canvas-record

canvas-record

Record a video in the browser or directly on the File System from a canvas (2D/WebGL/WebGPU) as MP4, WebM, MKV, GIF, PNG/JPG Sequence using WebCodecs and Wasm when available.

Installation

npm install canvas-record

Usage

import { Recorder, RecorderStatus, Encoders } from "canvas-record";
import createCanvasContext from "canvas-context";

// Setup
const pixelRatio = devicePixelRatio;
const width = 512;
const height = 512;
const { context, canvas } = createCanvasContext("2d", {
  width: width * pixelRatio,
  height: height * pixelRatio,
  contextAttributes: { willReadFrequently: true },
});
Object.assign(canvas.style, { width: `${width}px`, height: `${height}px` });

const mainElement = document.querySelector("main");
mainElement.appendChild(canvas);

// Animation
let canvasRecorder;

function render() {
  const width = canvas.width;
  const height = canvas.height;

  const t = canvasRecorder.frame / canvasRecorder.frameTotal || Number.EPSILON;

  context.clearRect(0, 0, width, height);
  context.fillStyle = "red";
  context.fillRect(0, 0, t * width, height);
}

const tick = async () => {
  render();

  if (canvasRecorder.status !== RecorderStatus.Recording) return;
  await canvasRecorder.step();

  if (canvasRecorder.status !== RecorderStatus.Stopped) {
    requestAnimationFrame(() => tick());
  }
};

canvasRecorder = new Recorder(context, { name: "canvas-record-example" });

// Start and encode frame 0
await canvasRecorder.start();

// Animate to encode the rest
tick(canvasRecorder);

API

Encoder comparison:

Encoder	Extension	Required Web API	WASM	Speed
WebCodecs	mp4 / webm / mkv	WebCodecs	✅ (embed, only for mp4)	Fast
H264MP4	mp4		✅ (embed)	Medium
FFmpeg	mp4 / webm	SharedArrayBuffer	✅ (need binary path)	Slow
GIF	gif	WebWorkers (wip)	❌	Fast
Frame	png / jpg	File System Access	❌	Fast
MediaCapture	mkv / webm	MediaStream	❌	Realtime

Note:

• WebCodecs 5-10x faster than H264MP4Encoder and 20x faster than FFmpeg (it needs to mux files after writing png to virtual FS)
• FFmpeg (mp4 and webm) and WebCodecs (mp4) have a AVC maximum frame size of 9437184 pixels. That's fine until a bit more than 4K 16:9 @ 30fps. So if you need 4K Square or 8K exports, be patient with H264MP4Encoder (which probably also has the 4GB memory limit) or use Frame encoder and mux them manually with FFmpeg CLI.
• WebCodecs is embedded from mp4-wasm for ease of use (FFmpeg will require encoderOptions.corePath)

Roadmap:

• add debug logging
• use WebWorkers for gifenc

Modules

index

Re-export Recorder, RecorderStatus, all Encoders and utils.

Classes

Recorder

Base Recorder class.

Functions

onStatusChangeCb(RecorderStatus)

A callback to notify on the status change. To compare with RecorderStatus enum values.

Typedefs

RecorderOptions : Object

Options for recording. All optional.

RecorderStartOptions : Object

Options for recording. All optional.

index

Re-export Recorder, RecorderStatus, all Encoders and utils.

Recorder

Base Recorder class.

Kind: global class Properties

Name	Type	Default	Description
[enabled]	boolean	true	Enable/disable pointer interaction and drawing.

• Recorder
  • new Recorder(context, options)
  • .start(startOptions)
  • .step()
  • .stop()
  • .dispose()

new Recorder(context, options)

Param	Type
context	RenderingContext
options	RecorderOptions

recorder.start(startOptions)

Start the recording by initializing and calling the initial step.

Kind: instance method of Recorder

Param	Type
startOptions	RecorderStartOptions

recorder.step()

Encode a frame and increment the time and the playhead. Calls await canvasRecorder.stop() when duration is reached.

Kind: instance method of Recorder

recorder.stop()

Stop the recording and return the recorded buffer. If options.download is set, automatically start downloading the resulting file. Is called when duration is reached or manually.

Kind: instance method of Recorder

recorder.dispose()

Clean up

Kind: instance method of Recorder

RecorderStatus : enum

Enum for recorder status

Kind: global enum Read only: true Example

// Check recorder status before continuing
if (canvasRecorder.status !== RecorderStatus.Stopped) {
  rAFId = requestAnimationFrame(() => tick());
}

onStatusChangeCb(RecorderStatus)

A callback to notify on the status change. To compare with RecorderStatus enum values.

Kind: global function

Param	Type	Description
RecorderStatus	number	the status

RecorderOptions : Object

Options for recording. All optional.

Kind: global typedef Properties

Name	Type	Default	Description
[name]	string	""""	A name for the recorder, used as prefix for the default file name.
[duration]	number	10	The recording duration in seconds. If set to Infinity, await canvasRecorder.stop() needs to be called manually.
[frameRate]	number	30	The frame rate in frame per seconds. Use await canvasRecorder.step(); to go to the next frame.
[download]	boolean	true	Automatically download the recording when duration is reached or when await canvasRecorder.stop() is manually called.
[extension]	boolean	"mp4"	Default file extension: infers which Encoder is selected.
[target]	string	""in-browser""	Default writing target: in-browser or file-system when available.
[encoder]	Object		A specific encoder. Default encoder based on options.extension: GIF > WebCodecs > H264MP4.
[encoderOptions]	Object	{}	See src/encoders or individual packages for a list of options.
[onStatusChange]	onStatusChangeCb

RecorderStartOptions : Object

Options for recording. All optional.

Kind: global typedef Properties

Name	Type	Description
[filename]	string	Overwrite the file name completely.

License

All MIT:

• mp4-wasm
• h264-mp4-encoder
• @ffmpeg/ffmpeg
• gifenc
• webm-muxer

MIT. See license file.