What motion estimation techniques that could estimate the motion of every pixel?
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.