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## Custom loss functions and metrics.
## Metrics taken from https://github.com/keras-team/keras/blob/53e541f7bf55de036f4f5641bd2947b96dd8c4c3/keras/metrics.py
from keras import backend as K
smooth = 1.
def dice_coef(y_true, y_pred):
y_true_f = K.flatten(y_true)
y_pred_f = K.flatten(y_pred)
intersection = K.sum(y_true_f * y_pred_f)
return (2. * intersection + smooth) / (K.sum(y_true_f) + K.sum(y_pred_f) + smooth)
def dice_coef_loss(y_true, y_pred):
return -dice_coef(y_true, y_pred)
def precision(y_true, y_pred):
"""Precision metric.
Only computes a batch-wise average of precision.
Computes the precision, a metric for multi-label classification of
how many selected items are relevant.
"""
true_positives = K.sum(K.round(K.clip(y_true * y_pred, 0, 1)))
predicted_positives = K.sum(K.round(K.clip(y_pred, 0, 1)))
precision = true_positives / (predicted_positives + K.epsilon())
return precision
def recall(y_true, y_pred):
"""Recall metric.
Only computes a batch-wise average of recall.
Computes the recall, a metric for multi-label classification of
how many relevant items are selected.
"""
true_positives = K.sum(K.round(K.clip(y_true * y_pred, 0, 1)))
possible_positives = K.sum(K.round(K.clip(y_true, 0, 1)))
recall = true_positives / (possible_positives + K.epsilon())
return recall
def fbeta_score(y_true, y_pred, beta=1):
"""Computes the F score.
The F score is the weighted harmonic mean of precision and recall.
Here it is only computed as a batch-wise average, not globally.
This is useful for multi-label classification, where input samples can be
classified as sets of labels. By only using accuracy (precision) a model
would achieve a perfect score by simply assigning every class to every
input. In order to avoid this, a metric should penalize incorrect class
assignments as well (recall). The F-beta score (ranged from 0.0 to 1.0)
computes this, as a weighted mean of the proportion of correct class
assignments vs. the proportion of incorrect class assignments.
With beta = 1, this is equivalent to a F-measure. With beta < 1, assigning
correct classes becomes more important, and with beta > 1 the metric is
instead weighted towards penalizing incorrect class assignments.
"""
if beta < 0:
raise ValueError('The lowest choosable beta is zero (only precision).')
# If there are no true positives, fix the F score at 0 like sklearn.
if K.sum(K.round(K.clip(y_true, 0, 1))) == 0:
return 0
p = precision(y_true, y_pred)
r = recall(y_true, y_pred)
bb = beta ** 2
fbeta_score = (1 + bb) * (p * r) / (bb * p + r + K.epsilon())
return fbeta_score
def fmeasure(y_true, y_pred):
"""Computes the f-measure, the harmonic mean of precision and recall.
Here it is only computed as a batch-wise average, not globally.
"""
return fbeta_score(y_true, y_pred, beta=1)
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