Line of sight

One of the more popular methods of determining wether something in the game world is visible (or targetable) is to input the x,y coords for the target and the player and then walk along a line between them. At each step on the line, the game checks to see if anything is in the world at these coords that would prevent a player's sight (like a wall).

Taken from wikipedia: (they will delete this code soon, I thought it might find a welcome home here)

An implementation of Bresenham s line algorithm in C follows. The plot() function is not shown and is assumed to render a single point of the line in the chosen color. This variation produces solid lines of a uniform color.

void drawline2d(int x0, int y0, int x1, int y1, int color)
{
       int i;
       int steep = 1;
       int sx, sy;  /* step positive or negative (1 or -1) */
       int dx, dy;  /* delta (difference in X and Y between points) */
       int e;
 
       /*
        * inline swap. On some architectures, the XOR trick may be faster
        */
       int tmpswap;
#define SWAP(a,b) tmpswap = a; a = b; b = tmpswap;
 
       /*
        * optimize for vertical and horizontal lines here
        */
        
       dx = abs(x1 - x0);
       sx = ((x1 - x0) > 0) ? 1 : -1;
       dy = abs(y1 - y0);
       sy = ((y1 - y0) > 0) ? 1 : -1;
       if (dy > dx) {
               steep = 0;
               SWAP(x0, y0);
               SWAP(dx, dy);
               SWAP(sx, sy);
       }
       e = (dy << 1) - dx;
       for (i = 0; i < dx; i++) {
               if (steep) {
                       plot(x0,y0,color);
               } else {
                       plot(y0,x0,color);
               }
               while (e >= 0) {
                       y0 += sy;
                       e -= (dx << 1);
               }
               x0 += sx;
               e += (dy << 1);
       }
}

A slightly more optimized version of the above (might be a bit harder to read, though):

void drawline2d(int x0, int y0, int x1, int y1, int color) {
   int i;
   int sx, sy;  /* step positive or negative (1 or -1) */
   int dx, dy;  /* delta (difference in X and Y between points) */
   int dx2, dy2;
   int e;
   int temp;
 
   dx = x1 - x0;
   sx = (dx > 0) ? 1 : -1;
   if (dx < 0)
       dx = -dx;
 
   dy = y1 - y0;
   sy = (dy > 0) ? 1 : -1;
   if (dy < 0)
       dy = -dy;
 
   dx2 = dx << 1; /* dx2 = 2 * dx */
   dy2 = dy << 1; /* dy2 = 2 * dy */
 
   if (dy <= dx) { /* steep */
       e = dy2 - dx;
 
       for (i = 0; i <= dx; ++i) {
           plot(x0, y0, color);
 
           while (e >= 0) {
               y0 += sy;
               e -= dx2;
           }
 
           x0 += sx;
           e += dy2;
       }
   } else {
       /* swap x0 <-> y0 */
       temp = x0;
       x0 = y0;
       y0 = temp;
 
       /* swap dx <-> dy */
       temp = dx;
       dx = dy;
       dy = temp;
 
       /* swap dx2 <-> dy2 */
       temp = dx2;
       dx2 = dy2;
       dy2 = temp;
 
       /* swap sx <-> sy */
       temp = sx;
       sx = sy;
       sy = temp;
 
       e = dy2 - dx;
 
       for (i = 0; i <= dx; ++i) {
           plot(y0, x0, color);
 
           while (e >= 0) {
               y0 += sy;
               e -= dx2;
           }
 
           x0 += sx;
           e += dy2;
       }
   }
}