this is code i downloaded for local binary pattern, i cant able to find what is B1? its not a variable ? pls help to find what it is?

a1 =  I2;
[m1,n1] = size(a1);
for i = 2:m1-1
    for j = 2:n1-1
        b1 = a1(i-1:i+1,j-1:j+1);
        B1(i-1:i+1,j-1:j+1) = LBP(b1);
    end

Answer 1

[Star Strider]

‘B1’ is a variable. It is a (3x3) matrix that is set to the corresponding values of whatever ‘LBP(b1)’ is.