Create macro to automatically place fading pals between two sources, and apply it to level selection menu daytime-based background pals (#12)

macros/gfx.asm

@@ -1,5 +1,9 @@
 ; Graphics macros
 
+DEF palred   EQUS "(1 << 0) *"
+DEF palgreen EQUS "(1 << 5) *"
+DEF palblue  EQUS "(1 << 10) *"
+
 MACRO assert_valid_rgb
 	rept _NARG
 		assert 0 <= (\1) && (\1) <= 31, "RGB channel must be 0-31"
@@ -11,13 +15,53 @@ MACRO RGB
 	rept _NARG / 3
 		assert_valid_rgb \1, \2, \3
 		dw palred (\1) + palgreen (\2) + palblue (\3)
+		if DEF(fade_src)
+			DEF {fade_src}_{d:color_index} EQU \1
+			DEF color_index += 1
+			DEF {fade_src}_{d:color_index} EQU \2
+			DEF color_index += 1
+			DEF {fade_src}_{d:color_index} EQU \3
+			DEF color_index += 1
+		endc
 		shift 3
 	endr
 ENDM
 
-DEF palred   EQUS "(1 << 0) *"
-DEF palgreen EQUS "(1 << 5) *"
-DEF palblue  EQUS "(1 << 10) *"
+MACRO rgbpals_fade_src
+	DEF fade_src EQUS \1
+	DEF color_index = 0
+ENDM
+
+MACRO rgbpals_fade_src_end
+	DEF {fade_src}_len = color_index
+	PURGE fade_src
+ENDM
+
+MACRO rgbpals_fade_apply
+	DEF fade_from EQUS \1
+	DEF fade_to EQUS \2
+	assert {fade_from}_len == {fade_to}_len, "fade_from pals and fade_to pals must be same length"
+	for i, \3
+		for j, 0, {fade_from}_len, 3
+			DEF rgbch_red = {j} + 0
+			DEF rgbch_green = {j} + 1
+			DEF rgbch_blue = {j} + 2
+			DEF palred_value = {fade_from}_{d:rgbch_red} + ({fade_to}_{d:rgbch_red} - {fade_from}_{d:rgbch_red}) * i / \3
+			DEF palgreen_value = {fade_from}_{d:rgbch_green} + ({fade_to}_{d:rgbch_green} - {fade_from}_{d:rgbch_green}) * i / \3
+			DEF palblue_value = {fade_from}_{d:rgbch_blue} + ({fade_to}_{d:rgbch_blue} - {fade_from}_{d:rgbch_blue}) * i / \3
+			dw palred (palred_value) + palgreen (palgreen_value) + palblue (palblue_value)
+		endr
+	endr
+	PURGE fade_from, fade_to, rgbch_red, rgbch_green, rgbch_blue, palred_value, palgreen_value, palblue_value
+ENDM
+
+/* MACRO rgbpals_fade_end
+	rept _NARG
+		for i,
+		endr
+		shift
+	endr
+ENDM */
 
 DEF palettes EQUS "* PALETTE_SIZE"
 DEF palette  EQUS "+ PALETTE_SIZE *"