Applying FocalLoss in fastai

[Daniel.R.Armstrong (Daniel Armstrong)]

That is really cool, I wanted to know how you can use pytorch functions. Are you thinking there is any downside in using pytorch functions?

I couldn’t it as a pre built function in v3, but i re-watched lesson 9 and it seem(I haven’t tried it) like all you would have to do is define it and use it as the loss function. Here is the part of the video he talked about focal loss.

the lecture 9 notes also talk from timlee also talk about focal loss.

@timlee

Focal Loss

class FocalLoss(BCE_Loss):
    def get_weight(self,x,t):
        alpha,gamma = 0.25,2.
        p = x.sigmoid()
        pt = p*t + (1-p)*(1-t)
        w = alpha*t + (1-alpha)*(1-t)
        return w * (1-pt).pow(gamma)

loss_f = FocalLoss(len(id2cat))