Hacker News Reader

Comment by fchollet | Hacker News Reader

> I'm sure we'll see the Circular Harmonic Transform for rotation invariant (up to a point) Deep Nets

A fairly easy way to introduce rotation invariance in DCNNS is to perform random rotations on the inputs during training. Likewise for scale invariance. Translation invariance is already introduced by the convolution operation itself.

The thing about deep learning, is that transform kernels are learned, not pre-computed as in classical signal processing. A DCNN will learn whatever convolution kernels it needs to perform the task at hand. I wouldn't be surprised if a DCNN trained on a classical signal processing task ended up rediscovering some well-known transform kernels originally derived from physical first principles...

View on HN

> A fairly easy way to introduce rotation invariance in DCNNS is to perform random rotations on the inputs during training. Likewise for scale invariance.

It is a bit silly to call these invariances, as different filter/kernel combinations will be activated when a rotated or scaled input is encountered, the individual filters are not rotation or scale invariant. The entire network can only deal with rotations and scales it encountered during training, whilst having to learn 'redundant features' to a certain extent.

It will get the job done for many tasks, but it's a brute force sort of approach that will complicate the learning process (i.e. more scales and rotations require more filters, thus needing a more complex network that is harder to train).

I think there's definitely a lot that can be learnt from (classical) signal processing in order to come up with a much more elegant and efficient solution.

> A fairly easy way to introduce rotation invariance in DCNNS is to perform random rotations on the inputs during training. Likewise for scale invariance. Translation invariance is already introduced by the convolution operation itself.

Just to be clear (and I'm sorry if I'm being pedantic), you're talking about invariance of two separate things. In the first case, you're talking about the invariance of the overall network, F(x), i.e. if R is a rotation operator, F(Rx) = F(x). The network's prediction does not change for a suitable set of R's.

On the other hand, convolution is a shift invariant operator, meaning it acts the same no matter its location. If Cx is the output of a convolutional layer and Sx is a shifted signal, then C(Sx) = S(Cx). This is not shift invariance of the output.

The shift invariance of the operator means the convolutional will detect features that resonate well with its kernel irrespective of the location of their location in the signal. However, this does not automatically guarantee that the network's prediction will be shift invariant, i.e. F(Sx) = F(x).

Reply on news.ycombinator.com