From 331c5c2deae09c5861db6519fc485db79018d7c6 Mon Sep 17 00:00:00 2001
From: David Schmenk <dschmenk@sbcglobal.net>
Date: Fri, 13 Dec 2024 13:32:44 -0800
Subject: [PATCH] Calc true NTSC color wheel values for RGB

---
 src/dhgr.tk/utils/dhgrrgb.pla | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/dhgr.tk/utils/dhgrrgb.pla b/src/dhgr.tk/utils/dhgrrgb.pla
index fea0d3b..445dc4c 100644
--- a/src/dhgr.tk/utils/dhgrrgb.pla
+++ b/src/dhgr.tk/utils/dhgrrgb.pla
@@ -73,12 +73,20 @@ def luv2rgb(l, u, v)#3 // l = fix 1.7, u, v = fix s.15; return fix 0.8 * 3
 end
 
 def calcChroma(angle)#0
-  var r, g, b
+  var l, r, g, b
   byte i
 
+  l = 256 - contrast
   for i = 0 to 3
     // Calculate and NTSC RGB for this DHGR pixel
-    r, g, b = luv2rgb(256 - contrast, cos(angle), sin(angle))
+    // According to what I could find out about the NTSC color wheel:
+    //   Red maxes at 103.5 degrees
+    //   Green maxes at 240.7 degrees
+    //   Blue maxes at 347.1 degrees
+    r = max(0, l + (cos(angle - 104) >> 7))
+    g = max(0, l + (cos(angle - 241) >> 7))
+    b = max(0, l + (cos(angle - 347) >> 7))
+    //r, g, b = luv2rgb(256 - contrast, cos(angle), sin(angle))
     ntscChroma[i*3 + RED] = (r + 2) >> 2
     ntscChroma[i*3 + GRN] = (g + 2) >> 2
     ntscChroma[i*3 + BLU] = (b + 2) >> 2