sneak/Shared/Line.cs

using Microsoft.Xna.Framework;
using System;

namespace SemiColinGames {
  public static class Line {
    public static Point[] Rasterize(Point p1, Point p2) {
      return Line.Rasterize(p1.X, p1.Y, p2.X, p2.Y);
    }

    // Rasterizes a line using Bresenham's line-drawing algorithm.
    //
    // References:
    // https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
    // http://members.chello.at/~easyfilter/bresenham.html
    // http://members.chello.at/~easyfilter/Bresenham.pdf (section 1.6)
    public static Point[] Rasterize(int x1, int y1, int x2, int y2) {
      int dx = Math.Abs(x2 - x1);
      int stepX = x1 < x2 ? 1 : -1;
      int dy = -Math.Abs(y2 - y1);
      int stepY = y1 < y2 ? 1 : -1;
      int error = dx + dy;
      int errorXY;  // Error value e_xy from the PDF.
      // The size of the output is the size of the longer dimension, plus one.
      int resultSize = Math.Max(dx, -dy) + 1;
      // TODO: an array or list should be passed in by the caller.
      var result = new Point[resultSize];

      int i = 0;
      result[0] = new Point(x1, y1);
      while (x1 != x2 || y1 != y2) {
        i++;
        errorXY = 2 * error;
        if (errorXY >= dy) {  // e_xy + e_x > 0
          error += dy;
          x1 += stepX;
        }
        if (errorXY <= dx) {  // e_xy + e_y < 0
          error += dx;
          y1 += stepY;
        }
        result[i] = new Point(x1, y1);
      }
      return result;
    }
  }
}