From 86c6530e4643ed0d9044ae682b8ebcac33bab0a3 Mon Sep 17 00:00:00 2001
From: Irmen de Jong <irmen@razorvine.net>
Date: Tue, 14 Nov 2023 20:40:48 +0100
Subject: [PATCH] palette: more accurate color conversion from 8 to 4 bits
 channels

set_rgb8(), color8to4(), channel8to4()
---
 compiler/res/prog8lib/cx16/palette.p8 | 27 +++++++++++++++++++--------
 docs/source/todo.rst                  |  1 -
 2 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/compiler/res/prog8lib/cx16/palette.p8 b/compiler/res/prog8lib/cx16/palette.p8
index ee49129d2..33c9174dd 100644
--- a/compiler/res/prog8lib/cx16/palette.p8
+++ b/compiler/res/prog8lib/cx16/palette.p8
@@ -42,19 +42,30 @@ palette {
         ubyte red
         ubyte greenblue
         repeat num_colors {
-            red = @(palette_bytes_ptr) >> 4
-            palette_bytes_ptr++
-            greenblue = @(palette_bytes_ptr) & %11110000
-            palette_bytes_ptr++
-            greenblue |= @(palette_bytes_ptr) >> 4    ; add Blue
-            palette_bytes_ptr++
-            cx16.vpoke(1, vera_palette_ptr, greenblue)
+            cx16.r1 = color8to4(palette_bytes_ptr)
+            palette_bytes_ptr+=3
+            cx16.vpoke(1, vera_palette_ptr, cx16.r1H)       ; $GB
             vera_palette_ptr++
-            cx16.vpoke(1, vera_palette_ptr, red)
+            cx16.vpoke(1, vera_palette_ptr, cx16.r1L)       ; $0R
             vera_palette_ptr++
         }
     }
 
+    sub color8to4(uword colorpointer) -> uword {
+        ; accurately convert 24 bits (3 bytes) RGB color, in that order in memory, to 16 bits $GB;$0R colorvalue
+        cx16.r1 = colorpointer
+        cx16.r0 = channel8to4(@(cx16.r1))         ; (red)   -> $00:0R
+        cx16.r0H = channel8to4(@(cx16.r1+1))<<4   ; (green) -> $G0:0R
+        cx16.r0H |= channel8to4(@(cx16.r1+2))     ; (blue)  -> $GB:0R
+        return cx16.r0
+    }
+
+    sub channel8to4(ubyte channelvalue) -> ubyte {
+        ; accurately convert a single 8 bit color channel value to 4 bits,  see https://threadlocalmutex.com/?p=48
+        return msb(channelvalue * $000f + 135)
+    }
+
+
     sub set_monochrome(uword screencolorRGB, uword drawcolorRGB) {
         vera_palette_ptr = $fa00
         cx16.vpoke(1, vera_palette_ptr, lsb(screencolorRGB))   ; G,B
diff --git a/docs/source/todo.rst b/docs/source/todo.rst
index 39475ae6d..809323916 100644
--- a/docs/source/todo.rst
+++ b/docs/source/todo.rst
@@ -2,7 +2,6 @@
 TODO
 ====
 
-- palette: mnore accurate conversion to 4 bit colorspage in set_rgb8, by doing 	(x * 15 + 135) >> 8, see https://threadlocalmutex.com/?p=48
 - fix the compiler crash on s1[n+1] = s1[2] where s1 is a subroutine param (regular variable works)
 - improve the working of %option merge: should be able to merge your own stuff into say textio. , and improve the docs about it too.
 - give error when using %option merge in module scope.