en/devices/camera/camera3_metadata.html

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<h2 id="metadata">Metadata support</h2>
<p> To support the saving of raw image files by the Android framework, substantial 
  metadata is required about the sensor's characteristics. This includes 
  information such as color spaces and lens shading functions.</p>
<p>Most of this information is a static property of the camera subsystem and can 
  therefore be queried before configuring any output pipelines or submitting any 
  requests. The new camera APIs greatly expand the information provided by the 
  getCameraInfo() method to provide this information to the application.</p>
<p>In addition, manual control of the camera subsystem requires feedback from the 
  assorted devices about their current state, and the actual parameters used in 
  capturing a given frame. The actual values of the controls (exposure time, frame 
  duration, and sensitivity) as actually used by the hardware must be included in 
  the output metadata. This is essential so that applications know when either 
  clamping or rounding took place, and so that the application can compensate for 
  the real settings used for image capture.</p>
<p>For example, if an application sets frame duration to 0 in a request, the HAL 
  must clamp the frame duration to the real minimum frame duration for that 
  request, and report that clamped minimum duration in the output result metadata.</p>
<p>So if an application needs to implement a custom 3A routine (for example, to 
  properly meter for an HDR burst), it needs to know the settings used to capture 
  the latest set of results it has received in order to update the settings for 
  the next request. Therefore, the new camera API adds a substantial amount of 
  dynamic metadata to each captured frame. This includes the requested and actual 
  parameters used for the capture, as well as additional per-frame metadata such 
  as timestamps and statistics generator output.</p>
<h2 id="per-setting">Per-setting control</h2>
<p> For most settings, the expectation is that they can be changed every frame, 
  without introducing significant stutter or delay to the output frame stream. 
  Ideally, the output frame rate should solely be controlled by the capture 
  request's frame duration field, and be independent of any changes to processing 
  blocks' configuration.  In reality, some specific controls are known to be slow 
  to change; these include the output resolution and output format of the camera 
  pipeline, as well as controls that affect physical devices, such as lens focus 
  distance. The exact requirements for each control set are detailed later.</p>
<h2 id="raw-sensor">Raw sensor data support</h2>
<p>In addition to the pixel formats supported by 
  the old API, the new API adds a requirement for support for raw sensor data 
  (Bayer RAW), both for advanced camera applications as well as to support raw
  image files.</p>

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