tf.keras.losses.CategoricalFocalCrossentropy

Computes the alpha balanced focal crossentropy loss.

Inherits From: Loss

tf.keras.losses.CategoricalFocalCrossentropy(
    alpha=0.25,
    gamma=2.0,
    from_logits=False,
    label_smoothing=0.0,
    axis=-1,
    reduction='sum_over_batch_size',
    name='categorical_focal_crossentropy'
)

Use this crossentropy loss function when there are two or more label classes and if you want to handle class imbalance without using class_weights. We expect labels to be provided in a one_hot representation.

According to Lin et al., 2018, it helps to apply a focal factor to down-weight easy examples and focus more on hard examples. The general formula for the focal loss (FL) is as follows:

FL(p_t) = (1 - p_t) ** gamma * log(p_t)

where p_t is defined as follows: p_t = output if y_true == 1, else 1 - output

(1 - p_t) ** gamma is the modulating_factor, where gamma is a focusing parameter. When gamma = 0, there is no focal effect on the cross entropy. gamma reduces the importance given to simple examples in a smooth manner.

The authors use alpha-balanced variant of focal loss (FL) in the paper: FL(p_t) = -alpha * (1 - p_t) ** gamma * log(p_t)

where alpha is the weight factor for the classes. If alpha = 1, the loss won't be able to handle class imbalance properly as all classes will have the same weight. This can be a constant or a list of constants. If alpha is a list, it must have the same length as the number of classes.

The formula above can be generalized to: FL(p_t) = alpha * (1 - p_t) ** gamma * CrossEntropy(y_true, y_pred)

where minus comes from CrossEntropy(y_true, y_pred) (CE).

Extending this to multi-class case is straightforward: FL(p_t) = alpha * (1 - p_t) ** gamma * CategoricalCE(y_true, y_pred)

In the snippet below, there is num_classes floating pointing values per example. The shape of both y_pred and y_true are (batch_size, num_classes).

alpha A weight balancing factor for all classes, default is 0.25 as mentioned in the reference. It can be a list of floats or a scalar. In the multi-class case, alpha may be set by inverse class frequency by using compute_class_weight from sklearn.utils.
gamma A focusing parameter, default is 2.0 as mentioned in the reference. It helps to gradually reduce the importance given to simple (easy) examples in a smooth manner.
from_logits Whether output is expected to be a logits tensor. By default, we consider that output encodes a probability distribution.
label_smoothing Float in [0, 1]. When > 0, label values are smoothed, meaning the confidence on label values are relaxed. For example, if 0.1, use 0.1 / num_classes for non-target labels and 0.9 + 0.1 / num_classes for target labels.
axis The axis along which to compute crossentropy (the features axis). Defaults to -1.
reduction Type of reduction to apply to the loss. In almost all cases this should be "sum_over_batch_size". Supported options are "sum", "sum_over_batch_size" or None.
name Optional name for the loss instance.

Examples:

Standalone usage:

y_true = [[0., 1., 0.], [0., 0., 1.]]
y_pred = [[0.05, 0.95, 0], [0.1, 0.8, 0.1]]
# Using 'auto'/'sum_over_batch_size' reduction type.
cce = keras.losses.CategoricalFocalCrossentropy()
cce(y_true, y_pred)
0.23315276
 
# Calling with 'sample_weight'.
cce(y_true, y_pred, sample_weight=np.array([0.3, 0.7]))
0.1632
 
# Using 'sum' reduction type.
cce = keras.losses.CategoricalFocalCrossentropy(
    reduction="sum")
cce(y_true, y_pred)
0.46631
 
# Using 'none' reduction type.
cce = keras.losses.CategoricalFocalCrossentropy(
    reduction=None)
cce(y_true, y_pred)
array([3.2058331e-05, 4.6627346e-01], dtype=float32)

Usage with the compile() API:

model.compile(optimizer='adam',
              loss=keras.losses.CategoricalFocalCrossentropy())

Methods

call

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call(
    y_true, y_pred
)

from_config

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@classmethod
from_config(
    config
)

get_config

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get_config()

__call__

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__call__(
    y_true, y_pred, sample_weight=None
)

Call self as a function.