Data Convention

Camera information and normalization

Besides multi-view RGB images, VolSDF needs cameras information in order to run. For each scan that we used, we supply a file named cameras.npz. The cameras.npz file contains for each image its assosiacted camera projection matrix (named "world_mat_{i}"), and a normalization matrix (named "scale_mat_{i}").

Camera projection matrix

A 3x4 camera projection matrix, P = K[R | t] projects points from 3D coordinates to image pixels by the formula: d[x; y; 1]=P[X;Y;Z;1] where K is a 3x3 calibration matrix, [R t] is 3x4 a world to camera Euclidean transformation, [X;Y;Z] is the 3D point, [x;y] is the 2D pixel coordinates of the projected point and d is the depth of the point. The input cameras.npz file contains the camera matrices, where P_i = cameras['world_mat_{i}'][:3, :] is a 3x4 matrix that projects points from the 3D world coordinates to the 2D coordinates of image i (intrinsics and extrinsics, i.e. P=K[R | t] ). Each "world_mat" matrix is a concatenation of the camera projection matrix with a row vector of [0,0,0,1] (which makes it a 4x4 matrix).

Normalization matrix

The cameras.npz contains also one normalization matrix named "scale_mat_{i}" (identical for all i) for changing the coordinates system such that the cameras and the region of interest are located inside a sphere with radius 3 located at the origin (more details are in the paper).

Preprocess new data

For converting BlendedMVS cameras format to ours (not required for the supplied scans), run :

cd data/preprocess/
python parse_cameras_blendedmvs.py --blendedMVS_path [BLENDED_MVS_PATH] --output_cameras_file [OUTPUT_CAMERAS_NPZ_FILE] --scan_ind [BLENDED_MVS_SCAN_ID]

In order to generate a normalization matrix for each scan, we used the input camera projection matrices. A script that demonstrates this process is presented in: data/preprocess/normalize_cameras.py. Note: in order to run the supplied scans, it is not required to run this script. For normalizing a given cameras.npz file run:

cd data/preprocess/
python normalize_cameras.py --input_cameras_file [INPUT_CAMERAS_NPZ_FILE] --output_cameras_file [OUTPUT_NORMALIZED_CAMERAS_NPZ_FILE] [--number_of_cams [NUMBER_OF_CAMERAS_LIMIT]]

where the last argument is optional and used for limiting the number of cameras such that only the first [NUMBER_OF_CAMERAS_LIMIT] cameras are considered, which is useful for the DTU dataset, where for scan_id<80 only the first 49 cameras out of 64 are used.

Parsing COLMAP cameras

It is possible to convert COLMAP cameras to our cameras format using Python. First the functions read_cameras_text,read_images_text, qvec2rotmat should be imported from https://github.com/colmap/colmap/blob/dev/scripts/python/read_write_model.py. Then the following Python code can be used:

cameras=read_cameras_text("output_sfm\\cameras.txt")
images=read_images_text("output_sfm\\images.txt")
K = np.eye(3)
K[0, 0] = cameras[1].params[0]
K[1, 1] = cameras[1].params[1]
K[0, 2] = cameras[1].params[2]
K[1, 2] = cameras[1].params[3]

cameras_npz_format = {}
for ii in range(len(images)):
    cur_image=images[ii]

    M=np.zeros((3,4))
    M[:,3]=cur_image.tvec
    M[:3,:3]=qvec2rotmat(cur_image.qvec)

    P=np.eye(4)
    P[:3,:] = K@M
    cameras_npz_format['world_mat_%d' % ii] = P
    
np.savez(
        "cameras_before_normalization.npz",
        **cameras_npz_format)

Note that you will have to normalize the cameras after running this code by running normalize_cameras.py as described above.

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