From be83329b2d7b6c3395c13bedcf151e0fffd3a958 Mon Sep 17 00:00:00 2001
From: Alexander Buhl <63983202+Buhlean@users.noreply.github.com>
Date: Wed, 14 Jul 2021 00:18:35 +0200
Subject: [PATCH] Implemented remaining 7/8 of ImageDrawLine

The existing code was correct for one octant, it now works for all 8
Added two internal functions, _ImageDrawLineHorizontal and _ImageDrawLineVertical, whithout which it would've been 4 times as much code.
---
 src/textures.c | 105 +++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 93 insertions(+), 12 deletions(-)

diff --git a/src/textures.c b/src/textures.c
index 9bef90c4c..15b5f6d6f 100644
--- a/src/textures.c
+++ b/src/textures.c
@@ -2415,21 +2415,102 @@ void ImageDrawPixelV(Image *dst, Vector2 position, Color color)
     ImageDrawPixel(dst, (int)position.x, (int)position.y, color);
 }
 
-// Draw line within an image
-void ImageDrawLine(Image *dst, int startPosX, int startPosY, int endPosX, int endPosY, Color color)
-{
-    int m = 2*(endPosY - startPosY);
-    int slopeError = m - (endPosX - startPosX);
+void _ImageDrawLineHorizontal(Image *dst, int startPosX, int startPosY, int endPosX, int endPosY, Color color){
 
-    for (int x = startPosX, y = startPosY; x <= endPosX; x++)
+    int     changeInX = (endPosX - startPosX);
+    int abs_changeInX = changeInX < 0 ? -changeInX : changeInX;
+    int     changeInY = (endPosY - startPosY);
+    int abs_changeInY = changeInY < 0 ? -changeInY : changeInY;
+    
+    // we need to go up or down depending (-1 or 1);
+    int y_directional_unit = changeInY < 0 ? -1 : 1;
+    
+    int A =     2* abs_changeInY;
+    int B = A - 2* abs_changeInX;
+    int P = A -    abs_changeInX;
+    
+    ImageDrawPixel(dst, startPosX, startPosY, color);
+    
+    for (int x = startPosX+1, y = startPosY; x <= endPosX; x += 1) 
     {
-        ImageDrawPixel(dst, x, y, color);
-        slopeError += m;
-
-        if (slopeError >= 0)
+        if (P >= 0) 
         {
-            y++;
-            slopeError -= 2*(endPosX - startPosX);
+            y += y_directional_unit;
+            P += B;
+        }
+        else
+        {
+            P += A;
+        }
+        
+        ImageDrawPixel(dst, x, y, color);
+    }
+}
+        
+void _ImageDrawLineVertical(Image *dst, int startPosX, int startPosY, int endPosX, int endPosY, Color color){
+    // same code as *Horizontal but with all x and y flipped
+    
+    int     changeInX = (endPosX - startPosX);
+    int abs_changeInX = changeInX < 0 ? -changeInX : changeInX;
+    int     changeInY = (endPosY - startPosY);
+    int abs_changeInY = changeInY < 0 ? -changeInY : changeInY;
+    
+    // we need to go left or right depending (-1 or 1)
+    int x_directional_unit = changeInX < 0 ? -1 : 1;
+    
+    int A =     2* abs_changeInX; 
+    int B = A - 2* abs_changeInY;
+    int P = A -    abs_changeInY;
+    
+    ImageDrawPixel(dst, startPosX, startPosY, color);
+    
+    for (int y = startPosY+1, x = startPosX; y <= endPosY; y += 1) 
+    {
+        if (P >= 0) 
+        {
+            x += x_directional_unit;
+            P += B;
+        }
+        else
+        {
+            P += A;
+        }
+        
+        ImageDrawPixel(dst, x, y, color);
+    }
+}
+        
+// Draw line within an image
+void ImageDrawLine (Image *dst, int startPosX, int startPosY, int endPosX, int endPosY, Color color){
+    // Using Bresenham's algorithm as described in 
+    // Drawing Lines with Pixels - Joshua Scott - March 2012
+    // https://classic.csunplugged.org/wp-content/uploads/2014/12/Lines.pdf
+    
+    int     changeInX = (endPosX - startPosX);
+    int abs_changeInX = changeInX < 0 ? -changeInX : changeInX;
+    int     changeInY = (endPosY - startPosY);
+    int abs_changeInY = changeInY < 0 ? -changeInY : changeInY;
+
+    if (abs_changeInY < abs_changeInX) 
+    {
+        if (changeInX > 0) 
+        {
+            _ImageDrawLineHorizontal(dst, startPosX, startPosY, endPosX, endPosY, color);
+        }
+        else
+        {
+            _ImageDrawLineHorizontal(dst, endPosX, endPosY, startPosX, startPosY, color);
+        }            
+    }
+    else 
+    {
+        if (changeInY > 0) 
+        {
+            _ImageDrawLineVertical(dst, startPosX, startPosY, endPosX, endPosY, color);
+        }
+        else
+        {
+            _ImageDrawLineVertical(dst, endPosX, endPosY, startPosX, startPosY, color);
         }
     }
 }