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api_gcps.rst

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The ground control points are a simple python dictionary that should follow a certain schema. The schema looks as follows:

{
    "src": [
        [int, int],
        [int, int],
        ...,
    ],
    "dst": [
        [float, float, Optional(float)],
        [float, float, Optional(float)],
        ...,
    ],
    "z_0": Optional(float),
    "h_ref": Optional(float),
    "crs": Optional(int, str)

}

The coordinates in the src field are simply the pixel coordinates in your video, where the GCPS are located. You can look these up by plotting the first frame with plt.imshow or storing the first frame to a file and open that in your favorite photo editor and count the pixels there.

dst contains the real-world coordinates, that belong to the same points, indicated in src. dst must therefore contain either 4 x, y (if the left situation is chosen) or 6 x, y, z coordinates (if the right situation is chosen.

In both cases you provide the points as a list of lists.

z_0 must be provided if 6 randomly placed points are used. If you intend to provide multiple videos with a locally measured water level, then also provide h_ref as explained above. In case you have used 4 x, y points at the water surface, then also provide z_0. With this information the perspective of the water surface is reinterpreted with each video, provided that a water level (as measured with the installed device) is provided by the user with each new video.

Note

For drone users that only make nadir aimed videos, we are considering to also make an option with only 2 GCPs possible. If you are interested in this, kindly make an issue in GitHub. For the moment we suggest to use the 4 control point option and leave z_0 and h_ref empty.

Finally a coordinate reference system (CRS) may be provided, that indicates in which CRS the survey was done if this is available. This is only useful if you also have provided a CRS when creating the camera configuration. If you for instance measure your control points in WGS84 lat-lon (EPSG code 4326) then pass crs=4326 and your coordinates will be automatically transformed to the local CRS used for your camera configuration.

A full example that supplies GCPs to the existing camera configuration in variable cam_config is shown below:

src = [
    [1335, 1016],
    [270, 659],
    [607, 214],
    [1257, 268]
]  # source coordinates on the frames of the movie
dst = [
    [6.0478836167, 49.8830484917],
    [6.047858455, 49.8830683367],
    [6.0478831833, 49.8830964883],
    [6.0479187017, 49.8830770317]
]  # destination locations in long/lat locations, as measured with RTK GNSS.
z_0 = 306.595  # measured water level in the projection of the GPS device
crs = 4326  # coordinate reference system of the GPS device, EPSG code 4326 is WGS84 longitude/latitude.

cam_config.set_gcps(src=src, dst=dst, z_0=z_0, crs=crs)

If you have not supplied camera_matrix and dist_coeffs to the camera configuration, then these can be optimized using the provided GCPs after these have been set using the following without any arguments.

cam_config.set_intrinsic()