Contour Line Visualization in Java

This project implements Contour Line Visualization using the Marching Squares algorithm in Java. It reads a 2D grid of floating-point values from a CSV file and renders contour lines (isolines) over a colorized background that represents scalar values. It also supports resolution-independent rendering with bilinear interpolation.

Introduction

Contour lines, also known as isolines, are used to represent regions of equal value within a 2D scalar field — commonly used in elevation maps, fluid dynamics, and medical imaging.

This implementation uses the Marching Squares algorithm to trace contours and visualize them with customizable resolution, color gradients, and image size. The project is self-contained and uses only standard Java libraries.

The system supports:

• CSV-based Input: A CSV file where the first line defines the contour step (scale), and the remaining lines contain float values.
• Color Mapping:
  • Values ≤ 0: light blue (water).
  • Values > 0 and < scale: green.
  • Values ≥ scale: gradient from yellow to red (based on relative intensity).
• Contour Generation: Curves are drawn at each scalar level based on the contour step.
• Bilinear Interpolation: Grid is scaled to match the output resolution with smooth value transitions.
• Custom Image Size and Styling: Define output image dimensions, stroke width, and scale factor per cell.

Usage Instructions

Requirements

• Java 8 or newer;
• No external dependencies (pure Java).

Example Usage

public class Main {
    public static void main(String[] args) throws IOException {
        new ContourLineBuilder()
            .setOutputSize(600, 600)
            .setScale(10)
            .setStrokeWidth(1)
            .loadCSV("dataset/sample.csv")
            .generate("contour_output.jpg");
    }
}

CSV Format

The first line defines the scale step (used for both coloring and contour thresholds). The following lines define a rectangular grid of decimal values:

0.5
1.0,1.2,1.5,1.3
1.5,1.8,2.0,1.7
...

Output

• Color background is computed from scalar values.
• Contour lines are overlaid in black for visibility.
• Output image is saved to target/output/.

Project Structure

• ContourLineBuilder.java — Main class to load CSV, compute interpolation and draw contours.
• MarchingSquares.java — Contains the implementation of the Marching Squares algorithm.
• ImageUtils.java — Utility methods for image byte conversion and JPG output.

License

The available source codes here are under the Apache License, version 2.0 (see the attached LICENSE file for more details). Any questions can be submitted to my email: carloswilldecarvalho@outlook.com.

References

[1] W. E. Lorensen and H. E. Cline, "Marching Cubes: A High Resolution 3D Surface Construction Algorithm", Proceedings of SIGGRAPH '87.

[2] P. Bourke, "Contour Plotting Using the Marching Squares Algorithm", 1994. Available at: http://paulbourke.net/geometry/polygonise/.

[3] M. De Smith, M. F. Goodchild, and P. A. Longley, "Geospatial Analysis: A Comprehensive Guide", 6th edition, 2020. Available at: https://www.spatialanalysisonline.com