CTCGreedyDecoder

Versioned name: CTCGreedyDecoder-1

Category: Sequence processing

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

Detailed description: 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.

Note: Regardless of the value of ctc_merge_repeated attribute, if the output index for a given batch and time step corresponds to the blank_index, no new element is emitted.

Attributes

• ctc_merge_repeated

  • Description: ctc_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 batch of sequences of type T_F and shape [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 - input tensor with sequence masks for each sequence in the batch of type T_F populated with values 0 and 1 and shape [T, N]. Required.

Output

• 1: Output tensor of type T_F and shape [N, T, 1, 1] which is filled with 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.

Types

• T_F: any supported floating-point type.

Example

<layer ... type="CTCGreedyDecoder" ...>
    <data ctc_merge_repeated="true" />
    <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>