data_mining:neural_network:transfer_learning [AE Wiki]

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====== Transfer learning ======

Using pre-trained models / their trained weights as a starting point to train a model for a different data set.

Use pre-trained net, initialize last layers with random weights and e.g. create own softmax layer, **freeze** parameters in previous layers.

Options: Train new layers of network, or even more layers.

Prereqs:

  * Task A and B have same input x
  * Much data for existing model/task A, for new model only few data
  * Low level features from A could be helpful for task B
  * Pre-trained model needs to generalize


Another trick:

  * Precompute output of frozen layers for all samples (save computation time later)

For larger set of samples:

  * Only freeze first layers, train last few layers (and replace softmax)

For large set of samples:

  * Use weights as initialization, then train (and replace softmax)
===== Image-based NNs =====

Pre-trained models which generalize can be used as a starting model, e.g. https://github.com/KaimingHe/deep-residual-networks ResNet models.

For CNNs more generic features are usually contained in the **earlier** layers.