tf.nn.ctc_loss

tf.nn.ctc_loss(labels, inputs, sequence_length, preprocess_collapse_repeated=False, ctc_merge_repeated=True, time_major=True) {#ctc_loss}

Computes the CTC (Connectionist Temporal Classification) Loss.

This op implements the CTC loss as presented in the article:

A. Graves, S. Fernandez, F. Gomez, J. Schmidhuber.Connectionist Temporal Classification: Labelling Unsegmented Sequence Datawith Recurrent Neural Networks. ICML 2006, Pittsburgh, USA, pp. 369-376.

http://www.cs.toronto.edu/~graves/icml_2006.pdf

Input requirements:

sequence_length(b) <= time for all b

max(labels.indices(labels.indices[:, 1] == b, 2))
  <= sequence_length(b) for all b.

Notes:

This class performs the softmax operation for you, so inputs shouldbe e.g. linear projections of outputs by an LSTM.

The inputs Tensor's innermost dimension size, num_classes, representsnum_labels + 1 classes, where num_labels is the number of true labels, andthe largest value (num_classes - 1) is reserved for the blank label.

For example, for a vocabulary containing 3 labels [a, b, c],num_classes = 4 and the labels indexing is {a: 0, b: 1, c: 2, blank: 3}.

Regarding the arguments preprocess_collapse_repeated andctc_merge_repeated:

If preprocess_collapse_repeated is True, then a preprocessing step runsbefore loss calculation, wherein repeated labels passed to the lossare merged into single labels. This is useful if the training labels comefrom, e.g., forced alignments and therefore have unnecessary repetitions.

If ctc_merge_repeated is set False, then deep within the CTC calculation,repeated non-blank labels will not be merged and are interpretedas individual labels. This is a simplified (non-standard) version of CTC.

Here is a table of the (roughly) expected first order behavior:

• preprocess_collapse_repeated=False, ctc_merge_repeated=True

  Classical CTC behavior: Outputs true repeated classes with blanks inbetween, and can also output repeated classes with no blanks inbetween that need to be collapsed by the decoder.

• preprocess_collapse_repeated=True, ctc_merge_repeated=False

  Never learns to output repeated classes, as they are collapsedin the input labels before training.

• preprocess_collapse_repeated=False, ctc_merge_repeated=False

  Outputs repeated classes with blanks in between, but generally does notrequire the decoder to collapse/merge repeated classes.

• preprocess_collapse_repeated=True, ctc_merge_repeated=True

  Untested. Very likely will not learn to output repeated classes.

Args:

• labels: An int32 SparseTensor.labels.indices[i, :] == [b, t] means labels.values[i] storesthe id for (batch b, time t).labels.values[i] must take on values in [0, num_labels).See core/ops/ctc_ops.cc for more details.
• inputs: 3-D float Tensor.If time_major == False, this will be a Tensor shaped: [batch_size x max_time x num_classes].If time_major == True (default), this will be a Tensor shaped: [max_time x batch_size x num_classes].The logits.
• sequence_length: 1-D int32 vector, size [batch_size].The sequence lengths.
• preprocess_collapse_repeated: Boolean. Default: False.If True, repeated labels are collapsed prior to the CTC calculation.
• ctc_merge_repeated: Boolean. Default: True.
• time_major: The shape format of the inputs Tensors.If True, these Tensors must be shaped [max_time, batch_size, num_classes].If False, these Tensors must be shaped [batch_size, max_time, num_classes].Using time_major = True (default) is a bit more efficient because it avoidstransposes at the beginning of the ctc_loss calculation. However, mostTensorFlow data is batch-major, so by this function also accepts inputsin batch-major form.

Returns:

A 1-D float Tensor, size [batch], containing the negative log probabilities.

Raises:

• TypeError: if labels is not a SparseTensor.