# Equal contribution criteria during image pyramid construction

In their paper on Image Pyramids, Burt and Adelson state the following about pyramid generation.

An additional constraint is called equal contribution. This stipulates that all nodes at a particular level must contribute the same total weight($=\frac{1}{4}$) to nodes at the next higher level. Let $\hat{w}(0)=a$,$\hat{w}(-1)=\hat{w}(1)=b$,$\hat{w}(-2)=\hat{w}(2)=c$. In this case, equal contribution requires that $a+2c = 2b$.

How is this weight condition derived and why is the weight $\frac{1}{4}$? For a larger context of the question, the screenshot of the relevant section in the paper can be seen below