openMVG_main_GlobalSfM

[GlobalACSfM] is based on the paper “Global Fusion of Relative Motions for Robust, Accurate and Scalable Structure from Motion.” published at ICCV 2013.

Multi-view structure from motion (SfM) estimates the position and orientation of pictures in a common 3D coordinate frame. When views are treated incrementally, this external calibration can be subject to drift, contrary to global methods that distribute residual errors evenly. Here the method propose a new global calibration approach based on the fusion of relative motions between image pairs.

Algorithm of the Global Structure from Motion

Require: internal camera calibration (possibly from EXIF data)
Require: pairwise geometry consistent point correspondences
Ensure: 3D point cloud
Ensure: camera poses

compute relative pairwise rotations
detect and remove false relative pairwise rotations
  - using composition error of triplet of relative rotations
compute the global rotation
  - using a dense least square and approximated rotations
compute relative translations
  - using triplet of views for stability and colinear motion support
compute the global translation
  - integration of the relative translation directions using a l-∞ method.
final structure and motion
  - link tracks validated per triplets and compute global structure by triangulation,
  - refine estimated parameter in a 2 step Bundle Adjustment
    - refine structure and translations
    - refine structure and camera parameters (rotations, translations).

Information and usage

The chain is designed to run on a sfm_data.json file and some pre-computed matches. The chain will only consider images with known approximate focal length. Image with invalid intrinsic id will be ignored.

$ openMVG_main_GlobalSfM -i Dataset/matches/sfm_data.json -m Dataset/matches/ -o Dataset/out_Global_Reconstruction/

Arguments description:

Required parameters:

  • [-i|–input_file]
    • a SfM_Data file
  • [-m|–matchdir]
    • path were geometric matches were stored
  • [-o|–outdir]
    • path where the output data will be stored

Optional parameters:

  • [-r|–rotationAveraging]

    • 1: L1 rotation averaging _[Chatterjee]
    • 2: (default) L2 rotation averaging _[Martinec]

  • [-t|–translationAveraging]

    • 1: (default) L1 translation averaging _[GlobalACSfM]
    • 2: L2 translation averaging _[Kyle2014]
    • 3: (default) SoftL1 minimization _[GlobalACSfM]
  • [-f|–refineIntrinsics]

    User can control exactly which parameter will be considered as constant/variable and combine them by using the ‘|’ operator.

    • ADJUST_ALL -> refine all existing parameters (default)
    • NONE -> intrinsic parameters are held as constant
    • ADJUST_FOCAL_LENGTH -> refine only the focal length
    • ADJUST_PRINCIPAL_POINT -> refine only the principal point position
    • ADJUST_DISTORTION -> refine only the distortion coefficient(s) (if any)
    • NOTE Options can be combined thanks to ‘|’:
      • ADJUST_FOCAL_LENGTH|ADJUST_PRINCIPAL_POINT -> refine the focal length & the principal point position
      • ADJUST_FOCAL_LENGTH|ADJUST_DISTORTION -> refine the focal length & the distortion coefficient(s) (if any)
      • ADJUST_PRINCIPAL_POINT|ADJUST_DISTORTION -> refine the principal point position & the distortion coefficient(s) (if any)

_[GlobalACSfM] default settings are “-r 2 -t 3”.

Tips

As a dense image network is required to perform global SfM it is required to detect more Regions points per image to ensure a high probability of matching.
Please use:
  • “-p HIGH” option on openMVG_main_ComputeFeatures
  • “-r .8” on openMVG_main_ComputeMatches.