Add colour-interpolate operator

core/modules/filters/colour-ops.js

@@ -52,6 +52,25 @@ exports["colour-contrast"] = makeParallelColourOperator(function (colours, opera
 	return colourContrasts;
 });
 
+exports["colour-interpolate"] = makeParallelColourOperator(function (colours, operator, options) {
+	// Special case for less than two colours
+	if(colours.length < 2) {
+		return [];
+	}
+	// Step through the indexes collecting the interpolated colours
+	var space = ((operator.suffixes || [])[0] || ["srgb"])[0],
+		rangefn = colours[0].range(colours[1],{space: space}),
+		outputColours = [];
+	$tw.utils.each(operator.operands,function(operand) {
+		// Get the index
+		var index = $tw.utils.parseNumber(operand);
+		// Calculate the interpolated colour
+		var colour = rangefn(index);
+		outputColours.push(colour.display().toString());
+	});
+	return outputColours;
+});
+
 function makeSerialColourOperator(fn) {
 	return function (source, operator, options) {
 		var results = [];

editions/test/tiddlers/tests/data/operators/colour/ColourInterpolate.tid

@@ -0,0 +1,19 @@
+title: Operators/Colour/ColourInterpolate
+description: colour-interpolate function
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\parsermode inline
+<$text text={{{ [subfilter{Filter}] }}}/>
++
+title: Filter
+
+=[[#5778d8]]
+=[[#d85757]]
++[colour-interpolate:oklch[0.2],[0.5],[0.99]join[,]]
++
+title: ExpectedResult
+
+oklch(60.08% 0.15344 290.55),oklch(60.956% 0.15699 325.23),oklch(62.386% 0.1628 381.88)

editions/tw5.com/tiddlers/filters/colour-interpolate.tid

@@ -0,0 +1,33 @@
+created: 20241117161528913
+modified: 20241117161528913
+tags: [[Filter Operators]] [[Colour Operators]]
+title: colour-interpolate Operator
+caption: colour-interpolate
+op-purpose: smoothly interpolate between a set of colours
+op-input: a selection of colour values
+op-suffix: the name of the colour space to be used for the interpolation such as "OkLCh" or "sRGB"
+op-parameter: one or more indexes where 0 is the first colour and 1 is the last colour and intermediate values are smoothly interpolated
+op-output: the values of the interpolated colours
+
+<<.from-version "5.3.7">> See [[Colour Palettes]] for background.
+
+The <<.op colour-interpolate>> operator is used to interpolate colour values in a chosen colour space.
+
+The full list of colour spaces that can be used for interpolation is given in [[Colour Spaces]].
+
+Note that at least two input colours must be provided for interpolation to occur.
+
+Note that indexes outside the range 0 to 1 will extrapolate from the provided colour values.
+
+For example, this filter expression will return 4 colours that smoothly blend from red to green in the OKLCH colour space:
+
+```
+red green +[colour-interpolate:oklch[0],[0.333],[0.666],[1]]
+```
+
+See also the following related operators:
+
+* <<.olink colour-get-oklch>> to retrieve a component of a colour value in the OKLCH colour space
+* <<.olink colour-lighten>> to lighten a colour value
+* <<.olink colour-darken>> to lighten a colour value
+