Versioned name: CTCGreedyDecoder-1

Category: Sequence processing

Short description: CTCGreedyDecoder performs greedy decoding on the logits given in input (best path).

Detailed description:

This operation is similar Reference

Given an input sequence $X$ of length $T$ , CTCGreedyDecoder assumes the probability of a length $T$ character sequence $C$ is given by

$p(C|X) = \prod_{t=1}^{T} p(c_{t}|X)$

Sequences in the batch can have different length. The lengths of sequences are coded as values 1 and 0 in the second input tensor sequence_mask. Value sequence_mask[j, i] specifies whether there is a sequence symbol at index i in the sequence i in the batch of sequences. If there is no symbol at j-th position sequence_mask[j, i] = 0, and sequence_mask[j, i] = 1 otherwise. Starting from j = 0, sequence_mass[j, i] are equal to 1 up to the particular index j = last_sequence_symbol, which is defined independently for each sequence i. For j > last_sequence_symbol, values in sequence_mask[j, i] are all zeros.

Attributes

merge_repeated
- Description: merge_repeated is a flag for merging repeated labels during the CTC calculation.
- Range of values: True or False
- Type: boolean
- Default value: True
- Required: no

Inputs

1: data - Input tensor with a batch of sequences. Type of elements is any supported floating point type. Shape of the tensor is [T, N, C], where T is the maximum sequence length, N is the batch size and C is the number of classes. Required.
2: sequence_mask - 2D input floating point tensor with sequence masks for each sequence in the batch. Populated with values 0 and 1. Shape of this input is [T, N]. Required.

Output

1: Output tensor with shape [N, T, 1, 1] and integer elements containing final sequence class indices. A final sequence can be shorter that the size T of the tensor, all elements that do not code sequence classes are filled with -1. Type of elements is floating point, but all values are integers.

Example

<layer ... type="CTCGreedyDecoder" ...>
    <input>
        <port id="0">
            <dim>20</dim>
            <dim>8</dim>
            <dim>128</dim>
       </port>
        <port id="1">
            <dim>20</dim>
            <dim>8</dim>
        </port>
    </input>
    <output>
        <port id="0">
            <dim>8</dim>
            <dim>20</dim>
            <dim>1</dim>
            <dim>1</dim>
       </port>
    </output>
</layer>