HTMLCanvasElement: toDataURL() method
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.
Syntax
js
toDataURL()
toDataURL(type)
toDataURL(type, encoderOptions)
Parameters
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. encoderOptions
Optional-
A
Number
between0
and1
indicating the image quality to be used when creating images using file formats that support lossy compression (such asimage/jpeg
orimage/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.
Return value
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.
Exceptions
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.
Examples
Given this <canvas>
element:
html
<canvas id="canvas" width="5" height="5"></canvas>
You can get a data-URL of the canvas with the following lines:
js
const canvas = document.getElementById("canvas");
const dataURL = canvas.toDataURL();
console.log(dataURL);
// "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNby
// blAAAADElEQVQImWNgoBMAAABpAAFEI8ARAAAAAElFTkSuQmCC"
Setting image quality with jpegs
js
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);
Example: Dynamically change images
You can use this technique in coordination with mouse events in order to dynamically change images (gray-scale vs. color in this example):
HTML
html
<img class="grayscale" src="myPicture.png" alt="Description of my picture" />
JavaScript
js
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);
}
}
Specifications
Specification |
---|
HTML Standard # dom-canvas-todataurl-dev |
Browser compatibility
BCD tables only load in the browser