pngQuant.ts
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/**
* @preserve
* Copyright 2015-2016 Igor Bezkrovnyi
* All rights reserved. (MIT Licensed)
*
* pngQuant.ts - part of Image Quantization Library
*/
import { AbstractDistanceCalculator } from "./abstractDistanceCalculator"
/**
* 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) : number {
const alphas = (a2 - a1) * this._whitePoint.a;
return this._colordifference_ch(r1 * this._whitePoint.r, r2 * this._whitePoint.r, alphas) +
this._colordifference_ch(g1 * this._whitePoint.g, g2 * this._whitePoint.g, alphas) +
this._colordifference_ch(b1 * this._whitePoint.b, b2 * this._whitePoint.b, alphas);
}
private _colordifference_ch(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,
white = black + alphas;
return black * black + white * white;
}
}