From c3c0481efb4d98df0a76cf57df65f8f165a4376d Mon Sep 17 00:00:00 2001
From: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Date: Tue, 12 Aug 2008 04:01:18 -0300
Subject: [PATCH] Use Bresenham to draw the line between faces

This solves the problem of the slope and direction when drawing lines.
---
 movie.c | 44 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 37 insertions(+), 7 deletions(-)

diff --git a/movie.c b/movie.c
index 42310e6..4184b82 100644
--- a/movie.c
+++ b/movie.c
@@ -22,6 +22,12 @@
 #include <SDL.h>
 #include <SDL_image.h>
 
+#define SWAP(x, y) do { \
+	x ^= y; y ^= x; x ^= y; \
+	} while (0)
+
+#define ABS(x) ((x) < 0 ? -(x) : (x))
+
 #define IS_CENTER(cx, cy, x, y) \
 	((x + WIDTH/2 == cx) && (y + HEIGHT/2 == cy))
 
@@ -34,19 +40,43 @@ SDL_Rect *
 GetNextPoint (void)
 {
   static SDL_Rect rect = {0, 0, WIDTH, HEIGHT};
-  static int cur = 0;
-  int dx, dy;
+  static int cur = -1;
+  static int inc, err, thre, swap;
+  static int x1, y1, x2, y2;
+  static int x, y;
   int next;
   next = (cur + 1) % (sizeof (points) / sizeof (SDL_Rect));
-  if (IS_CENTER (points[next].x, points[next].y, rect.x, rect.y))
+  if (IS_CENTER (points[next].x, points[next].y, rect.x, rect.y) || cur == -1)
   {
     cur = next;
     next = (cur + 1) % (sizeof (points) / sizeof (SDL_Rect));
+    err = 0;
+    swap = 0;
+    x1 = points[cur].x;
+    y1 = points[cur].y;
+    x2 = points[next].x;
+    y2 = points[next].y;
+    inc = y2 - y1;
+    thre = x2 - x1;
+    if (ABS (inc) > ABS (thre))
+    {
+      SWAP (inc, thre);
+      SWAP (x1, y1);
+      SWAP (x2, y2);
+      swap = 1;
+    }
+    x = x1;
+    y = y1;
+  }
+  rect.x = (swap ? y : x) - WIDTH/2;
+  rect.y = (swap ? x : y) - HEIGHT/2;
+  (x2 < x1) ? x-- : x++;
+  err += ABS (inc);
+  if (err >= ABS (thre))
+  {
+    err -= ABS (thre);
+    y += (inc < 0) ? -1 : 1;
   }
-  dx = (points[next].x > points[cur].x) ? 1 : -1;
-  dy = (points[next].y > points[cur].y) ? 1 : -1;
-  rect.x = (rect.x + dx) % WIDTH;
-  rect.y = (rect.y + dy) % HEIGHT;
   return &rect;
 }
 
-- 
2.20.1