序列标注、手写小写字母OCR数据集、双向RNN

MIT口语系统研究组Rob Kassel收集，斯坦福大学人工智能实验室Ben Taskar预处理OCR数据集(http://ai.stanford.edu/~btaskar/ocr/ )，包含大量单独手写小写字母，每个样本对应16X8像素二值图像。字线组合序列，序列对应单词。6800个，长度不超过14字母的单词。gzip压缩，内容用Tab分隔文本文件。Python csv模块直接读取。文件每行一个归一化字母属性，ID号、标签、像素值、下一字母ID号等。

下一字母ID值排序，按照正确顺序读取每个单词字母。收集字母，直到下一个ID对应字段未被设置为止。读取新序列。读取完目标字母及数据像素，用零图像填充序列对象，能纳入两个较大目标字母所有像素数据NumPy数组。

时间步之间共享softmax层。数据和目标数组包含序列，每个目标字母对应一个图像帧。RNN扩展，每个字母输出添加softmax分类器。分类器对每帧数据而非整个序列评估预测结果。计算序列长度。一个softmax层添加到所有帧：或者为所有帧添加几个不同分类器，或者令所有帧共享同一个分类器。共享分类器，权值在训练中被调整次数更多，训练单词每个字母。一个全连接层权值矩阵维数batch_size*in_size*out_size。现需要在两个输入维度batch_size、sequence_steps更新权值矩阵。令输入(RNN输出活性值)扁平为形状batch_size*sequence_steps*in_size。权值矩阵变成较大的批数据。结果反扁平化(unflatten)。

代价函数，序列每一帧有预测目标对，在相应维度平均。依据张量长度(序列最大长度)归一化的tf.reduce_mean无法使用。需要按照实际序列长度归一化，手工调用tf.reduce_sum和除法运算均值。

损失函数，tf.argmax针对轴2非轴1,各帧填充，依据序列实际长度计算均值。tf.reduce_mean对批数据所有单词取均值。

TensorFlow自动导数计算，可使用序列分类相同优化运算，只需要代入新代价函数。对所有RNN梯度裁剪，防止训练发散，避免负面影响。

训练模型，get_sataset下载手写体图像，预处理，小写字母独热编码向量。随机打乱数据顺序，分偏划分训练集、测试集。

单词相邻字母存在依赖关系(或互信息)，RNN保存同一单词全部输入信息到隐含活性值。前几个字母分类，网络无大量输入推断额外信息，双向RNN(bidirectional RNN)克服缺陷。
两个RNN观测输入序列，一个按照通常顺序从左端读取单词，另一个按照相反顺序从右端读取单词。每个时间步得到两个输出活性值。送入共享softmax层前，拼接。分类器从每个字母获取完整单词信息。tf.modle.rnn.bidirectional_rnn已实现。

实现双向RNN。划分预测属性到两个函数，只关注较少内容。_shared_softmax函数，传入函数张量data推断输入尺寸。复用其他架构函数，相同扁平化技巧在所有时间步共享同一个softmax层。rnn.dynamic_rnn创建两个RNN。
序列反转，比实现新反向传递RNN运算容易。tf.reverse_sequence函数反转帧数据中sequence_lengths帧。数据流图节点有名称。scope参数是rnn_dynamic_cell变量scope名称，默认值RNN。两个参数不同RNN，需要不同域。
反转序列送入后向RNN，网络输出反转，和前向输出对齐。沿RNN神经元输出维度拼接两个张量，返回。双向RNN模型性能更优。

 import gzipimport csvimport numpy as npfrom helpers import downloadclass OcrDataset:

 URL = 'http://ai.stanford.edu/~btaskar/ocr/letter.data.gz'def __init__(self, cache_dir):
 path = download(type(self).URL, cache_dir)
 lines = self._read(path)
 data, target = self._parse(lines)
 self.data, self.target = self._pad(data, target)

 @staticmethoddef _read(filepath):
 with gzip.open(filepath, 'rt') as file_:
 reader = csv.reader(file_, delimiter='	')
 lines = list(reader)return lines

 @staticmethoddef _parse(lines):
 lines = sorted(lines, key=lambda x: int(x[0]))
 data, target = [], []
 next_ = Nonefor line in lines:if not next_:
 data.append([])
 target.append([])else:assert next_ == int(line[0])
 next_ = int(line[2]) if int(line[2]) > -1 else None
 pixels = np.array([int(x) for x in line[6:134]])
 pixels = pixels.reshape((16, 8))
 data[-1].append(pixels)
 target[-1].append(line[1])return data, target

 @staticmethoddef _pad(data, target):
 max_length = max(len(x) for x in target)
 padding = np.zeros((16, 8))
 data = [x + ([padding] * (max_length - len(x))) for x in data]
 target = [x + ([''] * (max_length - len(x))) for x in target]return np.array(data), np.array(target)import tensorflow as tffrom helpers import lazy_propertyclass SequenceLabellingModel:def __init__(self, data, target, params):
 self.data = data
 self.target = target
 self.params = params
 self.prediction
 self.cost
 self.error
 self.optimize

 @lazy_propertydef length(self):
 used = tf.sign(tf.reduce_max(tf.abs(self.data), reduction_indices=2))
 length = tf.reduce_sum(used, reduction_indices=1)
 length = tf.cast(length, tf.int32)return length

 @lazy_propertydef prediction(self):
 output, _ = tf.nn.dynamic_rnn(
 tf.nn.rnn_cell.GRUCell(self.params.rnn_hidden),
 self.data,
 dtype=tf.float32,
 sequence_length=self.length,
 )# Softmax layer.max_length = int(self.target.get_shape()[1])
 num_classes = int(self.target.get_shape()[2])
 weight = tf.Variable(tf.truncated_normal(
 [self.params.rnn_hidden, num_classes], stddev=0.01))
 bias = tf.Variable(tf.constant(0.1, shape=[num_classes]))# Flatten to apply same weights to all time steps.output = tf.reshape(output, [-1, self.params.rnn_hidden])
 prediction = tf.nn.softmax(tf.matmul(output, weight) + bias)
 prediction = tf.reshape(prediction, [-1, max_length, num_classes])return prediction

 @lazy_propertydef cost(self):# Compute cross entropy for each frame.cross_entropy = self.target * tf.log(self.prediction)
 cross_entropy = -tf.reduce_sum(cross_entropy, reduction_indices=2)
 mask = tf.sign(tf.reduce_max(tf.abs(self.target), reduction_indices=2))
 cross_entropy *= mask# Average over actual sequence lengths.cross_entropy = tf.reduce_sum(cross_entropy, reduction_indices=1)
 cross_entropy /= tf.cast(self.length, tf.float32)return tf.reduce_mean(cross_entropy)

 @lazy_propertydef error(self):
 mistakes = tf.not_equal(
 tf.argmax(self.target, 2), tf.argmax(self.prediction, 2))
 mistakes = tf.cast(mistakes, tf.float32)
 mask = tf.sign(tf.reduce_max(tf.abs(self.target), reduction_indices=2))
 mistakes *= mask# Average over actual sequence lengths.mistakes = tf.reduce_sum(mistakes, reduction_indices=1)
 mistakes /= tf.cast(self.length, tf.float32)return tf.reduce_mean(mistakes)

 @lazy_propertydef optimize(self):
 gradient = self.params.optimizer.compute_gradients(self.cost)try:
 limit = self.params.gradient_clipping
 gradient = [
 (tf.clip_by_value(g, -limit, limit), v)if g is not None else (None, v)for g, v in gradient]except AttributeError:print('No gradient clipping parameter specified.')
 optimize = self.params.optimizer.apply_gradients(gradient)return optimizeimport randomimport tensorflow as tfimport numpy as npfrom helpers import AttrDictfrom OcrDataset import OcrDatasetfrom SequenceLabellingModel import SequenceLabellingModelfrom batched import batched

 params = AttrDict(
 rnn_cell=tf.nn.rnn_cell.GRUCell,
 rnn_hidden=300,
 optimizer=tf.train.RMSPropOptimizer(0.002),
 gradient_clipping=5,
 batch_size=10,
 epochs=5,
 epoch_size=50)def get_dataset():
 dataset = OcrDataset('./ocr')# Flatten images into vectors.dataset.data = dataset.data.reshape(dataset.data.shape[:2] + (-1,))# One-hot encode targets.target = np.zeros(dataset.target.shape + (26,))for index, letter in np.ndenumerate(dataset.target):if letter:
 target[index][ord(letter) - ord('a')] = 1dataset.target = target# Shuffle order of examples.order = np.random.permutation(len(dataset.data))
 dataset.data = dataset.data[order]
 dataset.target = dataset.target[order]return dataset# Split into training and test data.dataset = get_dataset()
 split = int(0.66 * len(dataset.data))
 train_data, test_data = dataset.data[:split], dataset.data[split:]
 train_target, test_target = dataset.target[:split], dataset.target[split:]# Compute graph._, length, image_size = train_data.shape
 num_classes = train_target.shape[2]
 data = tf.placeholder(tf.float32, [None, length, image_size])
 target = tf.placeholder(tf.float32, [None, length, num_classes])
 model = SequenceLabellingModel(data, target, params)
 batches = batched(train_data, train_target, params.batch_size)

 sess = tf.Session()
 sess.run(tf.initialize_all_variables())for index, batch in enumerate(batches):
 batch_data = batch[0]
 batch_target = batch[1]
 epoch = batch[2]if epoch >= params.epochs:breakfeed = {data: batch_data, target: batch_target}
 error, _ = sess.run([model.error, model.optimize], feed)print('{}: {:3.6f}%'.format(index + 1, 100 * error))

 test_feed = {data: test_data, target: test_target}
 test_error, _ = sess.run([model.error, model.optimize], test_feed)print('Test error: {:3.6f}%'.format(100 * error))import tensorflow as tffrom helpers import lazy_propertyclass BidirectionalSequenceLabellingModel:def __init__(self, data, target, params):
 self.data = data
 self.target = target
 self.params = params
 self.prediction
 self.cost
 self.error
 self.optimize

 @lazy_propertydef length(self):
 used = tf.sign(tf.reduce_max(tf.abs(self.data), reduction_indices=2))
 length = tf.reduce_sum(used, reduction_indices=1)
 length = tf.cast(length, tf.int32)return length

 @lazy_propertydef prediction(self):
 output = self._bidirectional_rnn(self.data, self.length)
 num_classes = int(self.target.get_shape()[2])
 prediction = self._shared_softmax(output, num_classes)return predictiondef _bidirectional_rnn(self, data, length):
 length_64 = tf.cast(length, tf.int64)
 forward, _ = tf.nn.dynamic_rnn(
 cell=self.params.rnn_cell(self.params.rnn_hidden),
 inputs=data,
 dtype=tf.float32,
 sequence_length=length,
 scope='rnn-forward')
 backward, _ = tf.nn.dynamic_rnn(
 cell=self.params.rnn_cell(self.params.rnn_hidden),
 inputs=tf.reverse_sequence(data, length_64, seq_dim=1),
 dtype=tf.float32,
 sequence_length=self.length,
 scope='rnn-backward')
 backward = tf.reverse_sequence(backward, length_64, seq_dim=1)
 output = tf.concat(2, [forward, backward])return outputdef _shared_softmax(self, data, out_size):
 max_length = int(data.get_shape()[1])
 in_size = int(data.get_shape()[2])
 weight = tf.Variable(tf.truncated_normal(
 [in_size, out_size], stddev=0.01))
 bias = tf.Variable(tf.constant(0.1, shape=[out_size]))# Flatten to apply same weights to all time steps.flat = tf.reshape(data, [-1, in_size])
 output = tf.nn.softmax(tf.matmul(flat, weight) + bias)
 output = tf.reshape(output, [-1, max_length, out_size])return output

 @lazy_propertydef cost(self):# Compute cross entropy for each frame.cross_entropy = self.target * tf.log(self.prediction)
 cross_entropy = -tf.reduce_sum(cross_entropy, reduction_indices=2)
 mask = tf.sign(tf.reduce_max(tf.abs(self.target), reduction_indices=2))
 cross_entropy *= mask# Average over actual sequence lengths.cross_entropy = tf.reduce_sum(cross_entropy, reduction_indices=1)
 cross_entropy /= tf.cast(self.length, tf.float32)return tf.reduce_mean(cross_entropy)

 @lazy_propertydef error(self):
 mistakes = tf.not_equal(
 tf.argmax(self.target, 2), tf.argmax(self.prediction, 2))
 mistakes = tf.cast(mistakes, tf.float32)
 mask = tf.sign(tf.reduce_max(tf.abs(self.target), reduction_indices=2))
 mistakes *= mask# Average over actual sequence lengths.mistakes = tf.reduce_sum(mistakes, reduction_indices=1)
 mistakes /= tf.cast(self.length, tf.float32)return tf.reduce_mean(mistakes)

 @lazy_propertydef optimize(self):
 gradient = self.params.optimizer.compute_gradients(self.cost)try:
 limit = self.params.gradient_clipping
 gradient = [
 (tf.clip_by_value(g, -limit, limit), v)if g is not None else (None, v)for g, v in gradient]except AttributeError:print('No gradient clipping parameter specified.')
 optimize = self.params.optimizer.apply_gradients(gradient)return optimize

参考资料：
《面向机器智能的TensorFlow实践》

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