FS#13970 - lcd_drawline() different depending on drawing direction try#2

commit dcdb539ca54e90700d4a6ce20350e34f700a96f7
Author: William Wilgus <[email protected]>
Date:   Wed Aug 5 11:40:02 2026 -0400

    FS#13970 - lcd_drawline() different depending on drawing direction try#2
    
    Last patch didn't swap both x and y so was wrong and messed up the coord pair
    I didn't want to add more swapping due to extra overhead
    
    ultimately the issue is that you have 1/2 pixel error (-1 0 1)
    and when doing the reverse line the error is on the wrong side
    so instead get the first point from the opposite dinc
    
    Change-Id: I141c4af36a601314f2b509a9ebfcfab3b5213bcd

diff --git a/firmware/drivers/lcd-bitmap-common.c b/firmware/drivers/lcd-bitmap-common.c
index c4009155fa..e1ebd7267b 100644
--- a/firmware/drivers/lcd-bitmap-common.c
+++ b/firmware/drivers/lcd-bitmap-common.c
@@ -911,6 +911,18 @@ void LCDFN(drawline)(int x1, int y1, int x2, int y2)
         dinc2 = (deltay - deltax) * 2;
         xinc1 = 1;
         yinc1 = 0;
+        if (x1 > x2)
+        {
+            xinc1 = -xinc1;
+            xinc2 = -xinc2;
+        }
+
+        if (y1 > y2)
+        {
+            d += dinc2;
+            yinc1 = -yinc1;
+            yinc2 = -yinc2;
+        }
     }
     else
     {
@@ -920,24 +932,20 @@ void LCDFN(drawline)(int x1, int y1, int x2, int y2)
         dinc2 = (deltax - deltay) * 2;
         xinc1 = 0;
         yinc1 = 1;
-    }
-    numpixels++; /* include endpoints */
-
-    if (x1 > x2)
-    {
-        /* swap their identities */
-        int x0 = x2;
-        x2 = x1;
-        x1 = x0;
-    }
+        if (x1 > x2)
+        {
+            d += dinc2;
+            xinc1 = -xinc1;
+            xinc2 = -xinc2;
+        }
 
-    if (y1 > y2)
-    {
-        /* swap their identities */
-        int y0 = y2;
-        y2 = y1;
-        y1 = y0;
+        if (y1 > y2)
+        {
+            yinc1 = -yinc1;
+            yinc2 = -yinc2;
+        }
     }
+    numpixels++; /* include endpoints */
 
     x = x1;
     y = y1;
-- 
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