Canvas Recorder

A blatant ripoff of Looper 😎

This is a small utility to record a canvas based animation of any sort. The tool can be used to run the animation in the browser until one is ready to record it. The setup works around four core methods depicted in the code below:

import { options, start, draw, getCanvas } from "canvas-recorder";

options( {
    size: [ 500, 500 ],
    frames: 30
} );

draw( ( context, delta ) => {

    // ... Do something here

} );

document.body.appendChild( getCanvas() );

start();

Additionally, canvas-recorder can also be used as a command line tool to merge the image sequence into a MP4 file format. See here

Note: The package is written in Typescript and ships with types. Use in JS or TS alike.

• Methods
  • options( settings: {} )
  • draw( ( context, time ) => {} )
  • start()
  • stop()
  • setup( ( context ) => {} )
  • cleanup( () => {} )
  • getCanvas(): HTMLCanvasElement
  • getContext(): CanvasRenderingContext2D
  • bootstrap( options?: {} )
  • Recorder
  • addFrame( canvas: HTMLCanvasElement ): Promise
  • getBundle(): JSZip
  • downloadBundle(): Promise
• WebGL
  • Context
• Cli Tool
  • Flags
  • Setup

Methods

options( settings: {} )

Used to set settings for the recording. In most cases calling options will be done before any frames are recorded as a first step of the program. Calling options while in between start() and stop() (while it is recording) calls is not permitted.

It takes one argument which is an object with the following possible settings:

• record: [Default: true] Enables/Disables the recording of all frames. Setting this to false is useful in development. Not recording any frames significantly speeds up the drawing cycles.
• clear: [Default: false] Clears the previous frame on every draw call.,
• size: [Default: [1024, 1024]] Sets the size of the canvas.
• frames: [Default: -1] Determines the amount of frames recorded. If set to -1 it will continue recording until a call to stop().
• onComplete: [Default <internal>] Function that is called when all frames are recorded and archived into a zip in form of a Blob. When not set, a download is triggered automatically.
• color: [Default: "white"] Sets the background color of every frame if clear is set to true.
• fps: [Default: 60] The framerate from which the elapsed time is calculated in record mode. Note that the recording won't happen in at this pace as it is no longer realtime.
• canvas: [Default: HTMLCanvasElement] Allows to use a specific canvas element for recording instead of creating an internal one.

draw( ( context, time ) => {} )

The draw method is the heart of the recorder. It takes on argument which is a callback. This callback will recieve two arguments at every invocation:

• context which is a CanvasRenderingContext2D associated with the Canvas. This context is generally used to draw the frame.
• time is the amount of milliseconds since the most recent start call. Using this time argument allows for the async recording to adhere to the animations fluidity. Do not calculate the time yourself, as the recording process is much slower than the desired framerate.

start()

Calling this will start the loop of the recorder.

stop()

Will terminate the loop. If the settings are set to record: true, calling stop will subsequently finalize all recorded frames and compress them in a ZIP archive. By default this ZIP will trigger a download to save all frames, unless onComplete is set with a costum function. If so, said function will recieve the ZIP in form of a Blob.

setup( ( context ) => {} )

This method will be called right before the frist draw call. The context is passed. This is especially useful in the WebGL implementation.

cleanup( () => {} )

This is a utility that can be used as a callback after the recording has terminated. This is especially useful when the recorder is used in frame mode. After the desired amount of frames this method will be called. Once this method is called all resources can be used freely and won't no longer be used by the recorder.

getCanvas(): HTMLCanvasElement

Returns the canvas being used by the recorder.

getContext(): CanvasRenderingContext2D

Returns the context attached to the canvas of the recorder.

bootstrap( options? )

Shorthand for inserting the canvas into the DOM as well as calling start(). This is particularly useful for short sketches.

Bootstrapping allows an options paramter. An object that has a single flag clear. Calling boostrap( { clear: true } ) will terminate previous sketch and remove the previous canvas from the DOM. This is helpful when one has an auto-reload with a undefined loop. By default bootstrap() does not clear.

Recorder

All methods are simply a shorthand for an instance of a Recorder. If one would rather instantiate the recorder themselves, maybe to run multiple recorders at once, do it like so:

import { Recorder } from "canvas-recorder";

const recorder = new Recorder();

recorder.options( {
    ...
} );

recorder.draw( ( context: CanvasRenderingContext2D, time: number ) => {
    ...
} );

recorder.start();

addFrame( canvas: HTMLCanvasElement ): Promise

In order to use the frame packaging without any of the utility methods listed above, one can use addFrame and the following methods to aggregate all frames manually. This me adds an PNG of the current frame to the bundle. One can add as many frames as one likes. Use the following methods to retrive the ZIP or download it.

getBundle(): JSZip

Returns the current bundle as a zip containing all frames. See JSZip Documentation for how to use it.

downloadBundle(): Promise

Downloads the current set of frames and resets the bundle. This is useful if you want to download lots of frames and don't want the zip to get too large. After calling download the next call to addFrame will be in a new bundle.

The following example shows how to use it without all helper methods.

import { addFrame, downloadBundle } from "canvas-recorder";

// ... canvas setup

context.fillStyle = "green";
context.fillRect( 10, 14, 200, 300 );

addFrame( canvas ).then( () => {
    downloadBundle();
} );

WebGL

The package is also avialble with webgl support. The API is quasi identical. In order to use it as a WebGL package change the import slightly

import triangle from "a-big-triangle";
import createShader from "gl-shader";
import { options, start, draw, getCanvas, setup } from "canvas-recorder/gl";

let shader;

options( {
    frames: 10,
    size: [ 100, 100 ]
} );

setup( ( gl: WebGLRenderingContext ) => {
    shader = createShader(
        gl,
        `
            precision mediump float;
            attribute vec2 position;

            varying vec2 uv;

            void main() {
                uv = position.xy;
                gl_Position = vec4( position.xy, 0.0, 1.0 );
            }
        `,
        `
            precision mediump float;
            varying vec2 uv;
            void main() {
                gl_FragColor = vec4( 1, 0, 0, 1 );
            }
        `,
    );
} );

draw( ( gl: WebGLRenderingContext ) => {

    shader.bind();

    triangle( gl );
} );

start();

Context

In this implementation, the context is always a WebGLRenderingContext instead of a CanvasRenderingContext2D.

Cli Tool

Tool to turn the image sequence into a movei format.

When installed globally, or through the use of a package.json, one can invoke the command canvas-recorder or alternatively use the alias ffmpy (pronounced: effeffempey) as a shorter command.

Unsurprisingly uses FFmpeg under the hood. It has a limited amount of possible options but sets defaults for all of them. Therefore the easiest usecase is calling the command in the directory of the image sequence with not flags

Flags

• -i, --input <dir> Path to the folder of the image sequence. Defaults to ..
• -r, --fps <num> Framerate used in the movie file. Defaults to 30.
• -o, --output <name> File name of the output. Defaults to out.mp4.

Setup

When installed globally, the commands are available everywhere. Alternatively, when installed locally in the project it can still be executed from the package.json

"scripts": {
    "merge": "canvas-recorder -i ./image-sequence/ -o film.mp4"
}