What motion estimation techniques that could estimate the motion of every pixel? I'm trying to estimate a motion from a video, however based on my finding, those methods usually meant for large motion and sparse in nature.

Optical flow: Sparse and fail to detect sub-pixel motion

Phase-based: Only applicable to global shutter camera for accurate displacement

Welcome any suggestion/discussion you guys have in mind that would allow me to determine sub-pixel motion for every pixel that applicable to rolling shutter camera! Thanks in advance!
You can use Farneback to estimate dense flow.  In OpenCV, the function call is calcOpticalFlowFarneback().

Please note that the initial flow is important and you can use one of OPTFLOW_FARNEBACK_GAUSSIAN or OPTFLOW_USE_INITIAL_FLOW. 

It depends on the depth of the pre calculation to estimate the segmentation before calculating the frame to frame flow.