More accurate 8-bit <=> 5-bit RGB color conversion (#1827)

include/gfx/rgba.hpp

@@ -18,10 +18,7 @@ struct Rgba {
 	    : red(rgba >> 24), green(rgba >> 16), blue(rgba >> 8), alpha(rgba) {}
 
 	static constexpr Rgba fromCGBColor(uint16_t color) {
-		constexpr auto _5to8 = [](uint8_t channel) -> uint8_t {
-			channel &= 0b11111; // For caller's convenience
-			return channel << 3 | channel >> 2;
-		};
+		constexpr auto _5to8 = [](uint8_t c) -> uint8_t { return ((c & 0b11111) * 255 + 15) / 31; };
 		return {
 		    _5to8(color),
 		    _5to8(color >> 5),

man/rgbasm-old.5

@@ -461,6 +461,24 @@ Previously we had
 .Pp
 Instead, now we have
 .Ql p ** q ** r == p ** (q ** r) .
+.Ss 8-bit and 5-bit color conversion
+Changed in 1.0.0.
+.Pp
+RGBGFX takes 8-bit RGB colors as its PNG input, and outputs 5-bit GBC colors.
+Its
+.Ql -r/--reverse
+mode does the opposite 5-bit to 8-bit conversion.
+Instead of the previous inaccurate conversions, we now do accurate rounding to the nearest equivalent.
+.Pp
+Previously to convert an 8-bit color channel to 5-bit, we truncated it as
+.Ql c >> 3 ;
+and to reverse a 5-bit color channel to 8-bit, we extended it as
+.Ql (c << 3) | (c >> 2) .
+.Pp
+Instead, now we round 8-bit to 5-bit as
+.Ql (c * 31 + 127) / 255 ,
+and round 5-bit to 8-bit as
+.Ql (c * 255 + 15) / 31 .
 .Sh BUGS
 These are misfeatures that may have been possible by mistake.
 They do not get deprecated, just fixed.

src/gfx/rgba.cpp

@@ -50,9 +50,10 @@ uint16_t Rgba::cgbColor() const {
 		g = reverse_curve[g];
 		b = reverse_curve[b];
 	} else {
-		r >>= 3;
-		g >>= 3;
-		b >>= 3;
+		constexpr auto _8to5 = [](uint8_t c) -> uint8_t { return (c * 31 + 127) / 255; };
+		r = _8to5(r);
+		g = _8to5(g);
+		b = _8to5(b);
 	}
 	return r | g << 5 | b << 10;
 }

test/gfx/alpha_embedded.err

@@ -1,4 +1,4 @@
-error: Failed to fit tile colors [$1527, $15cc, $1ab4] in specified palettes
+error: Failed to fit tile colors [$1527, $15cc, $1a93] in specified palettes
 note: The following palette was specified:
-        [$1ab4, $15cc]
+        [$1a93, $15cc]
 Conversion aborted after 1 error

test/gfx/at-file-ref.err

@@ -1,4 +1,4 @@
-error: Failed to fit tile colors [$6c8a, $7f55, $7fff] in specified palettes
+error: Failed to fit tile colors [$688b, $7f55, $7fff] in specified palettes
 note: The following palettes were specified:
         [$5f77, $213d, $41a6, $40ee]
         [$3f65, $36b3, $262a, $50b0]

test/gfx/bad_manual_pals.err

@@ -1,5 +1,5 @@
-error: Failed to fit tile colors [$6c8a, $7f55, $7fff] in specified palettes
+error: Failed to fit tile colors [$688b, $7f55, $7fff] in specified palettes
 note: The following palettes were specified:
         [$7fff, $7f55]
-        [$7fff, $6c8a]
+        [$7fff, $688b]
 Conversion aborted after 1 error

test/gfx/bg_fuse.err

@@ -1,3 +1,3 @@
-warning: Fusing colors #a9b9c9ff and #aabbccff into Game Boy color $66f5 [first seen at x: 1, y: 1]
+warning: Fusing colors #a9bacbff and #aabbccff into Game Boy color $66f5 [first seen at x: 1, y: 1]
 FATAL: Tile (0, 0) contains the background color (#aabbccff)!
 Conversion aborted after 1 error

test/gfx/empty_lines.out.pal

@@ -1 +1 @@
-妉U<EFBFBD><EFBFBD><EFBFBD>
+媓U<EFBFBD><EFBFBD><EFBFBD>

test/gfx/randtilegen.cpp

@@ -144,7 +144,7 @@ static void generate_palettes(uint16_t palettes[/* 60 */][4]) {
  * Expand a 5-bit color component to 8 bits with minimal bias
  */
 static uint8_t _5to8(uint8_t five) {
-	return five << 3 | five >> 2;
+	return (five * 255 + 15) / 31;
 }
 
 // Can't mark as `const`, as the array type is otherwise not compatible (augh)