Strecha Dataset Pose Conventions

The Strecha dataset is widely used for multi-view stereo vision and SLAM algorithm evaluation. Understanding the format of its camera poses and projection files is crucial for correctly using this dataset.

File Format

Each scene in the Strecha dataset typically contains a series of .camera files corresponding to image files. For example, for image 0001.jpg, its corresponding camera parameters are in the 0001.jpg.camera file.

Each .camera file contains the following information in order:

  1. Intrinsic matrix \(K\) (3 rows): A \(3 \times 3\) camera intrinsic matrix.

    \[\begin{split}K = \begin{bmatrix} f_x & s & c_x \\ 0 & f_y & c_y \\ 0 & 0 & 1 \end{bmatrix}\end{split}\]

    where \(f_x\), \(f_y\) are focal lengths, \(c_x\), \(c_y\) are principal point coordinates, \(s\) is the skew coefficient (usually 0).

  2. Empty line (1 row): An empty (zero) line used to separate intrinsics and extrinsics.

  3. Rotation matrix \(R^i_w\) (3 rows): A \(3 \times 3\) rotation matrix(row-major storage). This represents the rotation from world coordinate system to camera \(i\) coordinate system.

  4. Camera center \(\mathbf{t}^i_w\) (1 row): A \(3 \times 1\) translation vector(row vector storage). This represents the position of camera \(i\)’s center in the world coordinate system.

Global Camera Pose

According to the above file format, for camera \(i\), we directly read from the file:

  • Rotation: \(R^i_w\) (rotation from world coordinate system to camera \(i\) coordinate system)

  • Camera center: \(\mathbf{t}^i_w\) (3D coordinates of camera \(i\)’s center in the world coordinate system)

A 3D point \(P_w\) in the world coordinate system can be transformed to a point \(P_{c,i}\) in camera \(i\)’s coordinate system using the following formula:

\[P_i = R^i_w (P_w - \mathbf{t}^i_w)\]

This is equivalent to using the augmented matrix \([R^i_w | -R^i_w \mathbf{t}^i_w]\) for transformation.

Therefore, reading from Strecha ground truth files will load into PoSDK RwTw format.

Relative Camera Pose

Consistent with posdk conventions (see posdk.md), the Strecha dataset’s relative poses are also stored in RwTw format.