Flexible Automatic Motion Blending with Registration Curves
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This paper discusses how to do motion blending for several   
animations automatically. There are 2 really clever points in   
this paper.   
The first is the notice that any rotations around the "UP" axis,   
and any translations in the ground plane are really irrelevant,   
and can be ignored when blending. We can easily blend 2 walking   
motions  One of them walkig to the left and the other to the   
upper right corner.  It doesn't even matter if they are starting far apart.   
  
The second point is the notice that we can apply a Dynamic programming  
algorithm to find the optimal motion warping curve between 2 animations.  
This algorithm is the famous "Money Collector" algorithm, which is known to run  
in O(m*n) where m and n are the number of frames in the 2 animations respectively.  

The 2 points above are probably the most important points in blending 2 motions  
The paper discusses a third point, which is constraint matching. It basically says, we should  
take the intersection of constraints at any specific frame.  This makes sense, since when  
we have 2 motions one where both feet are on the ground, and the other with only 1 foot on  
the ground, the blended motion should also have just one foot on the ground.

With those 3 rules, we can now blend 2 motions automatically. The final clever point in the paper  
is how to select the global timing. It is selected in a way such that if we only pick one animation  
we advance at its pace.