keras SegNet使用池化索引（pooling indices）

keras中不能直接使用池化索引。最近学习到SegNet（网上许多错的，没有用池化索引），其中下采样上采样用到此部分。此处用到自定义层。

完整测试代码如下。

"""
@author: LiShiHang
@software: PyCharm
@file: utils.py
@time: 2018/12/18 14:58
"""
from keras.engine import Layer
import keras.backend as K


class MaxPoolingWithArgmax2D(Layer):

    def __init__(
            self,
            pool_size=(2, 2),
            strides=(2, 2),
            padding='same',
            **kwargs):
        super(MaxPoolingWithArgmax2D, self).__init__(**kwargs)
        self.padding = padding
        self.pool_size = pool_size
        self.strides = strides

    def call(self, inputs, **kwargs):
        padding = self.padding
        pool_size = self.pool_size
        strides = self.strides
        if K.backend() == 'tensorflow':
            ksize = [1, pool_size[0], pool_size[1], 1]
            padding = padding.upper()
            strides = [1, strides[0], strides[1], 1]
            output, argmax = K.tf.nn.max_pool_with_argmax(
                inputs,
                ksize=ksize,
                strides=strides,
                padding=padding)
        else:
            errmsg = '{} backend is not supported for layer {}'.format(
                K.backend(), type(self).__name__)
            raise NotImplementedError(errmsg)
        argmax = K.cast(argmax, K.floatx())
        return [output, argmax]

    def compute_output_shape(self, input_shape):
        ratio = (1, 2, 2, 1)
        output_shape = [
            dim // ratio[idx]
            if dim is not None else None
            for idx, dim in enumerate(input_shape)]
        output_shape = tuple(output_shape)
        return [output_shape, output_shape]

    def compute_mask(self, inputs, mask=None):
        return 2 * [None]


class MaxUnpooling2D(Layer):
    def __init__(self, up_size=(2, 2), **kwargs):
        super(MaxUnpooling2D, self).__init__(**kwargs)
        self.up_size = up_size

    def call(self, inputs, output_shape=None):

        updates, mask = inputs[0], inputs[1]
        with K.tf.variable_scope(self.name):
            mask = K.cast(mask, 'int32')
            input_shape = K.tf.shape(updates, out_type='int32')
            #  calculation new shape
            if output_shape is None:
                output_shape = (
                    input_shape[0],
                    input_shape[1] * self.up_size[0],
                    input_shape[2] * self.up_size[1],
                    input_shape[3])

            # calculation indices for batch, height, width and feature maps
            one_like_mask = K.ones_like(mask, dtype='int32')
            batch_shape = K.concatenate(
                [[input_shape[0]], [1], [1], [1]],
                axis=0)
            batch_range = K.reshape(
                K.tf.range(output_shape[0], dtype='int32'),
                shape=batch_shape)
            b = one_like_mask * batch_range
            y = mask // (output_shape[2] * output_shape[3])
            x = (mask // output_shape[3]) % output_shape[2]
            feature_range = K.tf.range(output_shape[3], dtype='int32')
            f = one_like_mask * feature_range

            # transpose indices & reshape update values to one dimension
            updates_size = K.tf.size(updates)
            indices = K.transpose(K.reshape(
                K.stack([b, y, x, f]),
                [4, updates_size]))
            values = K.reshape(updates, [updates_size])
            ret = K.tf.scatter_nd(indices, values, output_shape)
            return ret

    def compute_output_shape(self, input_shape):
        mask_shape = input_shape[1]
        return (
            mask_shape[0],
            mask_shape[1] * self.up_size[0],
            mask_shape[2] * self.up_size[1],
            mask_shape[3]
        )


if __name__ == '__main__':

    import keras
    import numpy as np

    # input = keras.layers.Input((4, 4, 3))
    # o = MaxPoolingWithArgmax2D()(input)
    # model = keras.Model(inputs=input, outputs=o)  # outputs=o
    # model.compile(optimizer="adam", loss='categorical_crossentropy')
    # x = np.random.randint(0, 100, (3, 4, 4, 3)) # 调试此处
    # m = model.predict(x) # 调试此处
    # print(m)

    input = keras.layers.Input((4, 4, 3))
    o = MaxPoolingWithArgmax2D()(input)
    o2 = MaxUnpooling2D()(o)
    model = keras.Model(inputs=input, outputs=o2)  # outputs=o
    model.compile(optimizer="adam", loss='categorical_crossentropy')
    x = np.random.randint(0, 100, (3, 4, 4, 3))  # 调试此处
    m = model.predict(x)  # 调试此处
    print(m)

感兴趣的可调试注释处。