FEAT: native for human perception weighted Euclidean distance between two RGB colors

Author: Oldes

src/core/n-image.c

@@ -184,6 +184,36 @@ typedef struct REBCLR {
 }
 
 
+/***********************************************************************
+**
+*/	REBNATIVE(color_distance)
+/*
+**	It's by design providing only the weighted version! The simple distance
+**  could be provided in more generic `distance` function.
+**
+**	https://www.compuphase.com/cmetric.htm
+**  https://observablehq.com/@luciyer/euclidian-distance-in-rgb-color-space
+**
+//	color-distance: native [
+//		"Human perception weighted Euclidean distance between two RGB colors"
+//		a [tuple!]
+//		b [tuple!]
+//	]
+***********************************************************************/
+{
+	REBCLR *val1 = (REBCLR*)VAL_TUPLE(D_ARG(1));
+	REBCLR *val2 = (REBCLR*)VAL_TUPLE(D_ARG(2));
+
+	long r = (long)val1->r - (long)val2->r;
+	long g = (long)val1->g - (long)val2->g;
+	long b = (long)val1->b - (long)val2->b;
+
+	long rmean = ((long)val1->r  + (long)val2->r ) / 2;
+	SET_DECIMAL(D_RET, sqrt((((512+rmean)*r*r)>>8) + 4*g*g + (((767-rmean)*b*b)>>8)));
+	return R_RET;
+}
+
+
 /***********************************************************************
 **
 */	REBNATIVE(tint)

src/tests/units/image-test.r3

@@ -363,6 +363,15 @@ FFFFFFDC1616212121212121
 
 ===end-group===
 
+===start-group=== "RGB color distance"
+--test-- "color-distance"
+	--assert 764.83 = round/to color-distance 0.0.0 255.255.255 0.01
+	--assert 569.97 = round/to color-distance 0.0.255 255.0.0 0.01
+	--assert 42.04  = round/to color-distance 200.105.200 225.105.200 0.01
+	--assert 41.3   = round/to color-distance 175.200.100 200.200.100 0.01
+	--assert 0.0    = round/to color-distance 200.200.100 200.200.100 0.01
+===end-group===
+
 
 ===start-group=== "Tint color"