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Loghi-core HTR

Loghi HTR is a system to generate text from images. It's part of the Loghi framework, which consists of several tools for layout analysis and HTR (Handwritten Text Recogntion).

Loghi HTR also works on machine printed text.

Table of Contents

  1. Installation
  2. Usage
  3. Creating Models
  4. API Usage Guide
  5. Model Visualizer Guide
  6. Frequently Asked Questions (FAQ)

Installation

This section provides a step-by-step guide to installing Loghi HTR and its dependencies.

Prerequisites

Ensure you have the following prerequisites installed or set up:

  • Ubuntu or a similar Linux-based operating system. The provided commands are tailored for such systems.

Important

The requirements listed in requirements.txt require a Python version > 3.9. This tensorflow version requires a Python version <= 3.11.

Steps

  1. Install Python 3
sudo apt-get install python3
  1. Clone and install CTCWordBeamSearch
git clone https://github.com/githubharald/CTCWordBeamSearch
cd CTCWordBeamSearch
python3 -m pip install .
  1. Clone the HTR repository and install its requirements
git clone https://github.com/knaw-huc/loghi-htr.git
cd loghi-htr
python3 -m pip install -r requirements.txt

With these steps, you should have Loghi HTR and all its dependencies installed and ready to use.

Usage

Setting Up

  1. (Optional) Organize Text Line Images

    While not mandatory, for better organization, you can place your text line images in a 'textlines' folder or any desired location. The crucial point is that the paths mentioned in 'lines.txt' should be valid and point to the respective images.

  2. Generate a 'lines.txt' File

    This file should contain the locations of the image files and their respective transcriptions. Separate each location and transcription with a tab.

Example of 'lines.txt' content:

/path/to/texline/1.png  This is a ground truth transcription
/path/to/texline/2.png  It can be generated from PageXML
/path/to/texline/3.png  And another textline

Command-Line Options and Config File Usage

Our tool provides various command-line options for stages such as training, validation, and inference. To simplify usage, especially for newcomers, we've introduced the option to run the script with a configuration file.

Using a Config File

Instead of using command-line arguments, you can specify parameters in a JSON configuration file. This is recommended for ease of use. To use a configuration file, run the script with:

python3 main.py --config_file "/path/to/config.json"

In the configs directory, we provide several minimal configuration files tailored to different use cases:

  • default.json: Contains default values for general use.
  • training.json: Configured specifically for training.
  • validation.json: Optimized for validation tasks.
  • inference.json: Set up for inference processes.
  • testing.json: Suitable for testing scenarios.
  • finetuning.json: Adjusted for fine-tuning purposes.

These files are designed to provide a good starting point. You can use and modify them as needed.

Overriding Config File Parameters with Command-Line Arguments

You can override specific config file parameters with command-line arguments. For example:

python3 main.py --config_file "/path/to/config.json" --gpu 1

This command will use settings from the config file but overrides the GPU setting to use GPU 1.

Available Command-Line Options

You can still use command-line arguments. Some of the options include --train_list, --do_validate, --learning_rate, --gpu, --batch_size, --epochs, etc. For a full list and descriptions, refer to the help command:

python3 main.py --help

Note

Ensure that the parameters (via config file or command-line arguments) are consistent and appropriate for your operation mode (training, validation, or inference).

Creating Models

In this project, we use the vgslify package to generate models from Variable-size Graph Specification Language (VGSL) strings. VGSL is a concise tool that enables the creation of complex neural network architectures tailored for variable-sized images. The vgslify package makes it easy to define models using a simple specification string and the --model argument.

You can either use a custom VGSL model via the --model argument or select one of the several predefined models provided by this project.

1. Using VGSLify with the --model Argument

The --model argument allows you to pass a VGSL string to define a custom model architecture. VGSLify then builds the corresponding model using the backend you specify (e.g., TensorFlow or PyTorch). For more details on how to write VGSL strings, check out the vgslify repository.

For example, you can generate a model with a convolutional layer, max-pooling layer, and a softmax output layer using the --model argument:

python3 src/main.py --model "None,None,64,1 Cr3,3,32 Mp2,2,2,2 O1s92" ...

2. Using Predefined Models

Alternatively, you can choose from several predefined models that are optimized for different tasks. One of the simplest models you can try is --model modelkeras, which is based on a similar model from the Keras Captcha OCR tutorial. You can use this by running the following:

python3 src/main.py --model modelkeras ...

3. Recommended Starting Models

A good starting point is the recommended model, which offers a balanced architecture for speed and accuracy. This model can be used with the following command:

python3 src/main.py --model recommended ...

4. Full Model Library

Here are the available predefined models:

  • modelkeras: A basic model inspired by the Keras Captcha OCR example.
  • model9 to model16: These models vary in complexity, depth, and the number of bidirectional LSTMs.
  • recommended: A well-balanced model for general tasks, incorporating convolutional layers, batch normalization, max pooling, and bidirectional LSTMs with dropout.

Each model is designed to tackle specific use cases and input/output configurations, and you can explore each by using the corresponding --model argument. For more details, refer to the VGSL specification or check out the available models in the model library within the project.

API Usage Guide

This guide walks you through the process of setting up and running the API, as well as how to interact with it.

1. Setting up the API

Navigate to the src/api directory in your project:

cd src/api

Starting the API

You have the choice to run the API using either gunicorn (recommended) or flask. To start the server:

Using gunicorn:

gunicorn 'app:create_app()'

Environment Variables Configuration

Before running the app, you must set several environment variables. The app fetches configurations from these variables:

Gunicorn Options:

GUNICORN_RUN_HOST        # Default: "127.0.0.1:8000": The host and port where the API should run.
GUNICORN_ACCESSLOG       # Default: "-": Access log settings.

Loghi-HTR Options:

LOGHI_MODEL_PATH         # Path to the model.
LOGHI_BATCH_SIZE         # Default: "256": Batch size for processing.
LOGHI_OUTPUT_PATH        # Directory where predictions are saved.
LOGHI_MAX_QUEUE_SIZE     # Default: "10000": Maximum size of the processing queue.
LOGHI_PATIENCE           # Default: "0.5": Maximum time to wait for new images before predicting current batch

Important Note: The LOGHI_MODEL_PATH must include a config.json file that contains at least the channels key, along with its corresponding model value. This file is expected to be automatically generated during the training or fine-tuning process of a model. Older versions of Loghi-HTR (< 1.2.10) did not do this automatically, so please be aware that our generic-2023-02-15 model lacks this file by default and is configured to use 1 channel.

GPU Options:

LOGHI_GPUS               # Default: "0": GPU configuration.

Security Options:

SECURITY_ENABLED         # Default: "false": Enable or disable API security.
SECURITY_KEY_USER_JSON   # JSON string with API key and associated user data.

You can set these variables in your shell or use a script. An example script to start a gunicorn server can be found in src/api/start_local_app.sh or src/api/start_local_app_with_security.sh for using security.

2. Interacting with the running API

Once the API is up and running, you can send HTR requests using curl. Here's how:

curl -X POST -F "image=@$input_path" -F "group_id=$group_id" -F "identifier=$filename" http://localhost:5000/predict

Replace $input_path, $group_id, and $filename with your respective file paths and identifiers. If you're considering switching the recognition model, use the model field cautiously:

  • The model field (-F "model=$model_path") allows for specifying which handwritten text recognition model the API should use for the current request.
  • To avoid the slowdown associated with loading different models for each request, it is preferable to set a specific model before starting your API by using the LOGHI_MODEL_PATH environment variable.
  • Only use the model field if you are certain that a different model is needed for a particular task and you understand its performance characteristics.

Warning

Continuous model switching with $model_path can lead to severe processing delays. For most users, it's best to set the LOGHI_MODEL_PATH once and use the same model consistently, restarting the API with a new variable only when necessary.

Optionally, you can add "whitelist=" fields to add extra metadata to your output. The field values will be used as keys to lookup values in the model config.

Security and Authentication:

If security is enabled, you need to first authenticate by obtaining a session key. Use the /login endpoint with your API key:

curl -v -X POST -H "Authorization: Bearer <your_api_key>" http://localhost:5000/login

Your session key will be returned in the header of the response. Once authenticated, include the received session key in the Authorization header for all subsequent requests:

curl -X POST -H "Authorization: Bearer <your_session_key>" -F "image=@$input_path" ... http://localhost:5000/predict

3. Server Health Check

To check the health of the server, simply run:

curl http://localhost:5000/health

This will respond with a 500 error, and an "unhealthy" status if one of the processes has crashed. Otherwise, it will respond with a 200 error, and a corresponding "healthy" status.

This guide should help you get started with the API. For advanced configurations or troubleshooting, please reach out for support.

Model Visualizer Guide

The following instructions will explain how to generate visualizations that can help describe an existing model's learned representations when provided with a sample image. The visualizer requires a trained model and a sample image (e.g. PNG or JPG):

Example time-step prediction

Fig.1 - Time-step Prediction Visualization.

Example Conv Visualization

Fig.2 - Convolutional Layer Activation Visualization.

1. Visualize setup

Navigate to the src/visualize directory in your project:

cd src/visualize

2. Start the visualizers

python3 main.py 
--model /path/to/existing/model 
--sample_image /path/to/sample/img

This will output various files into the visualize_plots directory:

  • A PDF sheet consisting of all made visualizations for the above call
  • Individual PNG and JPG files of these visualizations
  • A sample_image_preds.xslx which consist of a character prediction table for each prediction timestep. The highest probability is the character that was chosen by the model

Currently, the following visualizers are implemented:

  1. visualize_timestep_predictions: Takes the sample_image and simulates the model's prediction process for each time step, the top-3 most probable characters per timestep are displayed and the "cleaned" result is shown at the bottom.
  2. visualize_filter_activations: Display what the convolutional filters have learned after providing it with random noise + show the activation of conv filters for the sample_image. Each unique convolutional layer is displayed once.

Potential future implementations:

  • Implement a SHAP visualizer to show the parts of the image that influence the model's character prediction. Or a similar saliency plot.
  • Plot the raw Conv filters (e.g. a 3x3 filter)

Note: If a model has multiple Cr3,3,64 layers then only the first instance of this configuration is visualized)

3. (Optional parameters)

--do_detailed # Visualize all convolutional layers, not just the first instance of a conv layer
--dark_mode  # Plots and overviews are shown in dark mode (instead of light mode)
--num_filters_per_row # Changes the number of filters per row in the filter activation plots (default =6)
# NOTE: increasing the num_filters_per_row requires significant computing resources, you might experience an OOM.

FAQ

If you're new to using this tool or encounter issues, this FAQ section provides answers to common questions and problems. If you don't find your answer here, please reach out for further assistance.

How Can I Use One of the Loghi HTR Models in My Own Project?

To integrate a Loghi HTR model into your project, follow these steps:

  1. Obtain the Model: First, you need to get the HTR model file. This could be done by training a model yourself or downloading a pre-trained model here or here.

  2. Loading the Model for Inference:

    • Install TensorFlow in your project environment if you haven't already.

    • Load the model using TensorFlow's tf.keras.models.load_model function. Here's a basic code snippet to help you get started:

      import tensorflow as tf

model_file = 'path_to_your_model.keras'  # Replace with your model file path
model = tf.keras.models.load_model(model_file, compile=False)

    • Setting compile=False is crucial as it indicates the model is being loaded for inference, not training.

  3. Using the Model for Inference:

    • Once the model is loaded, you can use it to make predictions on handwritten text images.
    • Prepare your input data (images of handwritten text) according to the model's expected input format.
    • Use the model.predict() method to get the recognition results.

  4. Note on Training:

    • The provided model is pre-trained and configured for inference purposes.
    • If you wish to retrain or fine-tune the model, this must be done within the Loghi framework, as the model structure and training configurations are tailored to their system.

How can I determine the VGSL spec of a model I previously used?

If you've used one of our models and would like to know its VGSL specification, you can now use the vgslify package to generate the VGSL spec directly from your model. Follow the steps below:

  1. Load your model as usual (either from a saved file or from memory).
  2. Use the vgslify.utils.model_to_spec function to generate the VGSL spec string.

Example:

from vgslify.utils import model_to_spec
vgsl_spec_string = model_to_spec(model)
print(vgsl_spec_string)

Replace model with your loaded TensorFlow model.

How do I use replace_recurrent_layer?

The replace_recurrent_layer is a feature that allows you to replace the recurrent layers of an existing model with a new architecture defined by a VGSL string. To use it:

  1. Specify the model you want to modify using the --model argument.
  2. Provide the VGSL string that defines the new recurrent layer architecture with the --replace_recurrent_layer argument. The VGSL string describes the type, direction, and number of units for the recurrent layers. For example, "Lfs128 Lfs64" describes two LSTM layers with 128 and 64 units respectively, with both layers returning sequences.
  3. Execute your script or command, and the tool will replace the recurrent layers of your existing model based on the VGSL string you provided.

I'm getting the following error when I want to use replace_recurrent_layer: Input 0 of layer "lstm_1" is incompatible with the layer: expected ndim=3, found ndim=2. What do I do?

This error usually indicates that there is a mismatch in the expected input dimensions of the LSTM layer. Often, this is because the VGSL spec for the recurrent layers is missing the [s] argument, which signifies that the layer should return sequences.

To resolve this:

  • Ensure that your VGSL string for the LSTM layer has an s in it, which will make the layer return sequences. For instance, instead of "Lf128", use "Lfs128".
  • Re-run the script or command with the corrected VGSL string.