JavaScript 分形生成算法 - 为什么一个这么快？

Question

I'm trying to write a JavaScript fractal generation algorithm from first principles. I'm aware that there are many examples out there but I wanted to incorporate additional functionality to support both Mandelbrot and 'spinning' Julia with variants such as 'Burning Ship' and 'Tricorn'. With this in mind I implemented a lightweight Complex maths library ( again, I'm aware there are standard Complex js libraries out there but I wanted to build one from scratch as a learning exercise ).

I tested two alternate functions, one fractal using standard maths functions and the other fractalComplex using my Complex library methods. They both work fine, but I was surprised to find that the standard version is almost twice as fast as the Complex version. I was expecting some additional overhead but not that much!

Can anyone explain why? The Complex library is using the same maths constructs 'under the covers'. Is the additional overhead purely down to object creation?

The code is reproduced below (the input parms z and c are objects of the form {re, im} ).

function fractal(z, c, maxiter) {

    var i, za, re, im, re2, im2;
    c = (settype === JULIA ? c : z);

    // Iterate until abs(z) exceeds escape radius
    for (i = 0; i < maxiter; i += 1) {

        if (setvar === BURNING_SHIP) {
            re = Math.abs(z.re);
            im = -Math.abs(z.im);
        }
        else if (setvar === TRICORN) {
            re = z.re
            im = -z.im; // conjugate z
        }
        else { // Mandelbrot
            re = z.re;
            im = z.im;
        }

        re2 = re * re;
        im2 = im * im;
        z = { // z = z² + c
            re: re2 - im2 + c.re,
            im: 2 * im * re + c.im
        };

        za = re2 + im2 // abs(z)²
        if (za > 4) { // abs(z)² > radius²
            break;
        }
    }
    za = Math.sqrt(za); // abs(z)
    return { i, za };
}

function fractalComplex(z, c, maxiter, n, radius) {

    var i, za;
    c = (settype === JULIA ? c : z);

    // Iterate until abs(z) exceeds escape radius
    for (i = 0; i < maxiter; i += 1) {

        if (setvar === BURNING_SHIP) {
            z = new Complex(Math.abs(z.re), -Math.abs(z.im))
        }
        if (setvar === TRICORN) {
            z = z.conjugate()
        }

        z = z.quad(n, c); // z = zⁿ + c
        za = z.abs();
        if (za > radius) {
            break;
        }
    }
    return { i, za };
}

My "Complex lite" library is as follows:

// ------------------------------------------------------------------------
// A basic complex number library which implements the methods used for
// Mandelbrot and Julia Set generation.
// ------------------------------------------------------------------------
'use strict';

// Instantiate complex number object.
function Complex(re, im) {
  this.re = re; // real
  this.im = im; // imaginary
}

Complex.prototype = {

  're': 0,
  'im': 0,

  // Set value.
  'set': function (re, im) {
    this.re = re;
    this.im = im;
  },

  // Get magnitude.
  'abs': function () {
    return Math.sqrt(this.re * this.re + this.im * this.im);
  },

  // Get polar representation (r, θ); angle in radians.
  'polar': function () {
    return { r: this.abs(), θ: Math.atan2(this.im, this.re) };
  },

  // Get square.
  'sqr': function () {
    var re2 = this.re * this.re - this.im * this.im;
    var im2 = 2 * this.im * this.re;
    return new Complex(re2, im2);
  },

  // Get complex number to the real power n.
  'pow': function (n) {
    if (n === 0) { return new Complex(1, 0); }
    if (n === 1) { return this; }
    if (n === 2) { return this.sqr(); }
    var pol = this.polar();
    var rn = Math.pow(pol.r, n);
    var θn = n * pol.θ;
    return cart(rn, θn);
  },

  // Get conjugate.
  'conjugate': function () {
    return new Complex(this.re, -this.im);
  },

  // Get quadratic zⁿ + c.
  'quad': function (n, c) {
    var zn = this.pow(n);
    return new Complex(zn.re + c.re, zn.im + c.im);
  },

  // Rotate by angle in radians.
  'rotate': function (angle) {
    var pol = this.polar();
    angle += pol.θ;
    return new Complex(pol.r * Math.cos(angle), pol.r * Math.sin(angle));
  },

  // String in exponent format to specified significant figures.
  'toString': function (sig = 9) {
    return this.re.toExponential(sig) + " + " + this.im.toExponential(sig) + "i";
  },
}

// Convert polar (r, θ) to cartesian representation (re, im).
function cart(r, θ) {
  var re = r * Math.cos(θ);
  var im = r * Math.sin(θ);
  return new Complex(re, im);
}

Additional edit 20/12/2021 12:15:

For what it's worth, this is what I eventually settled on...

function fractal(p, c, maxiter) {

        var i, za, zre, zim, cre, cim, tmp;
        var lastre = 0;
        var lastim = 0;
        var per = 0;
        if (setmode === JULIA) {
            cre = c.re;
            cim = c.im;
            zre = p.re;
            zim = p.im;
        }
        else { // Mandelbrot mode
            cre = p.re;
            cim = p.im;
            zre = 0;
            zim = 0;
        }

        // Iterate until abs(z) exceeds escape radius
        for (i = 0; i < maxiter; i += 1) {

            if (setvar === BURNING_SHIP) {
                zre = Math.abs(zre);
                zim = -Math.abs(zim);
            }
            else if (setvar === TRICORN) {
                zim = -zim; // conjugate z
            }

            // z = z² + c
            tmp = zre * zre - zim * zim + cre;
            zim = 2 * zre * zim + cim;
            zre = tmp;

            // Optimisation - periodicity check speeds
            // up processing of points within set
            if (PERIODCHECK) {
                if (zre === lastre && zim === lastim) {
                    i = maxiter;
                    break;
                }
                per += 1;
                if (per > 20) {
                    per = 0;
                    lastre = zre;
                    lastim = zim;
                }
            }
            // ... end of optimisation

            za = zre * zre + zim * zim // abs(z)²
            if (za > radius) { // abs(z)² > radius²
                break;
            }
        }
        return { i, za };
}

Answer 1

The following class might satisfy both performance concerns and proper encapsulation of the Complex object...

Notables:

• Where applicable, always return the this Complex object (ie, the instantiated object), which facilitates chaining. Eg,

  • x = new Complex( 10, 5 ).sqrThis().powThis( 1 );

• For every Complex method that returns a Complex object, configure two (2) methods:

  • A method, dubbed <method>This that operates directly on the this object and contains the function logic.
  • A method, dubbed <method> that clones a new Complex object from the this object, and then calls <method>This to execute the function logic on the clone.
  • This provides the developer the choice of methods that either updates the existing object or returns a new object.

• Internal calls to other Complex methods generally should use the <method>This version, as the initial call establishes whether to use the existing this object in-place, or to clone it. From there, all internal calls to the other Complex methods will either continue to operate on the this object or the clone.

 // ------------------------------------------------------------------------ // A basic complex number library which implements the methods used for // Mandelbrot and Julia Set generation. // ------------------------------------------------------------------------ 'use strict'; class Complex { constructor( reOrComplex, im ) { this.set( reOrComplex, im ); } set( reOrComplex, im ) { if ( reOrComplex instanceof Complex ) { this.re = reOrComplex.re; this.im = reOrComplex.im; } else { this.re = reOrComplex; this.im = im; } return this; } abs() { return Math.sqrt(this.re * this.re + this.im * this.im); } toPolar() { return { r: this.abs(), θ: Math.atan2(this.im, this.re) }; } sqrThis() { return this.set( this.re * this.re - this.im * this.im, 2 * this.im * this.re ); } sqr() { return new Complex( this ).sqrThis(); } powThis( n ) { if ( n === 0 ) { return this.set( 1, 0 ) }; if ( n === 1 ) { return this; } if ( n === 2 ) { return this.sqrThis(); } let polar = this.toPolar(); return this.toCartesianThis( Math.pow(polar.r, n), n * polar.θ ); } pow( n ) { return new Complex( this ).powThis( n ); } conjugateThis() { return this.set( this.re, -this.im); } conjugate() { return new Complex( this ).conjugateThis(); } quadraticThis( n, c ) { let zn = this.powThis( n ); return this.set( zn.re + c.re, zn.im + c.im ); } quadratic( n, c ) { return new Complex( this ).quadraticThis( n, c ); } rotateThis( deltaAngle ) { let polar = this.toPolar(); let angle = polar.θ + deltaAngle; return this.set( polar.r * Math.cos(angle), polar.r * Math.sin(angle) ); } rotate( deltaAngle ) { return new Complex( this ).rotateThis( deltaAngle ); } toString( sig = 9 ) { return this.re.toExponential( sig ) + " + " + this.im.toExponential( sig ) + "i"; } toCartesianThis( r, θ ) { return this.set( r * Math.cos( θ ), r * Math.sin( θ ) ); } } // Convert polar (r, θ) to cartesian representation (re, im). Complex.toCartesian = function ( r, θ ) { return new Complex().toCartesianThis( r, θ ); } let x = new Complex( 10, 5 ).sqrThis(); console.log( 'x = new Complex( 10, 5 ).sqrThis()' ); console.log( 'x is ', x ); let y = x.pow( 3 ); console.log ( 'y = x.pow( 3 )' ); console.log ( 'y is ', y ); x.sqr(); console.log ( 'x.sqr()' ); console.log ( 'x is still', x ); x.sqrThis(); console.log ( 'x.sqrThis()' ); console.log ( 'x is now', x );

In short, structuring a class this way provides two versions of the same method:

• One method which embodies the function logic and directly mutates the instantiated this object.
• And the other method which simply clones the instantiated this object, and then calls the associated method containing the function logic.