Table of Contents

Class GenerativeReplay<T>

Namespace
AiDotNet.ContinualLearning
Assembly
AiDotNet.dll

Implements Generative Replay (also known as Deep Generative Replay) for continual learning.

public class GenerativeReplay<T> : IContinualLearningStrategy<T>

Type Parameters

T

The numeric type for calculations.

Inheritance
GenerativeReplay<T>
Implements
Inherited Members

Remarks

For Beginners: Generative Replay uses a generative model (like a VAE or GAN) to create pseudo-examples from previous tasks instead of storing real examples. This enables rehearsal without storing actual data, which is useful for privacy-sensitive applications or when memory is limited.

How it works:

  1. Train a generative model alongside the main model (called the "solver").
  2. After each task, use the generator to create pseudo-examples of previous tasks.
  3. When learning new tasks, mix real new data with generated pseudo-examples.
  4. The generator is also trained on mixed data to maintain its ability to generate old examples.

Key Components:

  • Solver: The main model being trained on tasks.
  • Generator: Produces synthetic examples of previous tasks.
  • Scholar: Combined solver + generator system.

Advantages:

  • No need to store real examples (privacy-preserving).
  • Constant memory regardless of number of tasks.
  • Can generate unlimited pseudo-examples for rehearsal.

Reference: Shin, H., Lee, J.K., Kim, J., and Kim, J. "Continual Learning with Deep Generative Replay" (2017). NeurIPS.

Constructors

GenerativeReplay(int, double, double, int?)

Initializes a new instance of the GenerativeReplay class.

public GenerativeReplay(int replayBatchSize = 32, double replayRatio = 0.5, double lambda = 1, int? seed = null)

Parameters

replayBatchSize int

Number of pseudo-examples to generate per batch (default: 32).

replayRatio double

Ratio of replay samples to new samples (default: 0.5).

lambda double

Weight for replay loss contribution (default: 1.0).

seed int?

Random seed for reproducibility (default: null for random).

Properties

Lambda

Gets the regularization strength parameter (lambda) for loss-based continual learning.

public double Lambda { get; set; }

Property Value

double

Remarks

For Beginners: Lambda controls how strongly the strategy prevents forgetting. A higher lambda means the network is more conservative about changing weights important for previous tasks, but this might make it harder to learn new tasks effectively.

Typical values range from 100 to 10000, depending on the complexity of tasks and how important it is to preserve old knowledge versus learning new knowledge.

ReplayBatchSize

Gets the replay batch size.

public int ReplayBatchSize { get; }

Property Value

int

ReplayRatio

Gets the replay ratio.

public double ReplayRatio { get; }

Property Value

double

TaskCount

Gets the number of tasks processed.

public int TaskCount { get; }

Property Value

int

Methods

AfterTask(INeuralNetwork<T>, (Tensor<T> inputs, Tensor<T> targets), int)

Processes information after completing training on a task.

public void AfterTask(INeuralNetwork<T> network, (Tensor<T> inputs, Tensor<T> targets) taskData, int taskId)

Parameters

network INeuralNetwork<T>

The neural network that was trained.

taskData (Tensor<T> grad1, Tensor<T> grad2)

Data from the completed task for computing importance measures.

taskId int

The identifier for the completed task.

Remarks

For Beginners: This method is called after you finish training on a task. It allows the strategy to compute and store information about what the network learned, which will be used to protect this knowledge when learning future tasks.

For example, in Elastic Weight Consolidation (EWC), this computes the Fisher Information Matrix to identify which weights are most important for the completed task.

BeforeTask(INeuralNetwork<T>, int)

Prepares the strategy before starting to learn a new task.

public void BeforeTask(INeuralNetwork<T> network, int taskId)

Parameters

network INeuralNetwork<T>

The neural network that will be trained.

taskId int

The identifier for the upcoming task (0-indexed).

Remarks

For Beginners: This method is called before you start training on a new task. It allows the strategy to capture the network's current state or prepare any necessary data structures for protecting knowledge from previous tasks.

For example, in Learning without Forgetting (LwF), this might store the network's predictions on the new task's inputs before training begins, so we can later encourage the network to maintain similar predictions.

ComputeLoss(INeuralNetwork<T>)

Computes the regularization loss to prevent forgetting previous tasks.

public T ComputeLoss(INeuralNetwork<T> network)

Parameters

network INeuralNetwork<T>

The neural network being trained.

Returns

T

The regularization loss value that should be added to the task loss.

Remarks

For Beginners: This method calculates an additional loss term that penalizes the network for deviating from its learned knowledge of previous tasks. You add this to your regular task loss during training:

var totalLoss = taskLoss + strategy.ComputeLoss(network);

For example, in EWC, this returns a penalty proportional to how much important weights have changed from their optimal values for previous tasks. Larger changes to important weights result in higher loss, discouraging the network from forgetting.

CreateMixedBatch(Tensor<T>, Tensor<T>, int)

Creates a mixed training batch combining new data with generated replay data.

public (Tensor<T> inputs, Tensor<T> targets) CreateMixedBatch(Tensor<T> currentInputs, Tensor<T> currentTargets, int batchSize)

Parameters

currentInputs Tensor<T>

Current task inputs.

currentTargets Tensor<T>

Current task targets.

batchSize int

Total batch size.

Returns

(Tensor<T> grad1, Tensor<T> grad2)

Mixed batch of current and replay data.

GenerateReplaySamples()

Generates pseudo-examples for replay using the generator.

public (Tensor<T>? inputs, Tensor<T>? targets) GenerateReplaySamples()

Returns

(Tensor<T> grad1, Tensor<T> grad2)

Tuple of generated inputs and targets, or nulls if generator not available.

ModifyGradients(INeuralNetwork<T>, Vector<T>)

Modifies the gradient to prevent catastrophic forgetting.

public Vector<T> ModifyGradients(INeuralNetwork<T> network, Vector<T> gradients)

Parameters

network INeuralNetwork<T>

The neural network being trained.

gradients Vector<T>

The gradients from the current task loss.

Returns

Vector<T>

Modified gradients that protect previous task knowledge.

Remarks

For Beginners: Some continual learning strategies work by modifying the gradients (the update directions for weights) rather than adding a loss term. This method takes the gradients computed from the current task and modifies them to avoid interfering with previously learned tasks.

For example, in Gradient Episodic Memory (GEM), if a gradient would hurt performance on stored examples from previous tasks, it's projected to the closest gradient that doesn't interfere with those examples.

If a strategy doesn't use gradient modification, this should return the gradients unchanged.

Reset()

Resets the strategy, clearing all stored task information.

public void Reset()

Remarks

For Beginners: This method clears all the information the strategy has accumulated about previous tasks. After calling this, the network will be free to learn new tasks without any constraints from previously learned tasks.

Use this when you want to start fresh or when you're done with a sequence of tasks and want to begin a new independent sequence.

SetGenerator(IGenerativeModel<T>)

Sets the generative model used for replay.

public void SetGenerator(IGenerativeModel<T> generator)

Parameters

generator IGenerativeModel<T>

The generative model (VAE, GAN, etc.).

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