fix mu bug

this is only important when the local image is not locally flat and the mu vector is not near zero (though, it usually is very close to zero). However, this was a clear bug that caused poor handling in regions with gradients across the local image.

src/perstar.jl

@@ -265,7 +265,7 @@ function build_cov!(cov::Array{T,2},μ::Array{T,1},cx::Int,cy::Int,bimage::Array
                 @inbounds cov[i,j] = t
                 @inbounds cov[j,i] = t
                 if i == j
-                    @inbounds μ[i] = μ1μ2
+                    @inbounds μ[i] = sqrt(μ1μ2)
                 end
             end
         end