chainer.functions.triplet(anchor, positive, negative, margin=0.2, reduce='mean')[source]

Computes triplet loss.

It takes a triplet of variables as inputs, \(a\), \(p\) and \(n\): anchor, positive example and negative example respectively. The triplet defines a relative similarity between samples. Let \(N\) and \(K\) denote mini-batch size and the dimension of input variables, respectively. The shape of all input variables should be \((N, K)\).

\[L(a, p, n) = \frac{1}{N} \left( \sum_{i=1}^N \max \{d(a_i, p_i) - d(a_i, n_i) + {\rm margin}, 0\} \right)\]

where \(d(x_i, y_i) = \| {\bf x}_i - {\bf y}_i \|_2^2\).

The output is a variable whose value depends on the value of the option reduce. If it is 'no', it holds the elementwise loss values. If it is 'mean', this function takes a mean of loss values.

  • anchor (Variable or N-dimensional array) – The anchor example variable. The shape should be \((N, K)\), where \(N\) denotes the minibatch size, and \(K\) denotes the dimension of the anchor.

  • positive (Variable or N-dimensional array) – The positive example variable. The shape should be the same as anchor.

  • negative (Variable or N-dimensional array) – The negative example variable. The shape should be the same as anchor.

  • margin (float) – A parameter for triplet loss. It should be a positive value.

  • reduce (str) – Reduction option. Its value must be either 'mean' or 'no'. Otherwise, ValueError is raised.


A variable holding a scalar that is the loss value calculated by the above equation. If reduce is 'no', the output variable holds array whose shape is same as one of (hence both of) input variables. If it is 'mean', the output variable holds a scalar value.

Return type



This cost can be used to train triplet networks. See Learning Fine-grained Image Similarity with Deep Ranking for details.


>>> anchor = np.array([[-2.0, 3.0, 0.5], [5.0, 2.0, -0.5]]).astype(np.float32)
>>> pos = np.array([[-2.1, 2.8, 0.5], [4.9, 2.0, -0.4]]).astype(np.float32)
>>> neg = np.array([[-2.1, 2.7, 0.7], [4.9, 2.0, -0.7]]).astype(np.float32)
>>> F.triplet(anchor, pos, neg)
>>> y = F.triplet(anchor, pos, neg, reduce='no')
>>> y.shape
>>> y.array
array([0.11000005, 0.17      ], dtype=float32)
>>> F.triplet(anchor, pos, neg, margin=0.5)  # harder penalty