canvas的ImageData怎么使用

canvas的ImageData 對象

ImageData對象中存儲著canvas對象真實的像素數(shù)據(jù)，它包含以下幾個只讀屬性：

width

圖片寬度，單位是像素

height

圖片高度，單位是像素

data

Uint8ClampedArray類型的一維數(shù)組，包含著RGBA格式的整型數(shù)據(jù)，范圍在0至255之間（包括255）。

創(chuàng)建一個ImageData對象

去創(chuàng)建一個新的，空白的ImageData對象，你應(yīng)該會使用createImageData() 方法。

var myImageData = ctx.createImageData(width, height);

上面代碼創(chuàng)建了一個新的具體特定尺寸的ImageData對象。所有像素被預(yù)設(shè)為透明黑。

得到場景像素數(shù)據(jù)

為了獲得一個包含畫布場景像素數(shù)據(jù)的ImageData對像，你可以用getImageData()方法：

var myImageData = ctx.getImageData(left, top, width, height);

這個方法會返回一個ImageData對象，它代表了畫布區(qū)域的對象數(shù)據(jù)，此畫布的四個角落分別表示為(left, top), (left + width, top), (left, top + height), 以及(left + width, top + height)四個點(diǎn)。這些坐標(biāo)點(diǎn)被設(shè)定為畫布坐標(biāo)空間元素。

在場景中寫入像素數(shù)據(jù)

你可以用putImageData()方法去對場景進(jìn)行像素數(shù)據(jù)的寫入。

ctx.putImageData(myImageData, dx, dy);

dx和dy參數(shù)表示你希望在場景內(nèi)左上角繪制的像素數(shù)據(jù)所得到的設(shè)備坐標(biāo)。

例如，為了在場景內(nèi)左上角繪制myImageData代表的圖片，你可以寫如下的代碼：

ctx.putImageData(myImageData, 0, 0);
toDataURL 將canvas轉(zhuǎn)為 data URI格式

有如下< canvas>元素

<canvas id="canvas" width="5" height="5"></canvas>
var canvas = document.getElementById("canvas");
var dataURL = canvas.toDataURL();
console.log(dataURL);
// "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNby
// blAAAADElEQVQImWNgoBMAAABpAAFEI8ARAAAAAElFTkSuQmCC"
var fullQuality = canvas.toDataURL("image/jpeg", 1.0);
// data:image/jpeg;base64,/9j/4AAQSkZJRgABAQ...9oADAMBAAIRAxEAPwD/AD/6AP/Z"
var mediumQuality = canvas.toDataURL("image/jpeg", 0.5);
var lowQuality = canvas.toDataURL("image/jpeg", 0.1);
應(yīng)用一：顏色選擇器
var img = new Image();
img.src = 'haorooms.jpg';
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
img.onload = function() {
  ctx.drawImage(img, 0, 0);
  img.style.display = 'none';
};
var color = document.getElementById('color');
function pick(event) {
  var x = event.layerX;
  var y = event.layerY;
  var pixel = ctx.getImageData(x, y, 1, 1);
  var data = pixel.data;
  console.log(data);
  var rgba = 'rgba(' + data[0] + ',' + data[1] +
             ',' + data[2] + ',' + (data[3] / 255) + ')';
  color.style.background =  rgba;
  color.textContent = rgba;
}
canvas.addEventListener('mousemove', pick);

應(yīng)用二：視頻純色背景過濾

今天，我們用getImageData過濾掉純色背景。

let processor = {
  timerCallback: function() {
    if (this.video.paused    this.video.ended) {
      return;
    }
    this.computeFrame();
    let self = this;
    setTimeout(function () {
        self.timerCallback();
      }, 0);
  },
  doLoad: function() {
    this.video = document.getElementById("video");
    this.c1 = document.getElementById("c1");
    this.ctx1 = this.c1.getContext("2d");
    this.c2 = document.getElementById("c2");
    this.ctx2 = this.c2.getContext("2d");
    let self = this;
    this.video.addEventListener("play", function() {
        self.width = self.video.videoWidth / 2;
        self.height = self.video.videoHeight / 2;
        self.timerCallback();
      }, false);
  },
  computeFrame: function() {
    this.ctx1.drawImage(this.video, 0, 0, this.width, this.height);
    let frame = this.ctx1.getImageData(0, 0, this.width, this.height);
        let l = frame.data.length / 4;
    for (let i = 0; i < l; i++) {
      let r = frame.data[i * 4 + 0];
      let g = frame.data[i * 4 + 1];
      let b = frame.data[i * 4 + 2];
      if (g > 100 && r > 100 && b < 43)
        frame.data[i * 4 + 3] = 0;
    }
    this.ctx2.putImageData(frame, 0, 0);
    return;
  }
};

應(yīng)用三：圖片灰度和反相顏色

在這個例子里，我們遍歷所有像素以改變他們的數(shù)值。然后我們將被修改的像素數(shù)組通過putImageData()放回到畫布中去。invert函數(shù)僅僅是去減掉顏色的最大色值255.grayscale函數(shù)僅僅是用紅綠和藍(lán)的平均值。你也可以用加權(quán)平均，例如x = 0.299r + 0.587g + 0.114b這個公式。

var img = new Image();
img.src = 'rhino.jpg';
img.onload = function() {
  draw(this);
};
function draw(img) {
  var canvas = document.getElementById('canvas');
  var ctx = canvas.getContext('2d');
  ctx.drawImage(img, 0, 0);
  img.style.display = 'none';
  var imageData = ctx.getImageData(0,0,canvas.width, canvas.height);
  var data = imageData.data;
  var invert = function() {
    for (var i = 0; i < data.length; i += 4) {
      data[i]     = 225 - data[i];     // red
      data[i + 1] = 225 - data[i + 1]; // green
      data[i + 2] = 225 - data[i + 2]; // blue
    }
    ctx.putImageData(imageData, 0, 0);
  };
  var grayscale = function() {
    for (var i = 0; i < data.length; i += 4) {
      var avg = (data[i] + data[i +1] + data[i +2]) / 3;
      data[i]     = avg; // red
      data[i + 1] = avg; // green
      data[i + 2] = avg; // blue
    }
    ctx.putImageData(imageData, 0, 0);
  };
  var invertbtn = document.getElementById('invertbtn');
  invertbtn.addEventListener('click', invert);
  var grayscalebtn = document.getElementById('grayscalebtn');
  grayscalebtn.addEventListener('click', grayscale);
}

應(yīng)用四-縮放和反鋸齒

var img = new Image();
img.src = 'haorooms.jpg';
img.onload = function() {
  draw(this);
};
function draw(img) {
  var canvas = document.getElementById('canvas');
  var ctx = canvas.getContext('2d');
  ctx.drawImage(img, 0, 0);
  img.style.display = 'none';
  var zoomctx = document.getElementById('zoom').getContext('2d');
  var smoothbtn = document.getElementById('smoothbtn');
  var toggleSmoothing = function(event) {
    zoomctx.imageSmoothingEnabled = this.checked;
    zoomctx.mozImageSmoothingEnabled = this.checked;
    zoomctx.webkitImageSmoothingEnabled = this.checked;
    zoomctx.msImageSmoothingEnabled = this.checked;
  };
  smoothbtn.addEventListener('change', toggleSmoothing);
  var zoom = function(event) {
    var x = event.layerX;
    var y = event.layerY;
    zoomctx.drawImage(canvas,
                      Math.abs(x - 5),
                      Math.abs(y - 5),
                      10, 10,
                      0, 0,
                      200, 200);
  };
  canvas.addEventListener('mousemove', zoom);
}

應(yīng)用五-canvas手繪并下載圖片

window.addEventListener('load', function(ev) {
    var sourceimage = document.querySelector('img');
    var canvas = document.querySelector('canvas');
    var link = document.querySelector('a');
    var context = canvas.getContext('2d');
    var mouseX = 0, mouseY = 0,
        width = 300, height = 300,
        mousedown = false;
    canvas.width = width;
    canvas.height = height;
    context.fillStyle = 'hotpink'; 
    function draw(ev) {
      if (mousedown) {
        var x = ev.layerX;
        var y = ev.layerY;
        x = (Math.ceil(x / 10) * 10) - 10;
        y = (Math.ceil(y / 5) * 5) - 5;
        context.fillRect(x, y, 10, 5);
      }
    }
    var link = document.createElement('a');
        link.innerHTML = '下載圖片';
        link.href = "#";
        link.download = "haorooms.png";
    document.body.insertBefore(link, canvas);
    canvas.addEventListener('mouseover', function(ev) {
      document.body.classList.add('painted');
    }, false);
    canvas.addEventListener('mousemove', draw, false);
    canvas.addEventListener('mousedown', function(ev) {
      mousedown = true;
    }, false );
    canvas.addEventListener('mouseup', function(ev) {
      link.href = canvas.toDataURL();
      mousedown = false;
    }, false );
  } ,false);

相信看了這些案例你已經(jīng)掌握了方法，更多精彩請關(guān)注php中文網(wǎng)其它相關(guān)文章！

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