Digital Ink Features

This is a reference implementation of digital ink features for behavior characterization with wrappers for Rust and Python as part of our paper "A categorisation and implementation of digital pen features for behaviour characterisation" (https://arxiv.org/abs/1810.03970).

Implementation

We distribute pre-compiled binaries which were written in Rust, these can be found in the bin folder. We also provide wrappers for Python inside the python folder and for Rust inside the rust folder. We support 3 input modes for digital ink data:

• Loading a .json file from a given filepath (for the specification of the JSON format see below).
• Loading a JSON string (for the specification of the JSON format see below).
• Loading ink in native format of the Python/Rust language. Inside the data folder we provide samples of digital ink encoded in our digital ink JSON format which is explained below.

Requirements

The example code was tested on Windows 10 (x64) with Python 3.7.

Python

Quick example taken from python/main.py, it loads a file and calculates the 2 specified features:

import digital_ink_features

feature_selection = [
    "weber_feature_05_compactness",
    "willems_feature_24_duration"
]

sample_file = '../data/sample.json'
file_result = digital_ink_features.calculate_features_file(sample_file, feature_selection, {})
print(file_result)

Rust

Quick example taken from rust/src/main.rs, it loads a file and calculates the 2 specified features:

let feature_selection = vec!(
    String::from("weber_feature_05_compactness"),
    String::from("willems_feature_24_duration")
);

let file_result = features_file(String::from("../data/sample.json"), feature_selection.clone(), HashMap::new());
println!("{:?}", file_result);

Features

A full list of features including their mathematical definition can be found in the Appendix section of our technical report (https://arxiv.org/abs/1810.03970). We provide reference implementations for the following feature sets:

• rubine
• weber
• hbf49
• willems

As you can see in the example code, you can select which features to compute by supplying a list of keys to the binary. A full list of feature keys is provided in the feature_keys.txt file. The structure of each key is as follows:

{feature set}_feature_{index}_{acronym}

Examples:

weber_feature_05_compactness
rubine_feature_01_cosine_initial_angle
weber_feature_02_length
hbf49_feature_13_downstrokes_trajectory_proportion
hbf49_feature_22_kperpendicularity
willems_feature_24_duration

As per their definition some of these features might require additional arguments which can be provided individually per feature if necessary, otherwise default values will be used. Examples of how to configure these arguments can be found in the example code.

JSON digital ink format

We provide 2 types of containers for digital ink data: stroke and sketch. Objects of type stroke contain 4 arrays of data that have to be of same length: x coordinates as float, y coordinates as float, timestamp for timestamps as long and pressure values as float. The type field let's us distinguish between stroke and sketch as JSON itself is untyped. The meta field can be used for additional information, such as labels. Be aware that it has to be present event if empty, otherwise parsing errors will occur.

Stroke

In general a stroke is represented as follows:

{
    "type": "stroke",
    "meta": {
        "key_1": value_1,
        "key_2": value_2,
        ...
    },
    "x": [
        x_1: float,
        x_2: float,
        ...
        x_n: float
    ],
    "y": [
        y_1: float,
        y_2: float,
        ...
        y_n: float
    ],
    "timestamp": [
        timestamp_1: long,
        timestamp_2: long,
        ...
        timestamp_n: long
    ],
    "pressure": [
        pressure_1: float,
        pressure_2: float,
        ...
        pressure_n: float
    ]
}

Note that fields x and y are required, while timestamp and pressure are optional. Be aware that these still need to be present if you want to calculate respective features that require these values. timestamp values can be either relative offset values, e.g., starting at 0 or alternatively absolute values such as unix timestamps.

Here is an example with actual values:

{
    "type": "stroke",
    "meta": {},
    "x": [0.0, 1.0, 1.0, 0.0, 0.0],
    "y": [0.0, 0.0, 1.0, 1.0, 0.0],
    "timestamp": [1, 2, 3, 4, 5],
    "pressure": [1.0, 1.0, 1.0, 1.0, 1.0]
}

Sketch

A sketch is a ordered collection of multiple stroke objects:

{
    "type": "sketch",
    "meta": {
        "key_1": value_1,
        "key_2": value_2,
        ...
    },
    "strokes": [
        <stroke_1>,
        <stroke_2>
        ...
        <stroke_n>
    ]
}

Here is an example of a sketch with sample value:

{
    "type": "sketch",
    "meta": {},
    "strokes": [
        {
            "type": "stroke",
            "meta": {},
            "x": [0.0, 1.0, 1.0, 0.0, 0.0],
            "y": [0.0, 0.0, 1.0, 1.0, 0.0],
            "timestamp": [1, 2, 3, 4, 5],
            "pressure": [1.0, 1.0, 1.0, 1.0, 1.0]
        },
        {
            "type": "stroke",
            "meta": {},
            "x": [1.0, 1.0, 1.0, 1.0, 1.0],
            "y": [0.0, 0.0, 0.0, 0.0, 0.0],
            "timestamp": [1, 2, 3, 4, 5],
            "pressure": [0.4, 0.2, 0.45, 9.2134, 1.0]
        }
    ]
}

Citation

If you use our work in your research please cite the following technical report:

@misc{alex2018categorisation,
    title={A categorisation and implementation of digital pen features for behaviour characterisation},
    author={Alexander Prange and Michael Barz and Daniel Sonntag},
    year={2018},
    eprint={1810.03970},
    archivePrefix={arXiv},
    primaryClass={cs.CV}
}

License

This work is licensed under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0). You are free to use it in your research, but commercial use is prohibited. If in doubt please contact us about how you want to use our work.