122

是否可以查询 HTML Canvas 对象以获取特定位置的颜色?

4

10 回答 10

184

W3C 文档中有一个关于像素操作的部分。

这是一个关于如何反转图像的示例

var context = document.getElementById('myCanvas').getContext('2d');

// Get the CanvasPixelArray from the given coordinates and dimensions.
var imgd = context.getImageData(x, y, width, height);
var pix = imgd.data;

// Loop over each pixel and invert the color.
for (var i = 0, n = pix.length; i < n; i += 4) {
    pix[i  ] = 255 - pix[i  ]; // red
    pix[i+1] = 255 - pix[i+1]; // green
    pix[i+2] = 255 - pix[i+2]; // blue
    // i+3 is alpha (the fourth element)
}

// Draw the ImageData at the given (x,y) coordinates.
context.putImageData(imgd, x, y);
于 2009-03-20T17:06:52.647 回答
59

试试getImageData方法:

var data = context.getImageData(x, y, 1, 1).data;
var rgb = [ data[0], data[1], data[2] ];
于 2009-03-20T17:11:21.137 回答
15

是的,只要你有它的上下文。(查看如何在此处获取画布上下文。)

var imgData = context.getImageData(0,0,canvas.width,canvas.height)
// { data: [r,g,b,a,r,g,b,a,r,g,..], ... }

function getPixel(imgData, index) {
  var i = index*4, d = imgData.data
  return [d[i],d[i+1],d[i+2],d[i+3]] // Returns array [R,G,B,A]
}

// AND/OR

function getPixelXY(imgData, x, y) {
  return getPixel(imgData, y*imgData.width+x)
}



PS:如果您打算改变数据并将它们绘制回画布上,您可以使用subarray

var
  idt = imgData, // See previous code snippet
  a = getPixel(idt, 188411), // Array(4) [0, 251, 0, 255]
  b = idt.data.subarray(188411*4, 188411*4 + 4) // Uint8ClampedArray(4) [0, 251, 0, 255]

a[0] = 255 // Does nothing
getPixel(idt, 188411) // Array(4) [0, 251, 0, 255]

b[0] = 255 // Mutates the original imgData.data
getPixel(idt, 188411) // Array(4) [255, 251, 0, 255]

// Or use it in the function
function getPixel(imgData, index) {
  var i = index*4, d = imgData.data
  return imgData.data.subarray(i, i+4) // Returns subarray [R,G,B,A]
}

您可以在http://qry.me/xyscope/上进行试验,此代码在源代码中,只需在控制台中复制/粘贴即可。

于 2014-12-30T13:49:27.063 回答
10
function GetPixel(context, x, y)
{
    var p = context.getImageData(x, y, 1, 1).data; 
    var hex = "#" + ("000000" + rgbToHex(p[0], p[1], p[2])).slice(-6);  
    return hex;
}

function rgbToHex(r, g, b) {
    if (r > 255 || g > 255 || b > 255)
        throw "Invalid color component";
    return ((r << 16) | (g << 8) | b).toString(16);
}
于 2015-04-17T13:44:39.977 回答
8

是的,看看getImageData()。下面是一个使用 canvas 使用 JavaScript 破解 CAPTCHA 的示例:

JavaScript 中的 OCR 和神经网络

于 2009-03-20T17:07:09.137 回答
7

请注意,getImageData返回快照。含义是:

  • 更改将在以后生效putImageData
  • getImageData并且putImageData通话速度相对较慢
于 2010-12-31T07:51:00.580 回答
5
//Get pixel data
var imageData = context.getImageData(x, y, width, height);

//Color at (x,y) position
var color = [];
color['red'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 0];
color['green'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 1];
color['blue'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 2];
color['alpha'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 3];
于 2015-02-06T11:03:17.587 回答
2

您可以使用i << 2.

const data = context.getImageData(x, y, width, height).data;
const pixels = [];

for (let i = 0, dx = 0; dx < data.length; i++, dx = i << 2) {
    pixels.push({
        r: data[dx  ],
        g: data[dx+1],
        b: data[dx+2],
        a: data[dx+3]
    });
}
于 2018-07-15T10:36:08.313 回答
1

快速方便

使用以下实现本文中描述的快速方法并包含您需要的所有方法的类:readPixel, putPixel, get width/height。调用refresh()方法后类更新画布。示例求解二维波动方程的简单情况

class Screen{
  constructor(canvasSelector) {
    this.canvas = document.querySelector(canvasSelector);
    this.width  = this.canvas.width;
    this.height = this.canvas.height;
    this.ctx = this.canvas.getContext('2d');
    this.imageData = this.ctx.getImageData(0, 0, this.width, this.height);
    this.buf = new ArrayBuffer(this.imageData.data.length);
    this.buf8 = new Uint8ClampedArray(this.buf);
    this.data = new Uint32Array(this.buf);  
  }
  
  // r,g,b,a - red, gren, blue, alpha components in range 0-255
  putPixel(x,y,r,g,b,a=255) {
    this.data[y * this.width + x] = (a << 24) | (b << 16) | (g <<  8) | r;
  }
  
  readPixel(x,y) {
    let p= this.data[y * this.width + x]
    return [p&0xff, p>>8&0xff, p>>16&0xff, p>>>24];
  }

  refresh() {
    this.imageData.data.set(this.buf8);
    this.ctx.putImageData(this.imageData, 0, 0);
  }
}




// --------
// TEST
// --------

let s=new Screen('#canvas');

function draw() {

  for (var y = 1; y < s.height-1; ++y) {
    for (var x = 1; x < s.width-1; ++x) {      
      let a = [[1,0],[-1,0],[0,1],[0,-1]].reduce((a,[xp,yp])=> 
        a+= s.readPixel(x+xp,y+yp)[0]
      ,0);
      let v=a/2-tmp[x][y];
      tmp[x][y]=v<0 ? 0:v;
    }
  }

  for (var y = 1; y < s.height-1; ++y) {
    for (var x = 1; x < s.width-1; ++x) {  
      let v=tmp[x][y];
      tmp[x][y]= s.readPixel(x,y)[0];
      s.putPixel(x,y, v,v,v);
    }
  }

  s.refresh();
  window.requestAnimationFrame(draw)
}

// temporary 2d buffer ()for solving wave equation)
let tmp = [...Array(s.width)].map(x => Array(s.height).fill(0));

function move(e) { s.putPixel(e.x-10, e.y-10, 255,255,255);}

draw();
<canvas id="canvas" height="150" width="512" onmousemove="move(event)"></canvas>
<div>Move mouse on black box</div>

于 2019-02-01T11:35:28.630 回答
0

如果您想通过将像素坐标传递给函数来提取特定颜色的像素,这将派上用场:

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
function detectColor(x, y){
  data=ctx.getImageData(x, y, 1, 1).data;
  col={
    r:data[0],
    g:data[1],
    b:data[2]
  };
  return col;
}

x,y是要滤除颜色的坐标。

var color = detectColor(x, y)

颜色是对象,您将通过color.r, color.g,获得 RGB 值color.b

于 2018-10-13T18:38:11.757 回答