From 65eb2edc121a198ac6f02fc691152e4e46b19a08 Mon Sep 17 00:00:00 2001
From: jespergravgaard <jesper@balmangravgaard.dk>
Date: Sat, 3 Apr 2021 21:00:48 +0200
Subject: [PATCH] Added better rounding after division.

---
 src/main/kc/include/mega65-math.h         |  3 +++
 src/test/kc/examples/mega65/linedrawing.c | 21 +++++++++++----------
 2 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/src/main/kc/include/mega65-math.h b/src/main/kc/include/mega65-math.h
index 5abfad3e3..e2e16d78f 100644
--- a/src/main/kc/include/mega65-math.h
+++ b/src/main/kc/include/mega65-math.h
@@ -18,6 +18,9 @@ char * const MATH_BUSY = 0xd70f;
 // $D768-$D76F DIVOUT 64-bit output of MULTINA ÷ MULTINB
 // $D768-$D76B DIVOUT FRAC 32-bit output of MULTINA ÷ MULTINB
 signed char volatile * const MATH_DIVOUT_FRAC_CHAR0 = 0xd768;
+signed char volatile * const MATH_DIVOUT_FRAC_CHAR1 = 0xd769;
+signed char volatile * const MATH_DIVOUT_FRAC_CHAR2 = 0xd76a;
+signed char volatile * const MATH_DIVOUT_FRAC_CHAR3 = 0xd76b;
 signed int  volatile * const MATH_DIVOUT_FRAC_INT0  = 0xd768;
 signed int  volatile * const MATH_DIVOUT_FRAC_INT1  = 0xd76a;
 signed long volatile * const MATH_DIVOUT_FRAC_LONG0 = 0xd768;
diff --git a/src/test/kc/examples/mega65/linedrawing.c b/src/test/kc/examples/mega65/linedrawing.c
index 3c4a13e36..d70d6c2e3 100644
--- a/src/test/kc/examples/mega65/linedrawing.c
+++ b/src/test/kc/examples/mega65/linedrawing.c
@@ -216,9 +216,9 @@ void draw_line(int x1, int y1, int x2, int y2, unsigned char colour) {
       y2 = temp;
     }
 
-    // Use hardware divider to get the slope (add 1/2 to get better rounding )
-    *MATH_MULTINA_INT0 = dx*2+1;
-    *MATH_MULTINB_INT0 = dy*2;
+    // Use hardware divider to get the slope 
+    *MATH_MULTINA_INT0 = dx;
+    *MATH_MULTINB_INT0 = dy;
     *MATH_MULTINA_INT1 = 0;
     *MATH_MULTINB_INT1 = 0;
     
@@ -229,9 +229,9 @@ void draw_line(int x1, int y1, int x2, int y2, unsigned char colour) {
       lda MATH_DIVOUT_FRAC_INT1 @nooptimize
       lda MATH_DIVOUT_FRAC_INT1 @nooptimize
     }
-    // Slope is the most significant bytes of the fractional part
-    // of the division result
-    unsigned int slope = (unsigned int)*MATH_DIVOUT_FRAC_INT1;
+    // Slope is the most significant bytes of the fractional part of the division result
+    // Perform rounding by examining the next bit also
+    unsigned int slope = (unsigned int)*MATH_DIVOUT_FRAC_INT1 + (((char)*MATH_DIVOUT_FRAC_CHAR1&0x80)?1:0);
     unsigned int slope_init = 32768;
     unsigned long addr = GRAPHICS + (unsigned int)(x1/8) * 64 * 25 + (unsigned int)(y1*8) + (unsigned char)(x1&7);
     unsigned int count = (unsigned int)dy;
@@ -249,9 +249,9 @@ void draw_line(int x1, int y1, int x2, int y2, unsigned char colour) {
       y2 = temp;
     }
 
-    // Use hardware divider to get the slope (add 1/2 to get better rounding )
-    *MATH_MULTINA_INT0 = dy*2+1;
-    *MATH_MULTINB_INT0 = dx*2;
+    // Use hardware divider to get the slope 
+    *MATH_MULTINA_INT0 = dy;
+    *MATH_MULTINB_INT0 = dx;
     *MATH_MULTINA_INT1 = 0;
     *MATH_MULTINB_INT1 = 0;
 
@@ -264,7 +264,8 @@ void draw_line(int x1, int y1, int x2, int y2, unsigned char colour) {
     }
 
     // Slope is the most significant bytes of the fractional part of the division result
-    unsigned int slope = (unsigned int)*MATH_DIVOUT_FRAC_INT1;
+    // Perform rounding by examining the next bit also
+    unsigned int slope = (unsigned int)*MATH_DIVOUT_FRAC_INT1 + (((char)*MATH_DIVOUT_FRAC_CHAR1&0x80)?1:0);
     unsigned long addr = GRAPHICS + (unsigned int)(x1/8) * 64 * 25 + (unsigned int)(y1*8) + (unsigned char)(x1&7);
     unsigned int count = (unsigned int)dx;
     char is_slope_negative = ((y2 - y1) < 0) ? 1 : 0;