What is a Siamese Neural Network? | by Wanshun Wong | Oct, 2020


Loss Capabilities

Each contrastive loss and triplet loss are distance-based loss capabilities which might be primarily used for studying vector representations, and are sometimes used along with Siamese neural networks.

Contrastive Loss

Assume our dataset consists of various lessons of objects. For instance, the ImageNet dataset consists of pictures of vehicles, pictures of canine, and so on. Then for each pair of inputs (x₁, x₂),

  • If x₁ and x₂ belong to the identical class, we wish their vector representations to be related. Due to this fact we wish to decrease L = ‖v₁ − v₂‖².
  • Then again if x₁ and x₂ belong to totally different lessons, we wish ‖v₁ − v₂‖ to be giant. The time period we wish to decrease is
    L = max(0, m − ‖v₁ − v₂‖)², the place m is a hyperparameter known as margin. The thought of margin is that, when v₁ and v₂ are sufficiently totally different, L will already be Zero and can’t be additional minimized. Therefore the mannequin won’t waste efforts in additional separating v₁ and v₂, and can concentrate on different enter pairs as an alternative.

We will mix these two circumstances right into a single system:

L = y * v₁ − v₂‖² + (1 - y) * max(0, m − ‖v₁ − v₂‖)²

the place y = 1 if x₁ and x₂ belong to the identical class, y = Zero in any other case.

Triplet Loss

Triplet loss takes the above concept one step additional by contemplating triplets of inputs (xa, xp, xn). Right here xa is an anchor object, xp is a optimistic object (i.e. xa and xp belong to the identical class), and xn is a destructive object (i.e. xa and xn belong to totally different lessons). Our aim is to make the vector illustration va to be extra just like vp than to vn. The exact system is given by

L = max(0, m + ‖vavp‖ - ‖vavn‖)

the place m is the hyperparameter margin. Similar to the case for contrastive loss, margin determines when the distinction between ‖vavp‖ and ‖vavn‖ has change into sufficiently big, such that the mannequin will not alter its weights from this triplet.

For each contrastive loss and triplet loss, how we pattern the enter pairs (x₁, x₂) and the triplets (xa, xp, xn) from totally different lessons of objects has an amazing affect on the mannequin coaching course of. Ideally we wish enter pairs and triplets that aren’t too straightforward but additionally not too onerous for our mannequin.


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