rtc-videoproc

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

npm install rtc-videoproc
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rtc-videoproc

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.

NPM

unstable

Example Usage

var crel = require('crel');
var media = require('rtc-media');
var videoproc = require('rtc-videoproc');
var vid = crel('video');
var canvas = crel('canvas');

// create the processor
var processor = videoproc(vid, canvas, {
  filter: require('rtc-filter-grayscale')
});

// capture media and render
media().render(vid);

// add the canvas to the document
document.body.appendChild(canvas);

Using the Processing Pipeline

A processing pipeline has been included to assist with manipulating the canvas on the fly. To specify the filters to be used in the processing pipeline, this is done in the options accepted by videoproc. Either specifying an array of filters with the filters option or a single filter function with the filter option is fine. If you use both then the individual filter will be added filter list and used in series.

videoproc(srcVideo, targetCanvas, {
  filters: [
    require('rtc-filter-grayscale'),
    myCustomFilterFunction
  ]
});

Writing a Filter Function

Writing a filter function is very simple, and they use the following function signature:

function filter(imageData, tick) {
}

The imageData arg is an ImageData, and the tick argument refers to the tick that has been captured as part of the capture loop (be aware that it could be a high resolution timer value if rAF is being used).

If you are writing an analysis filter, then simply do what you need to do and exit the function. If you have written a filter that modifies the pixel data and you want this drawn back to the canvas then your filter must return true to tell rtc-videoproc that it should draw the imageData back to the canvas.

Listening for custom frame events

In addition to providing the opportunity to analyse and modify pixel data the rtc-videoproc module also provides the a custom frame event for detecting when a new frame has been drawn to the canvas.

A simple example can be found below:

var media = require('rtc-media');
var videoproc = require('rtc-videoproc');
var crel = require('crel');
var video = crel('video');

// set up the video processing pipeline
videoproc(video).on('frame', function(imageData, tick) {
  console.log('captured frame at: ' + tick);
});

// capture media and render to the video
media().render(video);

// add the canvas to the dom
document.body.appendChild(video);

NOTE: The frame event occurs after the filter pipeline has been run and and the imageData may have been modified from the original video frame.

A Note with Regards to CPU Usage

By default rtc-videoproc 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.

Reference

videoproc(src, target, opts?)

Create (or patch) a <canvas> element that will receive the video images from a video element. The following options are supported.

  • canvas - the canvas to draw video data to. If not supplied a new canvas element will be created.

  • video - the video element that will be used as the source of the video. If not supplied a new <video> element will be created.

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

  • greedy - Specify greedy: true if you want the videoproc module to run it's capture loop using setTimeout rather than requestAnimationFrame. Doing this will mean you application will continue to capture and process frames even when it's tab / window becomes inactive. This is usually the desired behaviour with video conferencing applications.

License(s)

Apache 2.0

Copyright 2014 National ICT Australia Limited (NICTA)

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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