Canvas has a non-planar homography in windy conditions

In windy conditions, the canvas becomes a curved surface rather than a plane and it becomes impossible to find other than a rough approximation of the homography.

The top image shows the canvas with wind from behind, causing the canvas to take on a curved, non-planar shape. Only a rough approximation of the planar homography can be found in this case. The bottom image shows the canvas when the wind has subdued temporarily, here the found homography more closely matches the shape of the goal region. Personal photos by author. September 2018.

Since a mapping between the coordinate space of the canvas in the picture and a rectangular and planar representation of the canvas (i.e. without perspective) relies on finding a good homography, windy conditions provide quite the challenge. Finding non-planar mappings is not impossible but seems much less reliable.