First image, i.e. previous frame in the video.
Second image of same size and type as f1, i.e. current frame in the video.
Optional pre-allocated flow buffer. If provided, has to be of same size as input images are, and with 2 channels (u, v).
Should the previous flow be used. If true prealloc is treated as previous flow, and has to satisfy size requirements.
Calculated flow field.
Evaluate dense optical flow method between two consecutive frames.