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Whether the camera device will trigger a precapture metering sequence when it processes this request.
This entry is normally set to IDLE, or is not included at all in the request settings. When included and set to START, the camera device will trigger the autoexposure precapture metering sequence.
The precapture sequence should triggered before starting a high-quality still capture for final metering decisions to be made, and for firing pre-capture flash pulses to estimate scene brightness and required final capture flash power, when the flash is enabled.
Normally, this entry should be set to START for only a single request, and the application should wait until the sequence completes before starting a new one.
The exact effect of auto-exposure (AE) precapture trigger depends on the current AE mode and state; see CaptureResult.ControlAeState for AE precapture state transition details.
On LEGACY-level devices, the precapture trigger is not supported; capturing a high-resolution JPEG image will automatically trigger a precapture sequence before the high-resolution capture, including potentially firing a pre-capture flash.
Possible values:
Optional - This value may be null on some devices.
Limited capability - Present on all camera devices that report being at least CameraMetadata.INFO_SUPPORTED_HARDWARE_LEVEL_LIMITED devices in the CameraCharacteristics.InfoSupportedHardwareLevel key