reference.js

/*
    This is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

// Set up audio context
window.AudioContext = window.AudioContext || window.webkitAudioContext;
const audioContext = new AudioContext();

/**
 * Retrieves audio from an external source, the initializes the drawing function
 * @param {String} url the url of the audio we'd like to fetch
 */
const drawAudio = url => {
  fetch(url)
    .then(response => response.arrayBuffer())
    .then(arrayBuffer => audioContext.decodeAudioData(arrayBuffer))
    .then(audioBuffer => draw(normalizeData(filterData(audioBuffer))));
};

/**
 * Filters the AudioBuffer retrieved from an external source
 * @param {AudioBuffer} audioBuffer the AudioBuffer from drawAudio()
 * @returns {Array} an array of floating point numbers
 */
const filterData = audioBuffer => {
  const rawData = audioBuffer.getChannelData(0); // We only need to work with one channel of data
  const samples = 70; // Number of samples we want to have in our final data set
  const blockSize = Math.floor(rawData.length / samples); // the number of samples in each subdivision
  const filteredData = [];
  for (let i = 0; i < samples; i++) {
    let blockStart = blockSize * i; // the location of the first sample in the block
    let sum = 0;
    for (let j = 0; j < blockSize; j++) {
      sum = sum + Math.abs(rawData[blockStart + j]); // find the sum of all the samples in the block
    }
    filteredData.push(sum / blockSize); // divide the sum by the block size to get the average
  }
  return filteredData;
};

/**
 * Normalizes the audio data to make a cleaner illustration 
 * @param {Array} filteredData the data from filterData()
 * @returns {Array} an normalized array of floating point numbers
 */
const normalizeData = filteredData => {
    const multiplier = Math.pow(Math.max(...filteredData), -1);
    return filteredData.map(n => n * multiplier);
}

/**
 * Draws the audio file into a canvas element.
 * @param {Array} normalizedData The filtered array returned from filterData()
 * @returns {Array} a normalized array of data
 */
const draw = normalizedData => {
  // set up the canvas
  const canvas = document.querySelector("canvas");
  const dpr = window.devicePixelRatio || 1;
  const padding = 20;
  canvas.width = canvas.offsetWidth * dpr;
  canvas.height = (canvas.offsetHeight + padding * 2) * dpr;
  const ctx = canvas.getContext("2d");
  ctx.scale(dpr, dpr);
  ctx.translate(0, canvas.offsetHeight / 2 + padding); // set Y = 0 to be in the middle of the canvas

  // draw the line segments
  const width = canvas.offsetWidth / normalizedData.length;
  for (let i = 0; i < normalizedData.length; i++) {
    const x = width * i;
    let height = normalizedData[i] * canvas.offsetHeight - padding;
    if (height < 0) {
        height = 0;
    } else if (height > canvas.offsetHeight / 2) {
        height = height > canvas.offsetHeight / 2;
    }
    drawLineSegment(ctx, x, height, width, (i + 1) % 2);
  }
};

/**
 * A utility function for drawing our line segments
 * @param {AudioContext} ctx the audio context 
 * @param {number} x  the x coordinate of the beginning of the line segment
 * @param {number} height the desired height of the line segment
 * @param {number} width the desired width of the line segment
 * @param {boolean} isEven whether or not the segmented is even-numbered
 */
const drawLineSegment = (ctx, x, height, width, isEven) => {
  ctx.lineWidth = 1; // how thick the line is
  ctx.strokeStyle = "#fff"; // what color our line is
  ctx.beginPath();
  height = isEven ? height : -height;
  ctx.moveTo(x, 0);
  ctx.lineTo(x, height);
  ctx.arc(x + width / 2, height, width / 2, Math.PI, 0, isEven);
  ctx.lineTo(x + width, 0);
  ctx.stroke();
};

drawAudio('https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/shoptalk-clip.mp3');