 1*(1-px.a)
* px_w.rgb = px.rgb + 1*(1-px.a) // blend px on white
* px_w.a = px.a + 1*(1-px.a)
*
* px_b.rgb = px.rgb // difference same as in opaque RGB
* px_b.a = 1
* px_w.rgb = px.rgb - px.a // difference simplifies to formula below
* px_w.a = 1
*
* (px.rgb - px.a) - (py.rgb - py.a)
* (px.rgb - py.rgb) + (py.a - px.a)
*
*/
calculateRaw(
r1: number,
g1: number,
b1: number,
a1: number,
r2: number,
g2: number,
b2: number,
a2: number,
) {
const alphas = (a2 - a1) * this._whitePoint.a;
return (
this._colordifferenceCh(
r1 * this._whitePoint.r,
r2 * this._whitePoint.r,
alphas,
) +
this._colordifferenceCh(
g1 * this._whitePoint.g,
g2 * this._whitePoint.g,
alphas,
) +
this._colordifferenceCh(
b1 * this._whitePoint.b,
b2 * this._whitePoint.b,
alphas,
)
);
}
private _colordifferenceCh(x: number, y: number, alphas: number) {
// maximum of channel blended on white, and blended on black
// premultiplied alpha and backgrounds 0/1 shorten the formula
const black = x - y;
const white = black + alphas;
return black * black + white * white;
}
protected _setDefaults() {}
}