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

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 = `rgba(${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

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