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This is a simple parser that takes a string of text and determines what that means in the context of WebRTC.


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It provides a simple, textual way of describing your requirements for media capture. Trying to remember the structure of the constraints object is painful.


A simple text string is converted to an intermediate JS object representation, which can then be converted to a getUserMedia constraints data structure using a toConstraints() call.

For example, the following text input:

camera min:1280x720 max:1280x720 min:15fps max:25fps

Is converted into an intermedia representation (via the CaptureConfig utility class) that looks like the following:

  camera: 0,
  microphone: 0,
  res: {
    min: { w: 1280, h: 720 },
    max: { w: 1280, h: 720 }

  fps: {
    min: 15,
    max: 25

Which in turn is converted into the following media constraints for a getUserMedia call:

  audio: true,
  video: {
    mandatory: {},
    optional: [
      { minFrameRate: 15 },
      { maxFrameRate: 25 },
      { frameRate: { min: 15, max: 25 } },

      { minWidth: 1280 },
      { maxWidth: 1280 },
      { width: 1280 },

      { minHeight: 720 },
      { maxHeight: 720 },
      { height: 720 }

Experimental: Targeted Device Capture

While the rtc-captureconfig module itself doesn't contain any media identification logic, it is able to the sources information from a MediaStreamTrack.getSources call to generate device targeted constraints.

For instance, the following example demonstrates how we can request camera:1 (the 2nd video device on our local machine) when we are making a getUserMedia call:

// load in capture config
var capture = require('rtc-captureconfig');

// pull in the getusermedia helper module
// see:
var getUserMedia = require('getusermedia');

// get the sources
MediaStreamTrack.getSources(function(sources) {
  var constraints = capture('camera:1').toConstraints({ sources: sources });

  /* here is an example of what the generated constraints actually look like
  var constraints = {
    video: {
      mandatory: {},
      optional: [
        { sourceId: '30a3f6408175c22df739bcbf9573d841d9f99289' }

  // get user media
  getUserMedia(constraints, function(err, stream) {
    if (err) {
      return console.log('Could not capture stream: ', err);

    console.log('captured stream: ', stream);

It's worth noting that if the requested device does not exist on the machine (in the case above, if your machine only has a single webcam - as is common) then no device selection constraints will be generated (i.e. the standard { video: true, audio: true } constraints will be returned from the toConstraints call).

Experimental: Screen Capture

If you are working with chrome and serving content of a HTTPS connection, then you will be able to experiment with experimental getUserMedia screen capture.

In the simplest case, screen capture can be invoked by using the capture string of:


Which generates the following contraints:

  audio: false,
  video: {
    mandatory: {
      chromeMediaSource: 'screen'

    optional: []



This is a utility class that is used to update capture configuration details and is able to generate suitable getUserMedia constraints based on the configuration.


Update the camera configuration to the specified index


Update the microphone configuration to the specified index


Specify that we would like to capture the screen


Update a maximum constraint. If an fps constraint this will be directed to the maxfps modifier.


Update the maximum fps


Update a minimum constraint. This can be either related to resolution or FPS.


Update the minimum fps


Convert the internal configuration object to a valid media constraints representation. In compatible browsers a list of media sources can be passed through in the opts.sources to create contraints that will target a specific device when captured.

var media = require('rtc-media');
var capture = require('rtc-captureconfig');

// get the sources
MediaStreamTrack.getSources(function(sources) {
  // get the cameras
  var cameras = sources.filter(function(info) {
    return info && info.kind === 'video';

  // create videos
  var videos =, idx) {
    return media(capture('camera:' + idx).toConstraints({ sources: sources }));

  // render the videos
  videos.forEach(function(vid) {

"Internal" methods


Parse a resolution specifier (e.g. 1280x720) into a simple JS object (e.g. { w: 1280, h: 720 })


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

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.