Mobile Nets computational cost

When Andrew tried to compare the computational cost between the normal convolution and the mobile nets, for the mobile nets the cost for the depthwise step was (3 * 3) * (4 * 4 * 3).
I think I have a problem with that. Isn’t that the computation for just one slice and then we need to multiply again with 3 ?

Hi Muhammad-Elmallah,

As Andrew explains starting from 5:16 in the Mobilenet video, one filter is applied to one input channel only. The number of computations for a filter is (4x4)x(3x3) (16 positions times 3x3 computations). In total there are 3 filters, each of which is applied to one input channel only. So you end up with 3x(4x4)x(3x3) computations.

I hope this clarifies.