Screen Capture

5.1 MediaDevices Additions

partial interface MediaDevices {
  Promise<MediaStream> getDisplayMedia(optional DisplayMediaStreamConstraints constraints = {});
};

getDisplayMedia

Prompts the user for permission to live-capture their display.

The user agent MUST let the end-user choose which display surface to share out of all available choices every time, and MUST NOT use constraints to limit that choice. Instead, constraints MUST be applied to the media chosen by the user, only after they have made their selection. This prevents an application from influencing the selection of sources, see § 5.3 Unconstrained Display Surface Selection for details.

In the case of audio, the user agent MAY present the end-user with audio sources to share. Which choices are available to choose from is up to the user agent, and the audio source(s) are not necessarily the same as the video source(s). An audio source may be a particular application, window, browser, the entire system audio or any combination thereof. Unlike getUserMedia with regards to audio+video, the user agent is allowed not to return audio even if the audio constraint is present. If the user agent knows no audio will be shared for the lifetime of the stream it MUST NOT include an audio track in the resulting stream. The user agent MAY accept a request for audio and video by only returning a video track in the resulting stream, or it MAY accept the request by returning both an audio track and a video track in the resulting stream. The user agent MUST reject audio-only requests.

In addition to drawing from a different set of sources and requiring user selection, getDisplayMedia also differs from getUserMedia in that "granted" permissions cannot be persisted.

When the getDisplayMedia() method is called, the User Agent MUST run the following steps:

1. If the method call is not triggered by user activation, return a promise rejected with a DOMException object whose name attribute has the value InvalidStateError.

2. Let constraints be the method's first argument.

3. If constraints.video is false, return a promise rejected with a newly created TypeError.

4. For each member present in constraints whose value, CS, is a dictionary, run the following steps:

   1. If CS contains a member named advanced, return a promise rejected with a newly created TypeError.

   2. If CS contains a member whose name specifies a constrainable property applicable to display surfaces, and whose value in turn is a dictionary containing a member named either min or exact, return a promise rejected with a newly created TypeError.

   3. If CS contains a member whose name, failedConstraint specifies a constrainable property, constraint, applicable to display surfaces, and whose value in turn is a dictionary containing a member named max, and that member's value in turn is less than the constrainable property's floor value, then let failedConstraint be the name of the constraint, let message be either undefined or an informative human-readable message, and return a promise rejected with a new OverconstrainedError created by calling OverconstrainedError(failedConstraint, message).

5. Let requestedMediaTypes be the set of media types in constraints with either a dictionary value or a value of true.

6. If the current settings object's responsible document is NOT fully active, return a promise rejected with a DOMException object whose name attribute has the value InvalidStateError.

7. Let p be a new promise.

8. Run the following steps in parallel:

   1. For each media type T in requestedMediaTypes,

      1. If no sources of type T are available, reject p with a new DOMException object whose name attribute has the value NotFoundError.

      2. Retrieve the permission state for obtaining sources of type T in the current browsing context. If the permission state is "denied", jump to the step labeled PermissionFailure below.

   2. Optionally, e.g., based on a previously-established user preference, for security reasons, or due to platform limitations, jump to the step labeled Permission Failure below.

   3. Prompt the user to choose a display device, with a PermissionDescriptor named "display-capture", resulting in a set of provided media.

      The provided media MUST include precisely one track of each media type in requestedMediaTypes. The devices chosen MUST be the ones determined by the user. Once selected, the source of a MediaStreamTrack MUST NOT change.

      User Agents are encouraged to warn users against sharing browser display devices as well as monitor display devices where browser windows are visible, or otherwise try to discourage their selection on the basis that these represent a significantly higher risk when shared.

      If the result of the request is "granted", then for each device that is sourcing the provided media, using a stable and private id for the device, deviceId, set [[devicesLiveMap]][deviceId] to true, if it isn’t already true, and set the [[devicesAccessibleMap]][deviceId] to true, if it isn’t already true.

      The User Agent MUST NOT create a permission storage entry with a value of "granted".

      If the result is "denied", jump to the step labeled Permission Failure below. If the user never responds, this algorithm stalls on this step.

      If the user grants permission but a hardware error such as an OS/program/webpage lock prevents access, reject p with a new DOMException object whose name attribute has the value NotReadableError and abort these steps.

      If the result is "granted" but device access fails for any reason other than those listed above, reject p with a new DOMException object whose name attribute has the value AbortError and abort these steps.

   4. Let stream be the MediaStream object for which the user granted permission.

   5. Run the ApplyConstraints algorithm on all tracks in stream with the appropriate constraints. Should this fail, let failedConstraint be the result of the algorithm that failed, and let message be either undefined or an informative human-readable message, and then reject p with a new OverconstrainedError created by calling OverconstrainedError(failedConstraint, message).

   6. Resolve p with stream and abort these steps.

   7. Permission Failure: Reject p with a new DOMException object whose name attribute has the value NotAllowedError.

9. Return p.

5.2 Closed and Minimized Display Surfaces

A display surface that is being shared may temporarily or permanently become inaccessible to the application because of actions taken by the operating system or user agent. What makes a display surface considered inaccesible is outside the scope of this specification, but examples MAY include a monitor disconnecting or an application, window or browser closing or becoming minimized.

When display surface enters an inaccessible state that is not necessarily permanent, the user agent MUST queue a task that sets the muted state of the corresponding media track to true.

When display surface exits an inaccessible state and becomes accessible, the user agent MUST queue a task that sets the muted state of the corresponding media track to false.

When a display surface enters an inaccessible state that is permanent (such as the source application terminating), the user agent MUST queue a task that ends the corresponding media track.

A stream that was just returned by getDisplayMedia MAY contain tracks that are muted by default. Audio and video tracks belonging to the same stream MAY be muted/unmuted independently of one another.

5.3 Unconstrained Display Surface Selection

Not accepting constraints for source selection means that getDisplayMedia only provides fingerprinting surface that exposes whether audio, video or audio and video display sources are present.

5.4 Constrainable Properties for Captured Display Surfaces

Constraints serve a different purpose in getDisplayMedia than they do in getUserMedia. They do not aid discovery, instead they are applied only after user-selection.

This section define which constraints apply to getDisplayMedia tracks; constraints defined for getUserMedia do not apply unless listed here.

Some of these constraints enable user agent processing like downscaling and frame decimation, as well as display-specific features. Others enable observation of inherent properties of a user-selected display surface, as capabilities and settings.

The following new and existing MediaStreamTrack Constrainable Properties are defined to apply to the user-selected video display surface, with the following behavior:

The following new and existing MediaStreamTrack Constrainable Properties are defined to apply to the user-selected audio sources, with the following behavior:

When inherent properties of the underlying source of a user-selected display surface change, for example in response to the end-user resizing a captured window, and these changes render the capabilities and/or settings of one or more constrainable properties outdated, the user agent MUST queue a task to run the following step:

1. Update all affected constrainable properties at the same time.

   If this causes an "overconstrained" situation, then the user agent MUST ignore the culprit constraints for as long as they overconstrain. The user agent MUST NOT mute the track, and the user agent MUST NOT fire the overconstrained event.

dictionary DisplayMediaStreamConstraints {
  (boolean or MediaTrackConstraints) video = true;
  (boolean or MediaTrackConstraints) audio = false;
};

partial dictionary MediaTrackSupportedConstraints {
  boolean displaySurface = true;
  boolean logicalSurface = true;
  boolean cursor = true;
  boolean restrictOwnAudio = true;
};

partial dictionary MediaTrackConstraintSet {
  ConstrainDOMString displaySurface;
  ConstrainBoolean logicalSurface;
  ConstrainDOMString cursor;
  ConstrainBoolean restrictOwnAudio;
};

partial dictionary MediaTrackSettings {
  DOMString displaySurface;
  boolean logicalSurface;
  DOMString cursor;
};

enum DisplayCaptureSurfaceType {
  "monitor",
  "window",
  "application",
  "browser"
};

enum CursorCaptureConstraint {
  "never",
  "always",
  "motion"
};

5.5 Device Identifiers

Each potential source of capture is treated by this API as a discrete media source. However, display capture sources MUST NOT be enumerated by enumerateDevices, since this would reveal too much information about the host system.

Display capture sources therefore cannot be selected with the deviceId constraint, since their deviceIds are not exposed.

8.1 Capturing Logical or Visible Display Surfaces

Capture of logical display surfaces causes there to be a potential for content to be shared that a user is not made aware of. A logical display surface might render information that a user did not intend to expose. This can be more easily recognized if this information is visible. Such means are likely ineffectual against a machine, but a human recipient is less able to process content that appears only briefly.

Information that is not currently rendered to the screen SHOULD be obscured in captures unless the application has been specifically authorized to access that content (this might require elevated permissions).

How obscured areas of the logical display surface are captured to produce a visible display surface capture MAY vary. Some applications, like presentation software, benefit from having obscured portions of the screen render the image that appeared prior to being obscured. Freezing images can cause visual artifacts for changing content, or hide the fact that content is being obscured. Note that frozen portions of a capture can be incorrectly perceived as a bug. Alternatively, obscured areas might be replaced with content that marks them as being obscured, such as a grey color or hatching.

Some systems MAY only capture the logical display surface. Devices with small screens, for instance, do not typically have the concept of a window, and render applications in full screen modes only. These systems might provide a capture of an application that is not currently visible, which could be unusable without capturing the logical display surface.

An important consideration when capturing a window or other display surface that is partially transparent is that content from the background might be shared. A user agent MUST NOT capture content from the background of a captured display surface.

There is a risk that the user prompt be exposed to the web page for a short amount of time by the newly created MediaStreamTrack, for instance if the user selects the screen on which the user prompt is displayed. In the case the user prompt displays previews of the various surfaces available for selection, the user agent MUST NOT capture, for the newly created MediaStreamTrack, the previews that the user did not intend to share explicitly.

Capturing Audio

getDisplayMedia allows capturing audio alongside video, this poses privacy and security concern as this may expose additional information about system applications, and the set of shared audio sources are not necessarily the same as the set of shared video sources. For example, a user agent MAY be capturing the video of a window and capture the audio of the entire system, including applications unrelated to that window. The user agent MUST NOT share audio without active user consent. It is important that the user is aware of what content will be shared, including any possible audio. It is strongly recommended that the user is allowed to give consent to video but not audio, resulting in a video-only stream. This ensures that the request for audio is always optional and does not restrict the user's choices compared to a video-only request.

8.2 Authorizing Display Capture

This document provides recommends that implementations provide additional limitations on the mechanisms used to affirm user consent. These limitations are designed to mitigate the security and privacy risks that the API poses.

Two forms of consent interaction are described: active user consent and a range of elevated permissions. These are non-normative recommandations only.

8.3 Feedback and Interface During Capture

Implementations are advised to provide user feedback and control mechanisms similar to those offered users when sharing a camera or microphone, as recommended in [GETUSERMEDIA].

It is important that a user be aware that content is being shared when content is actively being captured. User agents are advised to display a prominent indicator while content is being captured. In addition to an indicator, a user agent is advised to provide a means to learn precisely what is being shared; while this capability is trivially provided by an application by rendering the captured content, this information allows a user to accurately assess what is being shared.

In addition to feedback mechanisms, a means to for the user to stop any active capture is advisable.