Use more accurate color ramp conversion method

src/reskit/tileset.rs

@@ -1,15 +1,45 @@
 use crate::reskit::utility;
-use std::error::Error;
+use std::{error::Error, ops::RangeInclusive};
 use std::fs;
 use std::fs::File;
 use std::io::Write;
 use image::{ GenericImageView, DynamicImage };
 
+/**
+ * Get a particular RGB component adjusted for the Mega Drive VDP colour ramp.
+ * See colour ramp at https://plutiedev.com/vdp-color-ramp
+ */
+fn rgb_component_to_ramp( component: u16 ) -> Result<u16, Box<dyn Error>> {
+	let ramp: [(RangeInclusive<u16>, (u16,u16)); 7] = [
+		(0..=52,    (0x0, 0x2)),
+		(52..=87,   (0x2, 0x4)),
+		(87..=116,  (0x4, 0x6)),
+		(116..=144, (0x6, 0x8)),
+		(144..=172,	(0x8, 0xA)),
+		(172..=206, (0xA, 0xC)),
+		(206..=255, (0xC, 0xE))
+	];
+
+	for i in 0..ramp.len() {
+		let ( ramp_range, ( cram_round_down, cram_round_up ) ) = &ramp[ i ];
+		if ramp_range.contains( &component ) {
+			let range_midpoint = ramp_range.start() + ( ( ramp_range.end() - ramp_range.start() ) / 2 );
+			return if component >= range_midpoint {
+				Ok( *cram_round_up )
+			} else {
+				Ok( *cram_round_down )
+			}
+		}
+	}
+
+	Err( "rgb component not in range 0-255" )?
+}
+
 pub fn color_to_palette( r: u16, g: u16, b: u16, palette: &mut [u16; 16] ) -> Result<u32, Box<dyn Error>> {
 	let final_val =
-		( ( r & 0x00F0 ) >> 4 ) |
-		( g & 0x00F0 ) |
-		( ( b & 0x00F0 ) << 4 );
+		( rgb_component_to_ramp( b )? << 8 ) |
+		( rgb_component_to_ramp( g )? << 4 ) |
+		( rgb_component_to_ramp( r )? );
 
 	// Does the color already exist?
 	for i in 0..palette.len() {