CMB polarization signal may be decomposed into gradient-like ( E ) and curl-like ( B ) mode .
We have investigated E/B decomposition in pixel space .
We find E/B mixing due to incomplete sky is localized in pixel-space , and negligible in the regions far away from the masked area .
By estimating the expected local leakage power , we have diagnosed ambiguous pixels .
Our criteria for ambiguous pixels ( i.e . r _ { c } ) is associated with the tensor-to-scalar ratio of B mode power spectrum , which the leakage power is comparable to .
By setting r _ { c } to a lower value , we may reduce leakage level , but reduce sky fraction at the same time .
Therefore , we have solved \partial \Delta C _ { l } / \partial r _ { c } = 0 , and obtained the optimal r _ { c } , which minimizes the estimation uncertainty , given a foreground mask and noise level .
We have applied our method to a simulated map blocked by a foreground ( diffuse + point source ) mask .
Our simulation shows leakage power is smaller than primordial ( i.e .
unlensed ) B mode power spectrum of tensor-to-scalar ratio r \sim 1 \times 10 ^ { -3 } at wide range of multipoles ( 50 \lesssim l \lesssim 2000 ) , while allowing us to retain sky fraction \sim 0.48 .