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import torch
import numpy as np
from . import metric
from .confusionmatrix import ConfusionMatrix
class IoU(metric.Metric):
"""Computes the intersection over union (IoU) per class and corresponding
mean (mIoU).
Intersection over union (IoU) is a common evaluation metric for semantic
segmentation. The predictions are first accumulated in a confusion matrix
and the IoU is computed from it as follows:
IoU = true_positive / (true_positive + false_positive + false_negative).
Keyword arguments:
- num_classes (int): number of classes in the classification problem
- normalized (boolean, optional): Determines whether or not the confusion
matrix is normalized or not. Default: False.
- ignore_index (int or iterable, optional): Index of the classes to ignore
when computing the IoU. Can be an int, or any iterable of ints.
"""
def __init__(self, num_classes, normalized=False, ignore_index=None):
super().__init__()
self.conf_metric = ConfusionMatrix(num_classes, normalized)
if ignore_index is None:
self.ignore_index = None
elif isinstance(ignore_index, int):
self.ignore_index = (ignore_index,)
else:
try:
self.ignore_index = tuple(ignore_index)
except TypeError:
raise ValueError("'ignore_index' must be an int or iterable")
def reset(self):
self.conf_metric.reset()
def add(self, predicted, target):
"""Adds the predicted and target pair to the IoU metric.
Keyword arguments:
- predicted (Tensor): Can be a (N, K, H, W) tensor of
predicted scores obtained from the model for N examples and K classes,
or (N, H, W) tensor of integer values between 0 and K-1.
- target (Tensor): Can be a (N, K, H, W) tensor of
target scores for N examples and K classes, or (N, H, W) tensor of
integer values between 0 and K-1.
"""
# Dimensions check
assert predicted.size(0) == target.size(0), \
'number of targets and predicted outputs do not match'
assert predicted.dim() == 3 or predicted.dim() == 4, \
"predictions must be of dimension (N, H, W) or (N, K, H, W)"
assert target.dim() == 3 or target.dim() == 4, \
"targets must be of dimension (N, H, W) or (N, K, H, W)"
# If the tensor is in categorical format convert it to integer format
if predicted.dim() == 4:
_, predicted = predicted.max(1)
if target.dim() == 4:
_, target = target.max(1)
self.conf_metric.add(predicted.view(-1), target.view(-1))
def value(self):
"""Computes the IoU and mean IoU.
The mean computation ignores NaN elements of the IoU array.
Returns:
Tuple: (IoU, mIoU). The first output is the per class IoU,
for K classes it's numpy.ndarray with K elements. The second output,
is the mean IoU.
"""
conf_matrix = self.conf_metric.value()
if self.ignore_index is not None:
for index in self.ignore_index:
conf_matrix[:, self.ignore_index] = 0
conf_matrix[self.ignore_index, :] = 0
true_positive = np.diag(conf_matrix)
false_positive = np.sum(conf_matrix, 0) - true_positive
false_negative = np.sum(conf_matrix, 1) - true_positive
# Just in case we get a division by 0, ignore/hide the error
with np.errstate(divide='ignore', invalid='ignore'):
iou = true_positive / (true_positive + false_positive + false_negative)
return iou, np.nanmean(iou) |