fast-reid/fastreid/layers/circle_softmax.py

# encoding: utf-8
"""
@author:  liaoxingyu
@contact: sherlockliao01@gmail.com
"""

import math

import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.nn import Parameter


class CircleSoftmax(nn.Module):
    def __init__(self, cfg, in_feat, num_classes):
        super().__init__()
        self.in_feat = in_feat
        self._num_classes = num_classes
        self.s = cfg.MODEL.HEADS.SCALE
        self.m = cfg.MODEL.HEADS.MARGIN

        self.weight = Parameter(torch.Tensor(num_classes, in_feat))
        nn.init.kaiming_uniform_(self.weight, a=math.sqrt(5))

    def forward(self, features, targets):
        sim_mat = F.linear(F.normalize(features), F.normalize(self.weight))
        alpha_p = torch.clamp_min(-sim_mat.detach() + 1 + self.m, min=0.)
        alpha_n = torch.clamp_min(sim_mat.detach() + self.m, min=0.)
        delta_p = 1 - self.m
        delta_n = self.m

        s_p = self.s * alpha_p * (sim_mat - delta_p)
        s_n = self.s * alpha_n * (sim_mat - delta_n)

        targets = F.one_hot(targets, num_classes=self._num_classes)

        pred_class_logits = targets * s_p + (1.0 - targets) * s_n

        return pred_class_logits

    def extra_repr(self):
        return 'in_features={}, num_classes={}, scale={}, margin={}'.format(
            self.in_feat, self._num_classes, self.s, self.m
        )