使用具有圆形掩码的局部二进制模式

In order to extract texture-based features; LBPs (Local Binary Patterns) are used in many background-foreground detection algorithms. It is easy to understand why those features are tolerant against the illumination. However; there is one of their structural properties that i could not comprehend. What is the exact reason for using a "circular mask" rather than a "rectangular one", when we compute the pattern? For instance, In order to extract texture-based features; LBP