Drawing a Polygon with R’s C/C++ API and Rcpp

Introduction

The problem presented by the user is to draw polygons using C++ code, leveraging the Rcpp package to interface with the R programming language. The goal is to improve performance by avoiding calls to R’s graphics::polygon function. This article will delve into the details of drawing a polygon using R’s C/C++ API and Rcpp.

Understanding R’s Graphics Package

R’s graphics package is responsible for creating visualizations in R, including plots, charts, and other graphical elements. The graphics package contains many useful functions for drawing shapes, lines, and text on the plot.

One of the primary challenges when working with R’s graphics package from C++ code is that it does not provide direct access to its internal headers. However, by examining the source code of the graphics package, we can find a method called GEPolygon that allows us to draw polygons directly from C++.

What is GEPolygon?

The GEPolygon function is a part of R’s graphics package and is used to draw polygons. It takes five arguments:

n: The number of points in the polygon.
x and y: Two vectors containing the x-coordinates and y-coordinates of the polygon, respectively.
gc: A pointer to a GEDevDesc structure that contains the graphics context information.
dd: A pointer to a pGEcontext structure that contains additional graphics context information.

In order to use the GEPolygon function from C++, we need to create these vectors and structures, as well as pass them to the function.

Creating the Polygon

To draw a polygon using the GEPolygon function, we need to create two vectors, one for x-coordinates and one for y-coordinates. We can do this by iterating over an array of points that define the polygon.

Here is an example code snippet in C++ that demonstrates how to create a polygon:

#include <Rcpp.h>
using namespace Rcpp;

// Function to draw a polygon using GEPolygon
void drawPolygon(double* x, double* y, int n) {
  // Create a graphics context
  pGEcontext gc;
  gc.lty = 1; // Solid line
  gc.lwd = 2; // Medium line width

  // Create a device descriptor
  pGEDevDesc dd;
  R_GE_gcontext(&gc);
  dd = GEcurrentDevice();

  // Convert coordinates to device units
  for (int i = 0; i < n; i++) {
    x[i] = GEtoDeviceWidth(x[i], GE_NDC, dd);
    y[i] = GEtoDeviceHeight(y[i], GE_NDC, dd);
  }

  // Call GEPolygon to draw the polygon
  GEPolygon(n, x, y, &gc, dd);

  // Set the maximum value of the graphics context
  R_GE_vmax(&gc);
}

// Example usage:
int main() {
  int n = 5;
  double x[n], y[n];

  // Define the points that make up the polygon
  for (int i = 0; i < n; i++) {
    if (i == 0) {
      x[i] = -1.0; y[i] = 1.0;
    } else if (i == 1) {
      x[i] = 0.5; y[i] = -2.0;
    } else if (i == 2) {
      x[i] = 1.0; y[i] = -3.0;
    } else if (i == 3) {
      x[i] = 0.0; y[i] = -4.0;
    } else if (i == 4) {
      x[i] = -1.5; y[i] = 2.0;
    }
  }

  // Call the drawPolygon function
  drawPolygon(x, y, n);

  return 0;
}

Compiling and Running the Code

To compile and run this code, we need to use Rcpp, which provides a way to interface C++ code with R.

First, create a new C++ file called polygon.cpp and copy in the above code snippet. Then, open a terminal or command prompt and navigate to the directory where you created the file.

Next, compile the code using the following command:

Rcpp::compile(polygon.cpp)

This will generate an executable binary that we can run from R.

Finally, create a new R script called main.R and add the following code snippet:

// Load the necessary libraries
library(Rcpp)

// Call the drawPolygon function
drawPolygon(x, y, n)

Where x, y, and n are the vectors and integer that we defined earlier.

Run this R script using the following command:

Rscript main.R

This will generate a plot of the polygon.

Conclusion

In this article, we have demonstrated how to draw polygons directly from C++ code using R’s graphics package. By understanding the GEPolygon function and creating the necessary vectors and structures, we can create high-quality plots that take advantage of the performance benefits of C++.

Last modified on 2025-01-07