Source code for common.vision.models.reid.loss
"""
Modified from https://github.com/yxgeee/MMT
@author: Baixu Chen
@contact: cbx_99_hasta@outlook.com
"""
import torch
import torch.nn as nn
import torch.nn.functional as F
def pairwise_euclidean_distance(x, y):
"""Compute pairwise euclidean distance between two sets of features"""
m, n = x.size(0), y.size(0)
dist_mat = torch.pow(x, 2).sum(1, keepdim=True).expand(m, n) + \
torch.pow(y, 2).sum(1, keepdim=True).expand(n, m).t() \
- 2 * torch.matmul(x, y.t())
# for numerical stability
dist_mat = dist_mat.clamp(min=1e-12).sqrt()
return dist_mat
def hard_examples_mining(dist_mat, identity_mat, return_idxes=False):
r"""Select hard positives and hard negatives according to `In defense of the Triplet Loss for Person
Re-Identification (ICCV 2017) <https://arxiv.org/pdf/1703.07737v2.pdf>`_
Args:
dist_mat (tensor): pairwise distance matrix between two sets of features
identity_mat (tensor): a matrix of shape :math:`(N, M)`. If two images :math:`P[i]` of set :math:`P` and
:math:`Q[j]` of set :math:`Q` come from the same person, then :math:`identity\_mat[i, j] = 1`,
otherwise :math:`identity\_mat[i, j] = 0`
return_idxes (bool, optional): if True, also return indexes of hard examples. Default: False
"""
# the implementation here is a little tricky, dist_mat contains pairwise distance between probe image and other
# images in current mini-batch. As we want to select positive examples of the same person, we add a constant
# negative offset on other images before sorting. As a result, images of the **same** person will rank first.
sorted_dist_mat, sorted_idxes = torch.sort(dist_mat + (-1e7) * (1 - identity_mat), dim=1,
descending=True)
dist_ap = sorted_dist_mat[:, 0]
hard_positive_idxes = sorted_idxes[:, 0]
# the implementation here is similar to above code, we add a constant positive offset on images of same person
# before sorting. Besides, we sort in ascending order. As a result, images of **different** persons will rank first.
sorted_dist_mat, sorted_idxes = torch.sort(dist_mat + 1e7 * identity_mat, dim=1,
descending=False)
dist_an = sorted_dist_mat[:, 0]
hard_negative_idxes = sorted_idxes[:, 0]
if return_idxes:
return dist_ap, dist_an, hard_positive_idxes, hard_negative_idxes
return dist_ap, dist_an
[docs]class CrossEntropyLossWithLabelSmooth(nn.Module):
r"""Cross entropy loss with label smooth from `Rethinking the Inception Architecture for Computer Vision
(CVPR 2016) <https://arxiv.org/pdf/1512.00567.pdf>`_.
Given one-hot labels :math:`labels \in R^C`, where :math:`C` is the number of classes,
smoothed labels are calculated as
.. math::
smoothed\_labels = (1 - \epsilon) \times labels + \epsilon \times \frac{1}{C}
We use smoothed labels when calculating cross entropy loss and this can be helpful for preventing over-fitting.
Args:
num_classes (int): number of classes.
epsilon (float): a float number that controls the smoothness.
Inputs:
- y (tensor): unnormalized classifier predictions, :math:`y`
- labels (tensor): ground truth labels, :math:`labels`
Shape:
- y: :math:`(minibatch, C)`, where :math:`C` is the number of classes
- labels: :math:`(minibatch, )`
"""
def __init__(self, num_classes, epsilon=0.1):
super(CrossEntropyLossWithLabelSmooth, self).__init__()
self.num_classes = num_classes
self.epsilon = epsilon
self.log_softmax = nn.LogSoftmax(dim=1).cuda()
def forward(self, y, labels):
log_prob = self.log_softmax(y)
labels = torch.zeros_like(log_prob).scatter_(1, labels.unsqueeze(1), 1)
labels = (1 - self.epsilon) * labels + self.epsilon / self.num_classes
loss = (- labels * log_prob).mean(0).sum()
return loss
[docs]class TripletLoss(nn.Module):
"""Triplet loss augmented with batch hard from `In defense of the Triplet Loss for Person Re-Identification
(ICCV 2017) <https://arxiv.org/pdf/1703.07737v2.pdf>`_.
Args:
margin (float): margin of triplet loss
normalize_feature (bool, optional): if True, normalize features into unit norm first before computing loss.
Default: False.
"""
def __init__(self, margin, normalize_feature=False):
super(TripletLoss, self).__init__()
self.margin = margin
self.normalize_feature = normalize_feature
self.margin_loss = nn.MarginRankingLoss(margin=margin).cuda()
def forward(self, f, labels):
if self.normalize_feature:
# equivalent to cosine similarity
f = F.normalize(f)
dist_mat = pairwise_euclidean_distance(f, f)
n = dist_mat.size(0)
identity_mat = labels.expand(n, n).eq(labels.expand(n, n).t()).float()
dist_ap, dist_an = hard_examples_mining(dist_mat, identity_mat)
y = torch.ones_like(dist_ap)
loss = self.margin_loss(dist_an, dist_ap, y)
return loss
class TripletLossXBM(nn.Module):
r"""Triplet loss augmented with batch hard from `In defense of the Triplet Loss for Person Re-Identification
(ICCV 2017) <https://arxiv.org/pdf/1703.07737v2.pdf>`_. The only difference from triplet loss lies in that
both features from current mini batch and external storage (XBM) are involved.
Args:
margin (float, optional): margin of triplet loss. Default: 0.3
normalize_feature (bool, optional): if True, normalize features into unit norm first before computing loss.
Default: False
Inputs:
- f (tensor): features of current mini batch, :math:`f`
- labels (tensor): identity labels for current mini batch, :math:`labels`
- xbm_f (tensor): features collected from XBM, :math:`xbm\_f`
- xbm_labels (tensor): corresponding identity labels of xbm_f, :math:`xbm\_labels`
Shape:
- f: :math:`(minibatch, F)`, where :math:`F` is the feature dimension
- labels: :math:`(minibatch, )`
- xbm_f: :math:`(minibatch, F)`
- xbm_labels: :math:`(minibatch, )`
"""
def __init__(self, margin=0.3, normalize_feature=False):
super(TripletLossXBM, self).__init__()
self.margin = margin
self.normalize_feature = normalize_feature
self.ranking_loss = nn.MarginRankingLoss(margin=margin)
def forward(self, f, labels, xbm_f, xbm_labels):
if self.normalize_feature:
# equivalent to cosine similarity
f = F.normalize(f)
xbm_f = F.normalize(xbm_f)
dist_mat = pairwise_euclidean_distance(f, xbm_f)
# hard examples mining
n, m = f.size(0), xbm_f.size(0)
identity_mat = labels.expand(m, n).t().eq(xbm_labels.expand(n, m)).float()
dist_ap, dist_an = hard_examples_mining(dist_mat, identity_mat)
# Compute ranking hinge loss
y = torch.ones_like(dist_an)
loss = self.ranking_loss(dist_an, dist_ap, y)
return loss
[docs]class SoftTripletLoss(nn.Module):
r"""Soft triplet loss from `Mutual Mean-Teaching: Pseudo Label Refinery for Unsupervised
Domain Adaptation on Person Re-identification (ICLR 2020) <https://arxiv.org/pdf/2001.01526.pdf>`_.
Consider a triplet :math:`x,x_p,x_n` (anchor, positive, negative), corresponding features are :math:`f,f_p,f_n`.
We optimize for a smaller distance between :math:`f` and :math:`f_p` and a larger distance
between :math:`f` and :math:`f_n`. Inner product is adopted as their similarity measure, soft triplet loss is thus
defined as
.. math::
loss = \mathcal{L}_{\text{bce}}(\frac{\text{exp}(f^Tf_p)}{\text{exp}(f^Tf_p)+\text{exp}(f^Tf_n)}, 1)
where :math:`\mathcal{L}_{\text{bce}}` means binary cross entropy loss. We denote the first term in above loss function
as :math:`T`. When features from another teacher network can be obtained, we can calculate :math:`T_{teacher}` as
labels, resulting in the following soft version
.. math::
loss = \mathcal{L}_{\text{bce}}(T, T_{teacher})
Args:
margin (float, optional): margin of triplet loss. If None, soft labels from another network will be adopted when
computing loss. Default: None.
normalize_feature (bool, optional): if True, normalize features into unit norm first before computing loss.
Default: False.
"""
def __init__(self, margin=None, normalize_feature=False):
super(SoftTripletLoss, self).__init__()
self.margin = margin
self.normalize_feature = normalize_feature
def forward(self, features_1, features_2, labels):
if self.normalize_feature:
# equal to cosine similarity
features_1 = F.normalize(features_1)
features_2 = F.normalize(features_2)
dist_mat = pairwise_euclidean_distance(features_1, features_1)
assert dist_mat.size(0) == dist_mat.size(1)
n = dist_mat.size(0)
identity_mat = labels.expand(n, n).eq(labels.expand(n, n).t()).float()
dist_ap, dist_an, ap_idxes, an_idxes = hard_examples_mining(dist_mat, identity_mat, return_idxes=True)
assert dist_an.size(0) == dist_ap.size(0)
triple_dist = torch.stack((dist_ap, dist_an), dim=1)
triple_dist = F.log_softmax(triple_dist, dim=1)
if self.margin is not None:
loss = (- self.margin * triple_dist[:, 0] - (1 - self.margin) * triple_dist[:, 1]).mean()
return loss
dist_mat_ref = pairwise_euclidean_distance(features_2, features_2)
dist_ap_ref = torch.gather(dist_mat_ref, 1, ap_idxes.view(n, 1).expand(n, n))[:, 0]
dist_an_ref = torch.gather(dist_mat_ref, 1, an_idxes.view(n, 1).expand(n, n))[:, 0]
triple_dist_ref = torch.stack((dist_ap_ref, dist_an_ref), dim=1)
triple_dist_ref = F.softmax(triple_dist_ref, dim=1).detach()
loss = (- triple_dist_ref * triple_dist).sum(dim=1).mean()
return loss
[docs]class CrossEntropyLoss(nn.Module):
r"""We use :math:`C` to denote the number of classes, :math:`N` to denote mini-batch
size, this criterion expects unnormalized predictions :math:`y\_{logits}` of shape :math:`(N, C)` and
:math:`target\_{logits}` of the same shape :math:`(N, C)`. Then we first normalize them into
probability distributions among classes
.. math::
y = \text{softmax}(y\_{logits})
.. math::
target = \text{softmax}(target\_{logits})
Final objective is calculated as
.. math::
\text{loss} = \frac{1}{N} \sum_{i=1}^{N} \sum_{j=1}^C -target_i^j \times \text{log} (y_i^j)
"""
def __init__(self):
super(CrossEntropyLoss, self).__init__()
self.log_softmax = nn.LogSoftmax(dim=1).cuda()
def forward(self, y, labels):
log_prob = self.log_softmax(y)
loss = (- F.softmax(labels, dim=1).detach() * log_prob).sum(dim=1).mean()
return loss
