rtc-canvas

Draw video frames to a canvas and analyse / modify pixel data

rtc-canvas

This is a small helper module that allows you to substitute a video element with a canvas element. This can be useful when you want to do pixel manipulation of the rendered images, or in situations when a video element does not behave as you expect.

This was primarily written to work with the rtc-media library so here's an example of how it works there:

var media = require('rtc-media');
var fakevid = require('rtc-canvas');
 
media().render(fakevid(document.body));

Normally, the media().render call will create a <video> element in the specified target container. In this case, however, rtc-canvas intercepts the request and creates it's own fake video element that is passed back to the render call.

A processing pipeline has been included to assist with manipulating the canvas on the fly. Adding a processor to the pipeline is simply a matter of adding a pipeline processor available on the returned fake video:

// add a processor 
canvas.pipeline.add(function(imageData) {
  // examine the pixel data 
 
  // if we've modified the pixel data and want to write that back 
  // to the canvas then we must return a truthy value 
  return true;
});

A more complete example is shown below:

var media = require('rtc-media');
var fakevideo = require('rtc-canvas');
var vid;
 
function handleDraw(imageData) {
  var channels = imageData.data;
  var rgb = [];
  var rgbAvg;
  var alpha;
  var ii;
 
  // iterate through the data 
  for (ii = channels.length; ii -= 4; ) {
    // get the rgb tuple 
    rgb = [channels[ii], channels[ii + 1], channels[ii + 2]];
 
    // get the alpha value 
    alpha = channels[ii + 3];
 
    // calculate the rgb average 
    rgbAvg = (rgb[0] + rgb[1] + rgb[2] ) / 3;
 
    // update the values to the rgb average 
    channels[ii] = channels[ii + 1] = channels[ii + 2] = rgbAvg;
  }
 
  return true;
}
 
// capture media 
media().render(vid = fakevideo(document.body));
 
// handle draw events on the fake video 
vid.pipeline.add(handleDraw);

By default rtc-canvas will draw at 25fps but this can be modified to capture at a lower frame rate for slower devices, or increased if you have a machine with plenty of grunt.

Create a fake video element for the specified target element.

  • fps - the redraw rate of the fake video (default = 25)

Copyright (c) 2013 National ICT Australia Limited (NICTA)

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