Matrix multiplication in OpenCV

You have to be careful when multiplying matrices, as there are two possible meanings of multiply.

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;

Bitwise multiplication is where each pixel in the output matrix is formed by multiplying that pixel 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);