What determines which capabilities are enumerated by the MediaStreamTrack.getCapabilities() method? - getusermedia

Question

I have been working on an application of getUserMedia and WebRTC that requires me to make use of information returned by the getConstraints(), getCapabilities(), and getSettings() methods of a MediaStreamTrack.
In testing, using getCapabilities() on the track corresponding to an external webcam might return something like...
aspectRatio: {max: 1920, min: 0.000925925925925926}
deviceId: ""
exposureCompensation: MediaSettingsRange {max: 2047, min: 3, step: 1}
exposureMode: (2) ["manual", "continuous"]
facingMode: []
focusMode: (2) ["manual", "continuous"]
frameRate: {max: 30, min: 1}
groupId: ""
height: {max: 1080, min: 1}
whiteBalanceMode: ["manual"]
width: {max: 1920, min: 1}
zoom: MediaSettingsRange {max: 5, min: 1, step: 1}
...but if focusDistance or colorTemperature (or other potential members listed here) are not in the capabilities list, then what determines the controllable settings for a camera or other media device?
Would the answer to this also apply to mobile devices, such as Android or iOS tablets or phones, that have integrated cameras and microphones?

Answer 1

The capabilities returned by track.getCapabilities() are the intersection of four things:
The track source. getUserMedia and getDisplayMedia tracks have different capabilities.
The track.kind. E.g. audio capabilities are not returned for video tracks, and vice versa.
The capabilities supported by the browser.
The capabilities supported by the device.
You learn what capabilities the browser supports like this:
console.log(navigator.mediaDevices.getSupportedConstraints());
You then typically rely on documentation and specification to know which capabilities apply to which method and kind.
Newer versions of browsers tend to support more capabilities. Not all browsers support getCapabilities() yet. Firefox, for instance, does not yet, even though it otherwise supports constraints.
For this reason, it is always best to check what's supported—as shown above—before relying on a capability.
To the specific capabilities you mention, focusDistance and colorTemperature, they are defined for tracks returned by getUserMedia, but defined in the mediacapture-image extension spec, so they should show up if they're supported. Their absence means they're not supported.