openvino/docs/ops/sequence/CTCGreedyDecoder_1.md

# CTCGreedyDecoder {#openvino_docs_ops_sequence_CTCGreedyDecoder_1}

**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 \f$X\f$ of length \f$T\f$, *CTCGreedyDecoder* assumes the probability of a length \f$T\f$ character sequence \f$C\f$ is given by
\f[
p(C|X) = \prod_{t=1}^{T} p(c_{t}|X)
\f]

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**

```xml
<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>
```