IMU Character Recognition with CNN + GNN

This project implements a state-of-the-art handwriting recognition pipeline using Convolutional Neural Networks (CNN) and Graph Neural Networks (GNN) in a Multi-Task Learning (MTL) framework. The approach jointly optimizes character classification and trajectory regression tasks. This novel architecture builds on the principles discussed in Joint Classification and Trajectory Regression of Online Handwriting Using a Multi-Task Learning Approach and achieves substantial improvements over baseline methods.

The current model achieves a classification accuracy of 99.74% on the OnHW Dataset, improving significantly over the baseline CNN model (94.8%). Furthermore, the trajectory prediction component achieves smoother and more accurate results compared to previous approaches.

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Features

1. Hybrid CNN-GNN Architecture

• CNN Component:
  • Extracts temporal and spatial features from the IMU sensor data.
• GNN Component:
  • Captures relational structures and interdependencies in multi-channel sensor data.

2. Multi-Task Learning (MTL)

• Simultaneously solves two tasks:
  • Character Classification: Maps IMU features to character labels.
  • Trajectory Regression: Reconstructs handwriting trajectories for visualization and analysis.

3. Symbol and Equation Recognition

• Extended pipeline supports classification of mathematical symbols and multi-character equations.

4. Advanced Loss Strategies

• Combines:
  • Cross-Entropy Loss for classification.
  • Distance, Spatio-Temporal, and Distribution-Based Losses for regression.

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Dataset

The OnHW Dataset includes:

• 13 IMU Channels: Data from accelerometers, gyroscope, magnetometer, and force sensors.
• Variable-Length Sequences: Handwriting samples of varying lengths.
• Ground Truth Labels: Characters, symbols, and trajectories.

The dataset is preprocessed to normalize IMU data and interpolate sequences to a uniform length.

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How to Use

1. Install Dependencies

pip install -r requirements.txt

2. Preprocess the Train the model

python cnn_gnn.py

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Analysis

Accuracy Comparison

Model	Classification Accuracy (%)
Baseline CNN	94.8
CNN + GNN	99.74

MTL Loss Combination Insights

The table below compares different combinations of loss functions for trajectory regression and character classification:

Loss Combination	Classification Accuracy (%)	Regression Error (RMSE)
MSE + CrossEntropy	86.69	0.1169
Huber + CrossEntropy	88.43	0.1533
MSE + PearsonCorrelation	87.03	0.1490
MSE + Wasserstein	88.15	0.1530

Error Analysis

Mismatched predictions primarily occur with visually similar characters. For example:

• P is often mistaken for D.
• g is mistaken for y.

To reduce errors, the following strategies are recommended:

• Data augmentation to increase diversity.
• Weighted losses focusing on difficult samples.

Loss and Accuracy Plots

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Suggestions for Further Improvements

1. Advanced Architectures

• Implement Graph Attention Networks (GAT) for improved relational modeling.
• Incorporate Transformer-based Models for enhanced sequence-to-sequence learning.

2. Data Augmentation

• Introduce simulated IMU data with noise, shifts, and rotations to improve generalization.

3. Real-Time Optimization

• Convert models to lightweight formats (e.g., TensorFlow Lite or ONNX) for deployment on resource-constrained devices.

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Visualization of Architecture

Training Process

%%{init: {'theme': 'forest'}}%%
flowchart TD
    A[IMU Data - Inertial MTS] -->|Extract Features| B[<b>CNN Trunk</b>]
    C[Relational Data - GNN Features] -->|Model Relationships| D[<b>GNN Trunk</b>]
    B --> E[<b>Feature Concatenation</b>]
    D --> E
    E -->|Forward Pass| F[Loss Calculation]
    F --> G[Gradient Update]
    G --> H[Updated Model Weights]

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Evaluation Process

%%{init: {'theme': 'forest'}}%%
flowchart TD
    A[Test IMU Data] -->|Inference| B[<b>CNN Trunk</b>]
    C[Test Relational Data] -->|Inference| D[<b>GNN Trunk</b>]
    B --> E[<b>Feature Concatenation</b>]
    D --> E
    E -->|Predictions| F[Classification Results]
    E -->|Predictions| G[Regression Results]

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Results Overview

%%{init: {'theme': 'forest'}}%%
flowchart TD
    A[Classification Results] -->|Accuracy| B[Final Character Prediction]
    C[Regression Results] -->|RMSE| D[Reconstructed Trajectory]
    B --> E[Performance Metrics]
    D --> E
    E --> F[Final Evaluation Report]

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Paper and References

This project builds upon the methodology presented in:

• Paper: Joint Classification and Trajectory Regression of Online Handwriting Using a Multi-Task Learning Approach.
• Citation:

@inproceedings{ott2022joint,
  title={Joint Classification and Trajectory Regression of Online Handwriting Using a Multi-Task Learning Approach},
  author={Ott, Felix and Rügamer, David and Heublein, Lucas and Bischl, Bernd and Mutschler, Christopher},
  booktitle={IEEE Winter Conference on Applications of Computer Vision (WACV)},
  pages={266--276},
  year={2022}
}

Resources

• Dataset: OnHW Dataset
• Paper: PDF

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License

License

This project is licensed under the InkShare IMU2Text License, granted by Vahi Software Solutions. The license allows academic, personal, and non-commercial use. Commercial use requires prior authorization.

For details, see the LICENSE file.

For inquiries about commercial use or licensing, please contact: vkosuri@inkshare.in.