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There are two possible cases of multiply for matrices .

Matrix multiplication is where two matrices are multiplied directly. This operation multiplies matrix A of size [a x b] with matrix B of size [b x c] to produce matrix C of size [a x c]. In OpenCV it is achieved using the simple * operator:

C = A * B

Element-wise multiplication is where each pixel (element) in the output matrix is formed by multiplying that pixel (element) in matrix A by its corresponding entry in matrix B. The input matrices should be the same size, and the output will be the same size as well. This is achieved using the mul() function:

output = A.mul(B);