CTCLoss#

Versioned name: CTCLoss-4

Category: Sequence processing

Short description: CTCLoss computes the CTC (Connectionist Temporal Classification) Loss.

Detailed description:

CTCLoss operation is presented in Connectionist Temporal Classification - Labeling Unsegmented Sequence Data with Recurrent Neural Networks: Graves et al., 2016

CTCLoss estimates likelihood that a target labels[i,:] can occur (or is real) for given input sequence of logits logits[i,:,:]. Briefly, CTCLoss operation finds all sequences aligned with a target labels[i,:], computes log-probabilities of the aligned sequences using logits[i,:,:] and computes a negative sum of these log-probabilities.

Input sequences of logits logits can have different lengths. The length of each sequence logits[i,:,:] equals logit_length[i]. A length of target sequence labels[i,:] equals label_length[i]. The length of the target sequence must not be greater than the length of corresponding input sequence logits[i,:,:]. Otherwise, the operation behaviour is undefined.

CTCLoss calculation scheme:

  1. Compute probability of j-th character at time step t for i-th input sequence from logits using softmax formula:

\[p_{i,t,j} = \frac{\exp(logits[i,t,j])}{\sum^{K}_{k=0}{\exp(logits[i,t,k])}}\]

  1. For a given i-th target from labels[i,:] find all aligned paths. A path S = (c1,c2,...,cT) is aligned with a target G=(g1,g2,...,gT) if both chains are equal after decoding. The decoding extracts substring of length label_length[i] from a target G, merges repeated characters in G in case preprocess_collapse_repeated equal to true and finds unique elements in the order of character occurrence in case unique equal to true. The decoding merges repeated characters in S in case ctc_merge_repeated equal to true and removes blank characters represented by blank_index. By default, blank_index is equal to C-1, where C is a number of classes including the blank. For example, in case default ctc_merge_repeated, preprocess_collapse_repeated, unique and blank_index a target sequence G=(0,3,2,2,2,2,2,4,3) of a length label_length[i]=4 is processed to (0,3,2,2) and a path S=(0,0,4,3,2,2,4,2,4) of a length logit_length[i]=9 is also processed to (0,3,2,2), where C=5. There exist other paths that are also aligned with G, for instance, 0,4,3,3,2,4,2,2,2. Paths checked for alignment with a target label[:,i] must be of length logit_length[i] = L_i. Compute probabilities of these aligned paths (alignments) as follows:

\[p(S) = \prod_{t=1}^{L_i} p_{i,t,ct}\]

  1. Finally, compute negative log of summed up probabilities of all found alignments:

\[CTCLoss = - \ln \sum_{S} p(S)\]

Note 1: This calculation scheme does not provide steps for optimal implementation and primarily serves for better explanation.

Note 2: This is recommended to compute a log-probability \(\ln p(S)\) for an aligned path as a sum of log-softmax of input logits. It helps to avoid underflow and overflow during calculation. Having log-probabilities for aligned paths, log of summed up probabilities for these paths can be computed as follows:

\[\ln(a + b) = \ln(a) + \ln(1 + \exp(\ln(b) - \ln(a)))\]

Attributes

  • preprocess_collapse_repeated

    • Description: preprocess_collapse_repeated is a flag for a preprocessing step before loss calculation, wherein repeated labels in labels[i,:] passed to the loss are merged into single labels.

    • Range of values: true or false

    • Type: boolean

    • Default value: false

    • Required: no

  • ctc_merge_repeated

    • Description: ctc_merge_repeated is a flag for merging repeated characters in a potential alignment during the CTC loss calculation.

    • Range of values: true or false

    • Type: boolean

    • Default value: true

    • Required: no

  • unique

    • Description: unique is a flag to find unique elements for a target labels[i,:] before matching with potential alignments. Unique elements in the processed labels[i,:] are sorted in the order of their occurrence in original labels[i,:]. For example, the processed sequence for labels[i,:]=(0,1,1,0,1,3,3,2,2,3) of length label_length[i]=10 will be (0,1,3,2) in case unique equal to true.

    • Range of values: true or false

    • Type: boolean

    • Default value: false

    • Required: no

Inputs

  • 1: logits - Input tensor with a batch of sequences of logits. Type of elements is T_F. Shape of the tensor is [N, T, C], where N is the batch size, T is the maximum sequence length and C is the number of classes including the blank. Required.

  • 2: logit_length - 1D input tensor of type T1 and of a shape [N]. The tensor must consist of non-negative values not greater than T. Lengths of input sequences of logits logits[i,:,:]. Required.

  • 3: labels - 2D tensor with shape [N, T] of type T2. A length of a target sequence labels[i,:] is equal to label_length[i] and must contain of integers from a range [0; C-1] except blank_index. Required.

  • 4: label_length - 1D tensor of type T1 and of a shape [N]. The tensor must consist of non-negative values not greater than T and label_length[i] <= logit_length[i] for all possible i. Required.

  • 5: blank_index - Scalar of type T2. Set the class index to use for the blank label. Default value is C-1. Optional.

Output

  • 1: Output tensor with shape [N], negative sum of log-probabilities of alignments. Type of elements is T_F.

Types

  • T_F: any supported floating-point type.

  • T1, T2: int32 or int64.

Example

<layer ... type="CTCLoss" ...>
    <input>
        <port id="0">
            <dim>8</dim>
            <dim>20</dim>
            <dim>128</dim>
        </port>
        <port id="1">
            <dim>8</dim>
        </port>
        <port id="2">
            <dim>8</dim>
            <dim>20</dim>
        </port>
        <port id="3">
            <dim>8</dim>
        </port>
        <port id="4">  <!-- blank_index value is: 120 -->
    </input>
    <output>
        <port id="0">
            <dim>8</dim>
        </port>
    </output>
</layer>