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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toDataURL below:

HTMLCanvasElement: toDataURL() method - Web APIs

Baseline Widely available

The HTMLCanvasElement.toDataURL() method returns a data URL containing a representation of the image in the format specified by the type parameter.

The desired file format and image quality may be specified. If the file format is not specified, or if the given format is not supported, then the data will be exported as image/png. In other words, if the returned value starts with data:image/png for any other requested type, then that format is not supported.

Browsers are required to support image/png; many will support additional formats including image/jpeg and image/webp.

The created image data will have a resolution of 96dpi for file formats that support encoding resolution metadata.

Warning: toDataURL() encodes the whole image in an in-memory string. For larger images, this can have performance implications, and may even overflow browsers' URL length limit when assigned to HTMLImageElement.src. You should generally prefer toBlob() instead, in combination with URL.createObjectURL().

toDataURL()
toDataURL(type)
toDataURL(type, quality)

type Optional

A string indicating the image format. The default type is image/png; this image format will be also used if the specified type is not supported.

quality Optional

A Number between 0 and 1 indicating the image quality to be used when creating images using file formats that support lossy compression (such as image/jpeg or image/webp). A user agent will use its default quality value if this option is not specified, or if the number is outside the allowed range.

A string containing the requested data URL.

If the height or width of the canvas is 0 or larger than the maximum canvas size, the string "data:," is returned.

SecurityError

The canvas's bitmap is not origin clean; at least some of its contents have or may have been loaded from a site other than the one from which the document itself was loaded.

Given this <canvas> element:

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

You can get a data-URL of the canvas with the following lines:

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

const fullQuality = canvas.toDataURL("image/jpeg", 1.0);
// data:image/jpeg;base64,/9j/4AAQSkZJRgABAQ…9oADAMBAAIRAxEAPwD/AD/6AP/Z"
const mediumQuality = canvas.toDataURL("image/jpeg", 0.5);
const lowQuality = canvas.toDataURL("image/jpeg", 0.1);

You can use this technique in coordination with mouse events in order to dynamically change images (gray-scale vs. color in this example):

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

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() {
  const images = document.getElementsByClassName("grayscale");
  const canvas = document.createElement("canvas");
  const ctx = canvas.getContext("2d");

  for (const colorImg of images) {
    const width = colorImg.offsetWidth;
    const height = colorImg.offsetHeight;
    canvas.width = width;
    canvas.height = height;
    ctx.drawImage(colorImg, 0, 0);
    const imgData = ctx.getImageData(0, 0, width, height);
    const pix = imgData.data;
    const pixLen = pix.length;
    for (let pixel = 0; pixel < pixLen; pixel += 4) {
      pix[pixel + 2] =
        pix[pixel + 1] =
        pix[pixel] =
          (pix[pixel] + pix[pixel + 1] + pix[pixel + 2]) / 3;
    }
    ctx.putImageData(imgData, 0, 0);
    const grayImg = new Image();
    grayImg.src = canvas.toDataURL();
    grayImg.onmouseover = showColorImg;
    colorImg.onmouseout = showGrayImg;
    ctx.clearRect(0, 0, width, height);
    colorImg.style.display = "none";
    colorImg.parentNode.insertBefore(grayImg, colorImg);
  }
}

• Data URLs in the HTTP reference.

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