媒体追踪约束 - Web API 接口参考 编辑

The MediaTrackConstraints dictionary is used to describe a set of capabilities and the value or values each can take on. A constraints dictionary is passed into applyConstraints() to allow a script to establish a set of exact (required) values or ranges and/or preferred values or ranges of values for the track, and the most recently-requested set of custom constraints can be retrieved by calling getConstraints().

For each constraint, you can typically specify an exact value you need, an ideal value you want, a range of acceptable values, and/or a value which you'd like to be as close to as possible. The specifics vary somewhat depending on the type of the constrainable property.

To learn more about how constraints work, see Capabilities, constraints, and settings.

Properties

Some combination—but not necessarily all—of the following properties will exist on the object.

Properties of all media tracks

deviceId

A ConstrainDOMString object specifying a device ID or an array of device IDs which are acceptable and/or required.

groupId

A ConstrainDOMString object specifying a group ID or an array of group IDs which are acceptable and/or required.

Properties of audio tracks

autoGainControl

A ConstrainBoolean object which specifies whether automatic gain control is preferred and/or required.

channelCount

A ConstrainLong specifying the channel count or range of channel counts which are acceptable and/or required.

echoCancellation

A ConstrainBoolean object specifying whether or not echo cancellation is preferred and/or required.

latency

A ConstrainDouble specifying the latency or range of latencies which are acceptable and/or required.

noiseSuppression

A ConstrainBoolean which specifies whether noise suppression is preferred and/or required.

sampleRate

A ConstrainLong specifying the sample rate or range of sample rates which are acceptable and/or required.

sampleSize

A ConstrainLong specifying the sample size or range of sample sizes which are acceptable and/or required.

volume

A ConstrainDouble specifying the volume or range of volumes which are acceptable and/or required.

Properties of image tracks

whiteBalanceMode

A String specifying one of "none", "manual", "sigle-shot", or "continuous".

exposureMode

A String specifying one of "none", "manual", "sigle-shot", or "continuous".

focusMode

A String specifying one of "none", "manual", "sigle-shot", or "continuous".

pointsOfInterest

The pixel coordinates on the sensor of one or more points of interest. This is either an object in the form { x:value, y:value } or an array of such objects, where value  is a double-precision integer.

exposureCompensation

A ConstrainDouble (a double-precision integer) specifying f-stop adjustment by up to ±3. 

colorTemperature

A ConstrainDouble (a double-precision integer) specifying a desired color temperature in degrees kelvin.

iso

A ConstrainDouble (a double-precision integer) specifying a desired iso setting.

brightness

A ConstrainDouble (a double-precision integer) specifying a desired brightness setting.

contrast

A ConstrainDouble (a double-precision integer) specifying the degree of difference between light and dark.

saturation

A ConstrainDouble (a double-precision integer) specifying the degree of color intensity.

sharpness

A ConstrainDouble (a double-precision integer) specifying the intensity of edges.

focusDistance

A ConstrainDouble (a double-precision integer) specifying distance to a focused object.

zoom

A ConstrainDouble (a double-precision integer) specifying the desired focal length.

torch

A Boolean whter the fill light continuously connected, meaning it stays on as long as the track is active.

Properties of video tracks

aspectRatio

A ConstrainDouble specifying the video aspect ratio or range of aspect ratios which are acceptable and/or required.

facingMode

A ConstrainDOMString object specifying a facing or an array of facings which are acceptable and/or required.

frameRate

A ConstrainDouble specifying the frame rate or range of frame rates which are acceptable and/or required.

height

A ConstrainLong specifying the video height or range of heights which are acceptable and/or required.

width

A ConstrainLong specifying the video width or range of widths which are acceptable and/or required.

Specifications

Specification	Status	Comment
Media Capture and Streams applyConstraints()	Candidate Recommendation	Initial definition.
MediaStream Image Capture applyConstraints()	Working Draft	Adds image constraints.

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

• Desktop
• Mobile

Feature	Chrome	Firefox (Gecko)	Internet Explorer	Opera	Safari
Basic support	59	?	?	46	?
deviceId	59	?	?	46	?
groupId	59	?	?	46	?
Audio track properties	未实现	?	?	未实现	?
Image track properties	63	?	?	50	?
Video track properties	59	?	?	46	?

Feature	Android Webview	Chrome for Android	Firefox Mobile (Gecko)	IE Mobile	Opera Mobile	Safari Mobile
Basic support	59	59	?	?	46	?
deviceId	59	59	?	?	46	?
groupId	59	59	?	?	46	?
Audio track properties	未实现	未实现	?	?	未实现	?
Image track properties	63	63	?	?	50	?
Video track properties	59	59	?	?	46	?

See also

• MediaDevices.getUserMedia()
• MediaStreamTrack.getConstraints()
• MediaStreamTrack.applyConstraints()
• MediaDevices.getSupportedConstraints()
• MediaTrackSupportedConstraints
• MediaStreamTrack.getSettings()