ContextCapture User Guide

Photogroups

Photogroup properties

Photogroup properties define the interior orientation of the camera. Accurate estimates of photogroup properties are required for 3D reconstruction. These estimates can either:
  • Be automatically estimated by ContextCapture by aerotriangulation, starting from some coarse estimates derived from EXIF metadata or using the ContextCapture camera database.
  • Be imported (when importing a block, or from the camera database, or from an OPT file).
  • Or entered manually.

Multi-camera rig

Specifies if the photogroup defines a camera device that is part of a multi-camera rig. The supported options are None or Main. For a multi-camera rig setup to be valid, all the photogroups of the block must belong to it and all of them must fulfill the requirements of such setup.

Camera model type

Two camera model types are supported:

  • Perspective: perspective projection combined with a classical Brown's distortion model, suited for rectilinear lens or lens with low distortion.
  • Fisheye: Suited for wide-angle or fish-eye lens with extreme lens distortion.

For more information about ContextCapture camera models, see doc/ContextCapture camera model.pdf.

Camera model band

Two camera model band are supported:

  • Visible: standard RGB camera capturing visible colors.
  • Thermal: Far Infra Red camera capturing temperature.

Sensor size

ContextCapture may need to know the size of your camera's sensor. The needed sensor size is the sensor's largest dimension. If your camera model is not already listed in our database, you will be asked to enter this information. If you are unsure of your camera's specifications, you can consult the Digital Photography Review or contact Bentley Systems technical support.

Note: When a camera model is not listed in our database, the interface proposes to send a request to Bentley Systems. We will reply to your email in a timely manner, and we will include the missing camera model in the database of the next ContextCapture release.
Note: An undefined sensor size may be acceptable in some specific cases, e.g. if the 35mm-equivalent focal length is known from EXIF metadata.

Focal length

For a newly created photogroup, ContextCapture can generally extract a coarse guess of the focal length in mm from EXIF metadata. Otherwise, you will be asked to enter this initial guess. Later, ContextCapture can automatically estimate the focal length more accurately by aerotriangulation.

We recommend to specify sensor size and/or focal length for each photogroup. If these properties are missing, ContextCapture will assume that the 35 mm equivalent focal length equals to 50 mm. If the correct focal length value differs significantly, the aerotriangulation may fail. In such cases it is required to specify initial camera properties manually.

For fisheye camera models, the focal length is ignored as soon as a fisheye focal matrix is defined (see below).

Fisheye focal matrix

For the camera model type Fisheye, the fisheye focal matrix overrides the focal length property to model asymmetric fisheye focals.

Not used for perspective camera models.

Principal point

For a newly created photogroup, ContextCapture considers that the principal point is by default at the center of the image. Later, ContextCapture can automatically estimate the principal point more accurately by aerotriangulation.

Distortion

For a newly created photogroup, ContextCapture considers that there is by default no lens distortion. Later, ContextCapture can automatically estimate lens distortion more accurately by aerotriangulation.

Not used for fisheye camera models.

Fisheye distortion

Distortion model for the fisheye camera models.

Not used for perspective camera models.

Aspect ratio/Skew

Properties used when pixels are not square.

  • Skew: coefficient defining the angle between x and y pixel axes.
  • Aspect ratio: different from 1 if the pixel are not square.

Manipulating a photogroup

Get from the camera database/add to the camera database

The camera model of photogroup can be added to or obtained from the camera database.

Accessing the camera database from a photogroup

See also Camera Database.

Export/import optical properties

The properties of a photogroup (including sensor size, focal length, distortion, and principal point) can be exported and re-imported to/from an OPT file.

This feature can be used to re-use a camera calibration.

To import or export the optical properties, right click on a photogroup and choose Export optical properties or Import optical properties from the context menu.

Ungroup/group photogroups

In some specific cases, the interior orientation of the camera must be estimated independently for each photo: for example, if photos were taken by a camera rig, or if the focal length varied slightly from one photo to the other during acquisition (zoom lens).

In such case, we recommend to prepare the dataset before adding photos in ContextCapture by placing each photo in a different subdirectory, to conform to our general recommendations on photogroups. However, it is also possible to add photos as is and use the Ungroup function: select all photogroups in the table, right click and choose Ungroup from the context menu to get one photogroup per photo.

See also Preparing the imagery dataset.

Parent topic: Photos