Question

I know I can pass all the element of the matrix and do that.

but maybe there is another option.

I want to do:

mat1 = mat2 * mat1;

each element in mat1 that become to negative or bigger than 255, I want to leave the previous element.

for example (the result is not true, this is only for the example):

mat1 = [10  25  12
        33  7   163
        232 13  77]

mat2 = [-1  2  -3
         4 -5   6
         -7 -8  9]

asume mat1 = mat2 * mat1 gives:

mat1 = [-77.32  59    298
        0       -33   12
        -600     256   80]

so I want to repair mat1 to be:

mat1 = [10   59  12
        0    7   12
        232  13  80]

Answer 1

If i understand you correctly, you have a matrix multiplication

mat2 * mat1

Best is to store the matrix multiplication result in a third matrix

mat3 = mat2 * mat1

With

idx = (mat3 < 0 | mat3 > 255)

you a matrix with ones where you want to replace and zero where not. Apply this index to replace the values in mat3 with the values of mat1

mat3(idx) = mat1(idx)

The result is a matrix in which every element smaller than 0 or bigger than 255 has been replaced with the value of mat1.