python – Tensorflow数据集API中的过采样功能

我想问一下,当前的数据集API是否允许实现过采样算法？我处理高度不平衡的阶级问题.我当时认为在数据集解析过程中对特定类进行过采样会很好,即在线生成.我已经看到了rejection_resample函数的实现,但是这会删除样本而不是复制它们,并且它减慢了批处理生成(当目标分布与初始分布大不相同时).我想要实现的是：举一个例子,看看它的类概率决定是否复制它.然后调用dataset.shuffle(…)dataset.batch(…)并获取迭代器.最好的(在我看来)方法是对低概率类进行过采样,并对最可能的类进行子采样.我想在网上做,因为它更灵活.

解决方法:

在问题#14451中已经解决了该问题.
只需在此处发布anwser,以使其对其他开发人员更加明显.

示例代码对低频率类进行过采样并对高频率类进行欠采样,其中class_target_prob在我的情况下只是均匀分布.我想从最近的手稿A systematic study of the class imbalance problem in convolutional neural networks中查看一些结论

通过调用以下方式完成特定类的过采样：

dataset = dataset.flat_map(
    lambda x: tf.data.Dataset.from_tensors(x).repeat(oversample_classes(x))
)

这是完整的代码片段,可以完成所有事情：

# sampling parameters
oversampling_coef = 0.9  # if equal to 0 then oversample_classes() always returns 1
undersampling_coef = 0.5  # if equal to 0 then undersampling_filter() always returns True

def oversample_classes(example):
    """
    Returns the number of copies of given example
    """
    class_prob = example['class_prob']
    class_target_prob = example['class_target_prob']
    prob_ratio = tf.cast(class_target_prob/class_prob, dtype=tf.float32)
    # soften ratio is oversampling_coef==0 we recover original distribution
    prob_ratio = prob_ratio ** oversampling_coef 
    # for classes with probability higher than class_target_prob we
    # want to return 1
    prob_ratio = tf.maximum(prob_ratio, 1) 
    # for low probability classes this number will be very large
    repeat_count = tf.floor(prob_ratio)
    # prob_ratio can be e.g 1.9 which means that there is still 90%
    # of change that we should return 2 instead of 1
    repeat_residual = prob_ratio - repeat_count # a number between 0-1
    residual_acceptance = tf.less_equal(
                        tf.random_uniform([], dtype=tf.float32), repeat_residual
    )

    residual_acceptance = tf.cast(residual_acceptance, tf.int64)
    repeat_count = tf.cast(repeat_count, dtype=tf.int64)

    return repeat_count + residual_acceptance


def undersampling_filter(example):
    """
    Computes if given example is rejected or not.
    """
    class_prob = example['class_prob']
    class_target_prob = example['class_target_prob']
    prob_ratio = tf.cast(class_target_prob/class_prob, dtype=tf.float32)
    prob_ratio = prob_ratio ** undersampling_coef
    prob_ratio = tf.minimum(prob_ratio, 1.0)

    acceptance = tf.less_equal(tf.random_uniform([], dtype=tf.float32), prob_ratio)

    return acceptance


dataset = dataset.flat_map(
    lambda x: tf.data.Dataset.from_tensors(x).repeat(oversample_classes(x))
)

dataset = dataset.filter(undersampling_filter)

dataset = dataset.repeat(-1)
dataset = dataset.shuffle(2048)
dataset = dataset.batch(32)

sess.run(tf.global_variables_initializer())

iterator = dataset.make_one_shot_iterator()
next_element = iterator.get_next()

更新#1

这是一个简单的jupyter notebook,它在玩具模型上实现了上述过采样/欠采样.