losses.py

import torch
import math

class ArcFace(torch.nn.Module):
    """ ArcFace (https://arxiv.org/pdf/1801.07698v1.pdf):
    """
    def __init__(self, s=64.0, margin=0.5):
        super(ArcFace, self).__init__()
        self.scale = s
        self.cos_m = math.cos(margin)
        self.sin_m = math.sin(margin)
        self.theta = math.cos(math.pi - margin)
        self.sinmm = math.sin(math.pi - margin) * margin
        self.easy_margin = False


    def forward(self, logits: torch.Tensor, labels: torch.Tensor):
        index = torch.where(labels != -1)[0]
        target_logit = logits[index, labels[index].view(-1)]

        sin_theta = torch.sqrt(1.0 - torch.pow(target_logit, 2))
        cos_theta_m = target_logit * self.cos_m - sin_theta * self.sin_m  # cos(target+margin)
        if self.easy_margin:
            final_target_logit = torch.where(
                target_logit > 0, cos_theta_m, target_logit)
        else:
            final_target_logit = torch.where(
                target_logit > self.theta, cos_theta_m, target_logit - self.sinmm)

        logits[index, labels[index].view(-1)] = final_target_logit
        logits = logits * self.scale
        return logits


class CosFace(torch.nn.Module):
    def __init__(self, s=64.0, m=0.40):
        super(CosFace, self).__init__()
        self.s = s
        self.m = m

    def forward(self, logits: torch.Tensor, labels: torch.Tensor):
        index = torch.where(labels != -1)[0]
        target_logit = logits[index, labels[index].view(-1)]
        final_target_logit = target_logit - self.m
        logits[index, labels[index].view(-1)] = final_target_logit
        logits = logits * self.s
        return logits