Soundcloud waveform nodes - api

Question

I was reading a article on soundcloud today about their waveforms and how they generate them by converting the highest volume point into a INT between 0 - 1.
After that I opened the console on chrome and then a track on Soundcloud, going through the networks tab (all files) there was no file returning a array of data to generate the html5 waveform, so my question is how do they do it without requesting the data?

Answer 1

Interesting question :) I'm no expert at HTML5's canvas, but I'm sure it has to do with that.
If you look at the DOM you'll see a structure like this:
<div class="sound__body">
<div class="sound__waveform">
<div class="waveform loaded">
<div class="waveform__layer waveform__scene">
<canvas aria-hidden="true" class="g-box-full sceneLayer" width="453" height="60"></canvas>
<canvas aria-hidden="true" class="g-box-full sceneLayer waveformCommentsNode loaded" width="453" height="60"></canvas>
<canvas aria-hidden="true" class="g-box-full sceneLayer" width="453" height="60"></canvas>
</div>
<div class="commentPlaceholder g-z-index-content">...</div>
<div class="commentPopover darkText smallAvatar small">...</div>
</div>
</div>
</div>
On my page I have four sounds. In my networkpanel I also have four of these:
https://wis.sndcdn.com/iGZOEq0vuemr_m.png
They are being sent as JSON, not as PNG!
And contain stuff like:
{"width":1800,"height":140,"samples":
[111,116,118,124,121,121,116,103,119,120,118,118,119,123,128,128,119,119,119,120,117,116,123,127,124,119,115,120,120,121,120,120,121,121,117,116,117,120,123,119,121,125,128,126,122,99,119,120,121,117,122,120,125,125,134,135,130,126,122,123,120,124,126,124,114,111,119,120,120,118,119,132,133,128,127,
...much more
...much more
122,120,125,125,134,135,130]}
I'm pretty sure this is the data being used to draw the waveform using canvas.

Answer 2

As far as i understand this process.
SoundCloud creates an image directly after the upload.
You can access it via the tracks endpoint.
SC.get('/tracks/159966669', function(sound) {
$('#result').append('<img src="' +sound.waveform_url+'"/>' );
});
I.e. http://jsfiddle.net/iambnz/fzm4mckd/
Then they use a script like that, written by (former) SoundCloud devs, http://waveformjs.org - which converts the image into floats.
Example call:
http://www.waveformjs.org/w?url=https%3A%2F%2Fw1.sndcdn.com%2FzVjqZOwCm71W_m.png&callback=callback_json1
Example response (extract)
callback_json1([0.07142857142857142,0.5428571428571428,0.7857142857142857,0.65,0.6142857142857143,0.6357142857142857,0.5428571428571428,0.6214285714285714,0.6357142857142857,0.6571428571428571,0.6214285714285714,0.5285714285714286,0.6642857142857143,0.5714285714285714,0.5,0.5,0.6,0.4857142857142857,0.4785714285714286,0.5714285714285714,0.6642857142857143,0.6071428571428571,0.6285714285714286,0.5928571428571429,0.6357142857142857,0.6428571428571429,0.5357142857142857,0.65,0.5857142857142857,0.5285714285714286,0.55,0.6071428571428571,0.65,0.6142857142857143,0.5928571428571429,0.6428571428571429,...[....]
See example here, more detailed on waveform.js
HTML
<div class="example-waveform" id="example2">
<canvas width="550" height="50"></canvas>
</div>
JS
SC.get('/tracks/159966669', function(sound) {
var waveform = new Waveform({
container: document.getElementById("example2"),
innerColor: "#666666"
});
waveform.dataFromSoundCloudTrack(sound);
});
http://jsfiddle.net/iambnz/ro1481ga/
See docs here: http://waveformjs.org/#endpoint
I hope this will help you a bit.