HTMLCanvasElement.toDataURL()

HTMLCanvasElement.toDataURL() 方法回傳含有圖像和參數設置特定格式的 data URIs (預設 PNG). 回傳的圖像解析度為 96 dpi.

  • 如果 canvas 的高度或是寬度為 0, 將會回傳字串 "data:,".
  • 如果要求的圖像類型並非 image/png, 但是回傳的類型卻是 data:image/png, 表示要求的圖像類型並不支援.
  • Chrome 也支援 image/webp 類型.

表達式

canvas.toDataURL(type, encoderOptions);

參數

type 選擇性

圖像格式的 DOMString. 預設為 image/png.

encoderOptions 選擇性

表示 image/jpeg 或是 image/webp 的圖像品質, 為0 到 1 之間的 Number. 如果值不在上述範圍中, 將會使用預設值. 其他的會忽略.

回傳值

包含 data URI 的 DOMString.

範例

創建 <canvas> 元素:

<canvas id="canvas" width="5" height="5"></canvas>

可以使用下面的方式獲取 data-URL:

var canvas = document.getElementById("canvas");
var dataURL = canvas.toDataURL();
console.log(dataURL);
// "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNby
// blAAAADElEQVQImWNgoBMAAABpAAFEI8ARAAAAAElFTkSuQmCC"

設置圖像的品質

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);

範例: 動態改變圖像

為了動態改變圖像, 可以與滑鼠事件一起使用 (gray-scale versus color in this example):

HTML

<img class="grayscale" src="myPicture.png" alt="Description of my picture" />

JavaScript

window.addEventListener("load", removeColors);

function showColorImg() {
  this.style.display = "none";
  this.nextSibling.style.display = "inline";
}

function showGrayImg() {
  this.previousSibling.style.display = "inline";
  this.style.display = "none";
}

function removeColors() {
  var aImages = document.getElementsByClassName("grayscale"),
    nImgsLen = aImages.length,
    oCanvas = document.createElement("canvas"),
    oCtx = oCanvas.getContext("2d");
  for (
    var nWidth, nHeight, oImgData, oGrayImg, nPixel, aPix, nPixLen, nImgId = 0;
    nImgId < nImgsLen;
    nImgId++
  ) {
    oColorImg = aImages[nImgId];
    nWidth = oColorImg.offsetWidth;
    nHeight = oColorImg.offsetHeight;
    oCanvas.width = nWidth;
    oCanvas.height = nHeight;
    oCtx.drawImage(oColorImg, 0, 0);
    oImgData = oCtx.getImageData(0, 0, nWidth, nHeight);
    aPix = oImgData.data;
    nPixLen = aPix.length;
    for (nPixel = 0; nPixel < nPixLen; nPixel += 4) {
      aPix[nPixel + 2] =
        aPix[nPixel + 1] =
        aPix[nPixel] =
          (aPix[nPixel] + aPix[nPixel + 1] + aPix[nPixel + 2]) / 3;
    }
    oCtx.putImageData(oImgData, 0, 0);
    oGrayImg = new Image();
    oGrayImg.src = oCanvas.toDataURL();
    oGrayImg.onmouseover = showColorImg;
    oColorImg.onmouseout = showGrayImg;
    oCtx.clearRect(0, 0, nWidth, nHeight);
    oColorImg.style.display = "none";
    oColorImg.parentNode.insertBefore(oGrayImg, oColorImg);
  }
}

規範

Specification
HTML Standard
# dom-canvas-todataurl-dev

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參見