用手写asm.js

Question

我试图将asm.js注释添加到perlin噪声函数中：

"use strict";

// Ported from Stefan Gustavson's java implementation
// http://staffwww.itn.liu.se/~stegu/simplexnoise/simplexnoise.pdf
// Read Stefan's excellent paper for details on how this code works.
//
// Sean McCullough banksean@gmail.com

function SimplexNoise(stdlib,foreign,heap) {
    "use asm";
    // heap should be at least 1024*4+12*3*4 bytes long
    var floor = stdlib.Math.floor,
        sqrt = stdlib.Math.sqrt,
        random = new stdlib.Float32Array(heap),
        buf = new stdlib.Int32Array(heap),
        F2 = 0.0, F3 = 0.0,
        G2 = 0.0, G3 = 0.0;
    F2 = (sqrt(3.0)-1.0)*0.5; F3 = 1.0/3.0;
    G2 = (3.0-sqrt(3.0))/6.0; G3 = 1.0/6.0;
    function init() {
        // before calling this function, set the first 256 floats in the heap to be random numbers between 0..1
        // the heap gets rewritten, but if you record the IV separately, the noise is reproducable
        for(var i=0; i<256; i++) {
            buf[i] = buf[i+256] = floor(random[i]*256.0) & 0xff;
            buf[i+512] = buf[i+512+256] = (buf[i] % 12) * 3 + 1024;
        }
        i = 1024;
        buf[i++] = 1;  buf[i++] = 1;  buf[i++] = 0;
        buf[i++] = -1; buf[i++] = 1;  buf[i++] = 0;
        buf[i++] = 1;  buf[i++] = -1; buf[i++] = 0;
        buf[i++] = -1; buf[i++] = -1; buf[i++] = 0;
        buf[i++] = 1;  buf[i++] = 0;  buf[i++] = 1;
        buf[i++] = -1; buf[i++] = 0;  buf[i++] = 1; 
        buf[i++] = 1;  buf[i++] = 0;  buf[i++] = -1;
        buf[i++] = -1; buf[i++] = 0;  buf[i++] = -1;
        buf[i++] = 0;  buf[i++] = 1;  buf[i++] = 1;
        buf[i++] = 0;  buf[i++] = -1; buf[i++] = 1;
        buf[i++] = 0;  buf[i++] = 1;  buf[i++] = -1;
        buf[i++] = 0;  buf[i++] = -1; buf[i++] = -1;
    }
    function noise2D(xin,yin) {
        xin = +xin; yin = +yin;
        var s = 0.0, i = 0, j = 0,
            t = 0.0,
            X0 = 0.0, Y0 = 0.0,
            x0 = 0.0, y0 = 0.0,
            i1 = 0, j1 = 0,
            x1 = 0.0, y1 = 0.0,
            x2 = 0.0, y2 = 0.0,
            ii = 0, jj = 0,
            gi0 = 0, gi1 = 0, gi2 = 0,
            t0 = 0.0, t1 = 0.0, t2 = 0.0,
            n0 = 0.0, n1 = 0.0, n2 = 0.0;
        // Skew the input space to determine which simplex cell we're in 
        s = (xin+yin)*F2; // Hairy factor for 2D 
        i = floor(xin+s); j = floor(yin+s);
        t = (i+j)*G2;
        X0 = i-t; Y0 = j-t; // Unskew the cell origin back to (x;y) space 
        x0 = xin-X0; y0 = yin-Y0; // The x;y distances from the cell origin 
        // For the 2D case; the simplex shape is an equilateral triangle. 
        // Determine which simplex we are in. 
        i1 = (x0>y0?1:0); j1 = (x0>y0?0:1); // Offsets for second (middle) corner of simplex in (i;j) coords 
        // A step of (1;0) in (i;j) means a step of (1-c;-c) in (x;y); and 
        // a step of (0;1) in (i;j) means a step of (-c;1-c) in (x;y); where 
        // c = (3-sqrt(3))/6 
        x1 = x0-i1+G2; y1 = y0-j1+G2; // Offsets for middle corner in (x;y) unskewed coords 
        x2 = x0-1+2*G2; y2 = y0-1+2*G2; // Offsets for last corner in (x;y) unskewed coords 
        // Work out the hashed gradient indices of the three simplex corners 
        ii = i & 255; jj = j & 255;
        gi0 = buf[ii+buf[jj]+512];
        gi1 = buf[ii+i1+buf[jj+j1]+512]; 
        gi2 = buf[ii+1+buf[jj+1]+512];
        // Calculate the contribution from the three corners 
        t0 = 0.5-x0*x0-y0*y0; t1 = 0.5-x1*x1-y1*y1; t2 = 0.5-x2*x2-y2*y2;
        n0 = t0<0.0? 0.0: t0*t0*t0*t0*(buf[gi0]*x0+buf[gi0+1]*y0);  // (x;y) of buf used for 2D gradient 
        n1 = t1<0.0? 0.0: t1*t1*t1*t1*(buf[gi1]*x1+buf[gi1+1]*y1);
        n2 = t2<0.0? 0.0: t2*t2*t2*t2*(buf[gi2]*x2+buf[gi2+1]*y2); 
        // Add contributions from each corner to get the final noise value. 
        // The result is scaled to return values in the interval [-1,1]. 
        return 70.0 * (n0 + n1 + n2);
    }
    return {
        init: init,
        noise2D: noise2D
    };
};

Firefox Javascript错误控制台只说：

警告: TypeError: asm.js类型错误: asm.js必须以返回导出语句结束源文件: perlin_simplex.js行:9

我已经搜索了错误可能是什么的帮助，只找到了https://github.com/zbjornson/human-asmjs，这通常是非常有用的，但在这种特殊情况下没有帮助。

我出什么问题了？这个perlin噪声函数是如何由人类来实现的呢？

Answer 1

问题在于以下几行：

F2 = (sqrt(3.0)-1.0)*0.5; F3 = 1.0/3.0;
G2 = (3.0-sqrt(3.0))/6.0; G3 = 1.0/6.0;

在asm.js模块的顶层范围内只允许声明。不允许在顶级作用域中定义变量。

根据等级库，您只能将文字值赋值给全局变量(以及其他导入语句)。因此，您也不能执行以下操作，因为F2、F3、G2和G3没有分配文字值：

var floor = stdlib.Math.floor,
    sqrt = stdlib.Math.sqrt,
    random = new stdlib.Float32Array(heap),
    buf = new stdlib.Int32Array(heap),
    F2 = (sqrt(3.0)-1.0)*0.5, F3 = 1.0/3.0,
    G2 = (3.0-sqrt(3.0))/6.0, G3 = 1.0/6.0;

因此，您需要将F2、F3、G2和G3的定义移到函数中(可能是init函数)。因此，您的代码应该如下所示：

function SimplexNoise(stdlib,foreign,heap) {
    "use asm";

    var floor = stdlib.Math.floor,
        sqrt = stdlib.Math.sqrt,
        random = new stdlib.Float32Array(heap),
        buf = new stdlib.Int32Array(heap),
        F2 = 0.0, F3 = 0.0,
        G2 = 0.0, G3 = 0.0;

    function init() {
        F2 = (sqrt(3.0)-1.0)*0.5; F3 = 1.0/3.0;
        G2 = (3.0-sqrt(3.0))/6.0; G3 = 1.0/6.0;
        // rest of init
    }

    function noise2D(xin,yin) {
        // function body
    }

    return {
        init: init,
        noise2D: noise2D
    };
};

人工asm.js确实告诉了您这个问题，但它们给出了数组的示例，而不是简单的变量：

请注意，不能在函数之外修改这些类型化数组：

function MyModule(stdlib, foreign, heap) {
  var arr = new stdlib.Int8Array(heap);
  arr[0] = 1; // "asm.js must end with a return export statement"
  // ...
}

相反，可以这样做：

function MyModule(stdlib, foreign, heap) {
  var arr = new stdlib.Int8Array(heap);
  function init() {
    arr[0] = 1;
  }

  return {
    init: init
  };
}

这就对了。asm.js不是很痛吗？