Source code for mmseg.models.losses.lovasz_loss

# Copyright (c) OpenMMLab. All rights reserved.
"""Modified from https://github.com/bermanmaxim/LovaszSoftmax/blob/master/pytor
ch/lovasz_losses.py Lovasz-Softmax and Jaccard hinge loss in PyTorch Maxim
Berman 2018 ESAT-PSI KU Leuven (MIT License)"""

import mmcv
import torch
import torch.nn as nn
import torch.nn.functional as F

from ..builder import LOSSES
from .utils import get_class_weight, weight_reduce_loss


def lovasz_grad(gt_sorted):
    """Computes gradient of the Lovasz extension w.r.t sorted errors.

    See Alg. 1 in paper.
    """
    p = len(gt_sorted)
    gts = gt_sorted.sum()
    intersection = gts - gt_sorted.float().cumsum(0)
    union = gts + (1 - gt_sorted).float().cumsum(0)
    jaccard = 1. - intersection / union
    if p > 1:  # cover 1-pixel case
        jaccard[1:p] = jaccard[1:p] - jaccard[0:-1]
    return jaccard


def flatten_binary_logits(logits, labels, ignore_index=None):
    """Flattens predictions in the batch (binary case) Remove labels equal to
    'ignore_index'."""
    logits = logits.view(-1)
    labels = labels.view(-1)
    if ignore_index is None:
        return logits, labels
    valid = (labels != ignore_index)
    vlogits = logits[valid]
    vlabels = labels[valid]
    return vlogits, vlabels


def flatten_probs(probs, labels, ignore_index=None):
    """Flattens predictions in the batch."""
    if probs.dim() == 3:
        # assumes output of a sigmoid layer
        B, H, W = probs.size()
        probs = probs.view(B, 1, H, W)
    B, C, H, W = probs.size()
    probs = probs.permute(0, 2, 3, 1).contiguous().view(-1, C)  # B*H*W, C=P,C
    labels = labels.view(-1)
    if ignore_index is None:
        return probs, labels
    valid = (labels != ignore_index)
    vprobs = probs[valid.nonzero().squeeze()]
    vlabels = labels[valid]
    return vprobs, vlabels


def lovasz_hinge_flat(logits, labels):
    """Binary Lovasz hinge loss.

    Args:
        logits (torch.Tensor): [P], logits at each prediction
            (between -infty and +infty).
        labels (torch.Tensor): [P], binary ground truth labels (0 or 1).

    Returns:
        torch.Tensor: The calculated loss.
    """
    if len(labels) == 0:
        # only void pixels, the gradients should be 0
        return logits.sum() * 0.
    signs = 2. * labels.float() - 1.
    errors = (1. - logits * signs)
    errors_sorted, perm = torch.sort(errors, dim=0, descending=True)
    perm = perm.data
    gt_sorted = labels[perm]
    grad = lovasz_grad(gt_sorted)
    loss = torch.dot(F.relu(errors_sorted), grad)
    return loss


def lovasz_hinge(logits,
                 labels,
                 classes='present',
                 per_image=False,
                 class_weight=None,
                 reduction='mean',
                 avg_factor=None,
                 ignore_index=255):
    """Binary Lovasz hinge loss.

    Args:
        logits (torch.Tensor): [B, H, W], logits at each pixel
            (between -infty and +infty).
        labels (torch.Tensor): [B, H, W], binary ground truth masks (0 or 1).
        classes (str | list[int], optional): Placeholder, to be consistent with
            other loss. Default: None.
        per_image (bool, optional): If per_image is True, compute the loss per
            image instead of per batch. Default: False.
        class_weight (list[float], optional): Placeholder, to be consistent
            with other loss. Default: None.
        reduction (str, optional): The method used to reduce the loss. Options
            are "none", "mean" and "sum". This parameter only works when
            per_image is True. Default: 'mean'.
        avg_factor (int, optional): Average factor that is used to average
            the loss. This parameter only works when per_image is True.
            Default: None.
        ignore_index (int | None): The label index to be ignored. Default: 255.

    Returns:
        torch.Tensor: The calculated loss.
    """
    if per_image:
        loss = [
            lovasz_hinge_flat(*flatten_binary_logits(
                logit.unsqueeze(0), label.unsqueeze(0), ignore_index))
            for logit, label in zip(logits, labels)
        ]
        loss = weight_reduce_loss(
            torch.stack(loss), None, reduction, avg_factor)
    else:
        loss = lovasz_hinge_flat(
            *flatten_binary_logits(logits, labels, ignore_index))
    return loss


def lovasz_softmax_flat(probs, labels, classes='present', class_weight=None):
    """Multi-class Lovasz-Softmax loss.

    Args:
        probs (torch.Tensor): [P, C], class probabilities at each prediction
            (between 0 and 1).
        labels (torch.Tensor): [P], ground truth labels (between 0 and C - 1).
        classes (str | list[int], optional): Classes chosen to calculate loss.
            'all' for all classes, 'present' for classes present in labels, or
            a list of classes to average. Default: 'present'.
        class_weight (list[float], optional): The weight for each class.
            Default: None.

    Returns:
        torch.Tensor: The calculated loss.
    """
    if probs.numel() == 0:
        # only void pixels, the gradients should be 0
        return probs * 0.
    C = probs.size(1)
    losses = []
    class_to_sum = list(range(C)) if classes in ['all', 'present'] else classes
    for c in class_to_sum:
        fg = (labels == c).float()  # foreground for class c
        if (classes == 'present' and fg.sum() == 0):
            continue
        if C == 1:
            if len(classes) > 1:
                raise ValueError('Sigmoid output possible only with 1 class')
            class_pred = probs[:, 0]
        else:
            class_pred = probs[:, c]
        errors = (fg - class_pred).abs()
        errors_sorted, perm = torch.sort(errors, 0, descending=True)
        perm = perm.data
        fg_sorted = fg[perm]
        loss = torch.dot(errors_sorted, lovasz_grad(fg_sorted))
        if class_weight is not None:
            loss *= class_weight[c]
        losses.append(loss)
    return torch.stack(losses).mean()


def lovasz_softmax(probs,
                   labels,
                   classes='present',
                   per_image=False,
                   class_weight=None,
                   reduction='mean',
                   avg_factor=None,
                   ignore_index=255):
    """Multi-class Lovasz-Softmax loss.

    Args:
        probs (torch.Tensor): [B, C, H, W], class probabilities at each
            prediction (between 0 and 1).
        labels (torch.Tensor): [B, H, W], ground truth labels (between 0 and
            C - 1).
        classes (str | list[int], optional): Classes chosen to calculate loss.
            'all' for all classes, 'present' for classes present in labels, or
            a list of classes to average. Default: 'present'.
        per_image (bool, optional): If per_image is True, compute the loss per
            image instead of per batch. Default: False.
        class_weight (list[float], optional): The weight for each class.
            Default: None.
        reduction (str, optional): The method used to reduce the loss. Options
            are "none", "mean" and "sum". This parameter only works when
            per_image is True. Default: 'mean'.
        avg_factor (int, optional): Average factor that is used to average
            the loss. This parameter only works when per_image is True.
            Default: None.
        ignore_index (int | None): The label index to be ignored. Default: 255.

    Returns:
        torch.Tensor: The calculated loss.
    """

    if per_image:
        loss = [
            lovasz_softmax_flat(
                *flatten_probs(
                    prob.unsqueeze(0), label.unsqueeze(0), ignore_index),
                classes=classes,
                class_weight=class_weight)
            for prob, label in zip(probs, labels)
        ]
        loss = weight_reduce_loss(
            torch.stack(loss), None, reduction, avg_factor)
    else:
        loss = lovasz_softmax_flat(
            *flatten_probs(probs, labels, ignore_index),
            classes=classes,
            class_weight=class_weight)
    return loss


@LOSSES.register_module()
class LovaszLoss(nn.Module):
    """LovaszLoss.

    This loss is proposed in `The Lovasz-Softmax loss: A tractable surrogate
    for the optimization of the intersection-over-union measure in neural
    networks <https://arxiv.org/abs/1705.08790>`_.

    Args:
        loss_type (str, optional): Binary or multi-class loss.
            Default: 'multi_class'. Options are "binary" and "multi_class".
        classes (str | list[int], optional): Classes chosen to calculate loss.
            'all' for all classes, 'present' for classes present in labels, or
            a list of classes to average. Default: 'present'.
        per_image (bool, optional): If per_image is True, compute the loss per
            image instead of per batch. Default: False.
        reduction (str, optional): The method used to reduce the loss. Options
            are "none", "mean" and "sum". This parameter only works when
            per_image is True. Default: 'mean'.
        class_weight (list[float] | str, optional): Weight of each class. If in
            str format, read them from a file. Defaults to None.
        loss_weight (float, optional): Weight of the loss. Defaults to 1.0.
        loss_name (str, optional): Name of the loss item. If you want this loss
            item to be included into the backward graph, `loss_` must be the
            prefix of the name. Defaults to 'loss_lovasz'.
    """

    def __init__(self,
                 loss_type='multi_class',
                 classes='present',
                 per_image=False,
                 reduction='mean',
                 class_weight=None,
                 loss_weight=1.0,
                 loss_name='loss_lovasz'):
        super(LovaszLoss, self).__init__()
        assert loss_type in ('binary', 'multi_class'), "loss_type should be \
                                                    'binary' or 'multi_class'."

        if loss_type == 'binary':
            self.cls_criterion = lovasz_hinge
        else:
            self.cls_criterion = lovasz_softmax
        assert classes in ('all', 'present') or mmcv.is_list_of(classes, int)
        if not per_image:
            assert reduction == 'none', "reduction should be 'none' when \
                                                        per_image is False."

        self.classes = classes
        self.per_image = per_image
        self.reduction = reduction
        self.loss_weight = loss_weight
        self.class_weight = get_class_weight(class_weight)
        self._loss_name = loss_name

    def forward(self,
                cls_score,
                label,
                weight=None,
                avg_factor=None,
                reduction_override=None,
                **kwargs):
        """Forward function."""
        assert reduction_override in (None, 'none', 'mean', 'sum')
        reduction = (
            reduction_override if reduction_override else self.reduction)
        if self.class_weight is not None:
            class_weight = cls_score.new_tensor(self.class_weight)
        else:
            class_weight = None

        # if multi-class loss, transform logits to probs
        if self.cls_criterion == lovasz_softmax:
            cls_score = F.softmax(cls_score, dim=1)

        loss_cls = self.loss_weight * self.cls_criterion(
            cls_score,
            label,
            self.classes,
            self.per_image,
            class_weight=class_weight,
            reduction=reduction,
            avg_factor=avg_factor,
            **kwargs)
        return loss_cls

    @property
    def loss_name(self):
        """Loss Name.

        This function must be implemented and will return the name of this
        loss function. This name will be used to combine different loss items
        by simple sum operation. In addition, if you want this loss item to be
        included into the backward graph, `loss_` must be the prefix of the
        name.
        Returns:
            str: The name of this loss item.
        """
        return self._loss_name