Re: [sdw] Video Metadata Question: Use cases & requirements for recording camera attributes (#1137)

I've been thinking about a [WebVMT solution]( for a while. Here are my thoughts so far to get the discussion underway.

**Requirements & Use Cases**

Good practice should be to record camera attributes where possible and their omission should indicate that the values are unknown, though some assumptions may be permitted. Consideration should also be given to the limitations of resource-poor devices which may be capturing the source data.

Cameras can be classified as follows:
   * Mobile - able to change _location_ during capture;
   * Pivoting - able to change _orientation_ during capture.

Each classification can be considered in turn.

   1. Immobile, non-pivoting cameras, e.g. webcam, should specify their orientation if known. This can be done in the file header and should not be repeated in the file body. Location and orientation attributes should be included in the device configuration if suitable sensors are not included in the device.

   1. Immobile, pivoting cameras, e.g. security camera, should specify their orientation if known. This could be done in the file body with updates at key intervals, so instantaneous values can be calculated by interpolation. Location and orientation attributes should be included in the device configuration if suitable sensors are not included in the device, and there may be a requirement to record orientation relative to the pivot's 'front' direction.

   1. Mobile, non-pivoting cameras, e.g. dashcam or body-worn video, most often face forwards relative to the direction of motion, which should be the default orientation when no camera attributes are included. Cameras which face in another direction, e.g. vehicle reversing camera, should provide correct orientation details, which may raise a requirement for a relative orientation in the file header, e.g. relative direction = 180, to assist capture by resource-poor devices.

   1. Mobile, pivoting cameras, e.g. helicopters, drones and smartphones, can face in any direction relative to their direction of motion and should record their orientation if known, e.g. not all smartphones have gyros or compasses.

All cameras should have:

   1. An identifier, as there may be multiple cameras involved.
   1. The option to record orientation in two possible formats:

      1. World orientation - relative to north;
      1. Platform orientation - relative to the camera platform, e.g. front, back-left, etc. In this case, world orientation of the camera platform should also be included to allow parity with the former option.


[3GP video]( provides a good starting point for camera attributes, with the following:

Attribute | Description
-- | --
Digital zoom | Digital magnification scaling factor
Optical zoom | Optical magnification scaling factor
Pan indication | True or magnetic (north)
Pan | Angle from north in the horizontal plane
Tilt | Angle from the horizontal plane in the camera direction
Rotation | Angle from vertical in the camera direction

This only allows for a world orientation approach, but could be extended to include the platform orientation option.

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Received on Tuesday, 9 July 2019 18:01:38 UTC