The JsCanvasDemo macro allows you to create interactive HTML/JavaScript canvas demos easily.
These demos:
  • only start running when the user scrolls over them for the first time
  • stop automatically when they leave the viewport
so you can stuff as many of them as you want on a page, and they won't cause the reader's CPU to fry an egg.
\JsCanvasDemo[[
new class extends OurbigbookCanvasDemo {
  init() {
    super.init('hello');
    this.pixel_size_input = this.addInputAfterEnable(
      'Pixel size',
      {
        'min': 1,
        'type': 'number',
        'value': 1,
      }
    );
  }
  draw() {
    var pixel_size = parseInt(this.pixel_size_input.value);
    for (var x = 0; x < this.width; x += pixel_size) {
      for (var y = 0; y < this.height; y += pixel_size) {
        var b = ((1.0 + Math.sin(this.time * Math.PI / 16)) / 2.0);
        this.ctx.fillStyle =
          'rgba(' +
          (x / this.width) * 255 + ',' +
          (y / this.height) * 255 + ',' +
          b * 255 +
          ',255)'
        ;
        this.ctx.fillRect(x, y, pixel_size, pixel_size);
      }
    }
  }
}
]]
which renders as:
new class extends OurbigbookCanvasDemo {
  init() {
    super.init('hello');
    this.pixel_size_input = this.addInputAfterEnable(
      'Pixel size',
      {
        'min': 1,
        'type': 'number',
        'value': 1,
      }
    );
  }
  draw() {
    var pixel_size = parseInt(this.pixel_size_input.value);
    for (var x = 0; x < this.width; x += pixel_size) {
      for (var y = 0; y < this.height; y += pixel_size) {
        var b = ((1.0 + Math.sin(this.time * Math.PI / 16)) / 2.0);
        this.ctx.fillStyle =
          'rgba(' +
          (x / this.width) * 255 + ',' +
          (y / this.height) * 255 + ',' +
          b * 255 +
          ',255)'
        ;
        this.ctx.fillRect(x, y, pixel_size, pixel_size);
      }
    }
  }
}
And another one showing off some WebGL:
new class extends OurbigbookCanvasDemo {
  init() {
    super.init('webgl', {context_type: 'webgl'});
    this.ctx.viewport(0, 0, this.ctx.drawingBufferWidth, this.ctx.drawingBufferHeight);
    this.ctx.clearColor(0.0, 0.0, 0.0, 1.0);
    this.vertexShaderSource = `
#version 100
precision highp float;
attribute float position;
void main() {
  gl_Position = vec4(position, 0.0, 0.0, 1.0);
  gl_PointSize = 64.0;
}
`;

    this.fragmentShaderSource = `
#version 100
precision mediump float;
void main() {
  gl_FragColor = vec4(0.18, 0.0, 0.34, 1.0);
}
`;
    this.vertexShader = this.ctx.createShader(this.ctx.VERTEX_SHADER);
    this.ctx.shaderSource(this.vertexShader, this.vertexShaderSource);
    this.ctx.compileShader(this.vertexShader);
    this.fragmentShader = this.ctx.createShader(this.ctx.FRAGMENT_SHADER);
    this.ctx.shaderSource(this.fragmentShader, this.fragmentShaderSource);
    this.ctx.compileShader(this.fragmentShader);
    this.program = this.ctx.createProgram();
    this.ctx.attachShader(this.program, this.vertexShader);
    this.ctx.attachShader(this.program, this.fragmentShader);
    this.ctx.linkProgram(this.program);
    this.ctx.detachShader(this.program, this.vertexShader);
    this.ctx.detachShader(this.program, this.fragmentShader);
    this.ctx.deleteShader(this.vertexShader);
    this.ctx.deleteShader(this.fragmentShader);
    if (!this.ctx.getProgramParameter(this.program, this.ctx.LINK_STATUS)) {
      console.log('error ' + this.ctx.getProgramInfoLog(this.program));
      return;
    }
    this.ctx.enableVertexAttribArray(0);
    var buffer = this.ctx.createBuffer();
    this.ctx.bindBuffer(this.ctx.ARRAY_BUFFER, buffer);
    this.ctx.vertexAttribPointer(0, 1, this.ctx.FLOAT, false, 0, 0);
    this.ctx.useProgram(this.program);
  }
  draw() {
    this.ctx.clear(this.ctx.COLOR_BUFFER_BIT);
    this.ctx.bufferData(this.ctx.ARRAY_BUFFER, new Float32Array([Math.sin(this.time / 60.0)]), this.ctx.STATIC_DRAW);
    this.ctx.drawArrays(this.ctx.POINTS, 0, 1);
  }
}

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