Oh, when I read your description of the changes in the PR, I hadn't fully understood what you meant you were doing with the masked image.  Now I see.  This is certainly a neat way to avoid having lots of ```if``` statements checking for masked pixels everywhere, but I wonder how the actual tradeoff works for the timing.  Is the idea that the ```if``` statements are sufficiently time-consuming that it's worth just adding zero to your sums a lot of the time rather than actually checking for the presence of a masked pixel?