Stabilisation

In the context of a measurement with a fixed camera, the user must acquire the most stable video possible. However, this is not always obvious, especially during drone measurements with wind. To remedy this problem, ANDROMEDE is equipped with a module to stabilize the videos. It is possible to access it after loading a video by selecting Process>>Stabilization.

_images/stab.png — Stabilisation panel.

Once the module is open, two stabilization methods are available: stabilization with mask and stabilization with colored markers.

Mask Stabilisation

The stabilization with mask is the default method available on ANDROMEDE. The principle is to determine the displacement of the camera and to rectify it by analyzing the movement of supposedly fixed zones (thus mainly the banks). To access this method, simply select the appropriate option in the stabilization menu.

_images/menu_stab1.png — Banking Stabilization Tab.

To prevent flow from interfering with the analysis, the user can draw a mask to hide the free surface.

_images/masque_stab_berge.png — Blackout mask.

The detection of geometric primitives is based on the GoodFeaturestoTrack detection method whose number of detected objects is governed by the parameter Max corner. The displacement is computed by optical flow from the Lucas-Kanade algorithm. The trajectory described by the camera motion is then smoothed by sliding average in order to have a smooth correction. The parameter Smoothing radius” corresponds to the smoothing window in frame number.

This stabilization method does not require any particular reference points. The user must only ensure that the banks are visible when the video is taken.

NB: This method is mainly applicable for videos taken vertically from the free surface because it only corrects affine transformations (translations and rotations).

Color stabilisation

The stabilization with colored markers is more precise, faster and allows to correct the changes of perspectives as those induced by the changes of altitude of the drone. It also allows to stabilize videos which are not vertical. On the other hand, it requires the constant presence of 4 markers on the ground of the same color at any time of the video. To access them, switch to the corresponding option in the stabilization menu.

_images/menu_stab2.png — Color stabilization tab

The user has the possibility of drawing masks to select only the targets and thus avoid detecting other objects of the same color.

_images/stab_color.png — Selective mask

Ideally, you should choose a color that is not present in the study area. The color is determined by the user via the parameter Color and can be refined with the mouse. The parameters Blur size and Erode/dilate factor allow to smooth the image in order to improve the detection of the marks. This allows large ground marks to be interpreted as several ground marks. If the pixel size of the ground marks in the image is very small, the default values for these parameters at 1 relevant if the targets do not disappear during video recording. The “Smoothing radius” allows, as with the stabilization method with mask, to make a smoother correction and thus avoid a jerky rendering.