CanvasRenderingContext2D: putImageData() method
The CanvasRenderingContext2D.putImageData()
method of the Canvas 2D API paints data from the given ImageData
object
onto the canvas. If a dirty rectangle is provided, only the pixels from that rectangle
are painted. This method is not affected by the canvas transformation matrix.
Note: Image data can be retrieved from a canvas using the
getImageData()
method.
You can find more information about putImageData()
and general
manipulation of canvas contents in the article Pixel manipulation with canvas.
Syntax
putImageData(imageData, dx, dy)
putImageData(imageData, dx, dy, dirtyX, dirtyY, dirtyWidth, dirtyHeight)
Parameters
imageData
-
An
ImageData
object containing the array of pixel values. dx
-
Horizontal position (x coordinate) at which to place the image data in the destination canvas.
dy
-
Vertical position (y coordinate) at which to place the image data in the destination canvas.
dirtyX
Optional-
Horizontal position (x coordinate) of the top-left corner from which the image data will be extracted. Defaults to
0
. dirtyY
Optional-
Vertical position (y coordinate) of the top-left corner from which the image data will be extracted. Defaults to
0
. dirtyWidth
Optional-
Width of the rectangle to be painted. Defaults to the width of the image data.
dirtyHeight
Optional-
Height of the rectangle to be painted. Defaults to the height of the image data.
Return value
None (undefined
).
Exceptions
NotSupportedError
DOMException
-
Thrown if any of the arguments is infinite.
InvalidStateError
DOMException
-
Thrown if the
ImageData
object's data has been detached.
Examples
Understanding putImageData
To understand what this algorithm does under the hood, here is an implementation on top
of CanvasRenderingContext2D.fillRect()
.
HTML
<canvas id="canvas"></canvas>
JavaScript
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
function putImageData(
ctx,
imageData,
dx,
dy,
dirtyX,
dirtyY,
dirtyWidth,
dirtyHeight,
) {
const data = imageData.data;
const height = imageData.height;
const width = imageData.width;
dirtyX = dirtyX || 0;
dirtyY = dirtyY || 0;
dirtyWidth = dirtyWidth !== undefined ? dirtyWidth : width;
dirtyHeight = dirtyHeight !== undefined ? dirtyHeight : height;
const limitBottom = dirtyY + dirtyHeight;
const limitRight = dirtyX + dirtyWidth;
for (let y = dirtyY; y < limitBottom; y++) {
for (let x = dirtyX; x < limitRight; x++) {
const pos = y * width + x;
ctx.fillStyle = `rgb(${data[pos * 4 + 0]} ${data[pos * 4 + 1]}
${data[pos * 4 + 2]} / ${data[pos * 4 + 3] / 255})`;
ctx.fillRect(x + dx, y + dy, 1, 1);
}
}
}
// Draw content onto the canvas
ctx.fillRect(0, 0, 100, 100);
// Create an ImageData object from it
const imagedata = ctx.getImageData(0, 0, 100, 100);
// use the putImageData function that illustrates how putImageData works
putImageData(ctx, imagedata, 150, 0, 50, 50, 25, 25);
Result
Data loss due to browser optimization
Warning: Due to the lossy nature of converting to and from premultiplied alpha color values,
pixels that have just been set using putImageData()
might be returned to
an equivalent getImageData()
as different values.
JavaScript
const canvas = document.createElement("canvas");
canvas.width = 1;
canvas.height = 1;
const context = canvas.getContext("2d");
const imgData = context.getImageData(0, 0, canvas.width, canvas.height);
const pixels = imgData.data;
pixels[0 + 0] = 1;
pixels[0 + 1] = 127;
pixels[0 + 2] = 255;
pixels[0 + 3] = 1;
console.log("before:", pixels);
context.putImageData(imgData, 0, 0);
const imgData2 = context.getImageData(0, 0, canvas.width, canvas.height);
const pixels2 = imgData2.data;
console.log("after:", pixels2);
The output might look like:
before: Uint8ClampedArray(4) [ 1, 127, 255, 1 ] after: Uint8ClampedArray(4) [ 255, 255, 255, 1 ]
Specifications
Specification |
---|
HTML Standard # dom-context-2d-putimagedata-dev |
Browser compatibility
BCD tables only load in the browser
See also
- The interface defining this method:
CanvasRenderingContext2D
ImageData
objectCanvasRenderingContext2D.getImageData()
- Pixel manipulation with canvas