lambda=1.0 smoothness parameter for OpenCV 2
cv.CalcOpticalFlowHS is not the same as what= Matlab calls
With this example implementation in calcOptFlow.py I found that in OpenCV
lambda=0.001 gives results that are much more like Matlab–I can see the fine details.
OpenCV 3.x no longer has the original Horn Schunck optical flow
General issues with legacy optical flow estimation
Linearized Horn Schunck has problems with large displacements in the image, which can be resolved by a pyramidal algorithm, or by using the more modern optical flow algorithms in OpenCV 3.x.