[WIP] preserve missing channels through conversions

Author: nex3

lib/src/functions/color.dart

@@ -1092,12 +1092,11 @@ SassColor _transparentize(String name, List<Value> arguments) {
 /// `sassNull`, it defaults to the color's existing space.
 SassColor _colorInSpace(Value colorUntyped, Value spaceUntyped) {
   var color = colorUntyped.assertColor("color");
-  if (spaceUntyped == sassNull) return color;
-
-  var space = ColorSpace.fromName(
-      (spaceUntyped.assertString("space")..assertUnquoted("space")).text,
-      "space");
-  return color.space == space ? color : color.toSpace(space);
+  return spaceUntyped == sassNull
+      ? color
+      : color.toSpace(ColorSpace.fromName(
+          (spaceUntyped.assertString("space")..assertUnquoted("space")).text,
+          "space"));
 }
 
 /// Returns the color space named by [space], or throws a [SassScriptException]

lib/src/value/color.dart

@@ -546,9 +546,52 @@ class SassColor extends Value {
   /// This currently can't produce an error, but it will likely do so in the
   /// future when Sass adds support for color spaces that don't support
   /// automatic conversions.
-  SassColor toSpace(ColorSpace space) => this.space == space
-      ? this
-      : this.space.convert(space, channel0, channel1, channel2, alpha);
+  SassColor toSpace(ColorSpace space) {
+    if (this.space != space) {
+      return this.space.convert(space, channel0, channel1, channel2, alpha);
+    }
+
+    // Converting to a color space triggers powerless channel analysis, even if
+    // the target space is the same as the source. So for spaces that can have
+    // powerless channels, we need to check if those channels are indeed
+    // powerless and if so make sure they're marked as missing.
+    //
+    // We implement separate logic for each space rather than just checking all
+    // three channels for every space because we want converting a color to its
+    // own space to be as efficient as possible.
+    switch (space) {
+      case ColorSpace.hsl:
+        var needsMissing0 = !isChannel0Missing && isChannel0Powerless;
+        var needsMissing1 = !isChannel1Missing && isChannel1Powerless;
+        return needsMissing0 || needsMissing1
+            ? SassColor.forSpaceInternal(space, needsMissing0 ? null : channel0,
+                needsMissing1 ? null : channel1, channel2, alpha)
+            : this;
+
+      case ColorSpace.hwb:
+        return !isChannel0Missing && isChannel0Powerless
+            ? SassColor.forSpaceInternal(space, null, channel1, channel2, alpha)
+            : this;
+
+      case ColorSpace.lab:
+      case ColorSpace.oklab:
+      case ColorSpace.lch:
+      case ColorSpace.oklch:
+        var needsMissing1 = !isChannel1Missing && isChannel1Powerless;
+        var needsMissing2 = !isChannel2Missing && isChannel2Powerless;
+        return needsMissing1 || needsMissing2
+            ? SassColor.forSpaceInternal(
+                space,
+                channel0,
+                needsMissing1 ? null : channel1,
+                needsMissing2 ? null : channel2,
+                alpha)
+            : this;
+
+      default:
+        return this;
+    }
+  }
 
   /// Returns a copy of this color that's in-gamut in the current color space.
   SassColor toGamut() {

lib/src/value/color/space.dart

@@ -106,7 +106,6 @@ abstract class ColorSpace {
   /// https://www.w3.org/TR/css-color-4/#ok-lab
   static const ColorSpace oklab = OklabColorSpace();
 
-
   /// The Oklch color space.
   ///
   /// https://www.w3.org/TR/css-color-4/#ok-lab
@@ -198,8 +197,8 @@ abstract class ColorSpace {
   ///
   /// @nodoc
   @internal
-  SassColor convert(ColorSpace dest, double channel0, double channel1,
-      double channel2, double alpha) {
+  SassColor convert(ColorSpace dest, double? channel0, double? channel1,
+      double? channel2, double alpha) {
     var linearDest = dest;
     switch (dest) {
       case ColorSpace.hsl:
@@ -218,26 +217,32 @@ abstract class ColorSpace {
         break;
     }
 
-    double transformed0;
-    double transformed1;
-    double transformed2;
+    double? transformed0;
+    double? transformed1;
+    double? transformed2;
     if (linearDest == this) {
       transformed0 = channel0;
       transformed1 = channel1;
       transformed2 = channel2;
     } else {
-      var linear0 = toLinear(channel0);
-      var linear1 = toLinear(channel1);
-      var linear2 = toLinear(channel2);
+      var linear0 = toLinear(channel0 ?? 0);
+      var linear1 = toLinear(channel1?? 0);
+      var linear2 = toLinear(channel2??0);
       var matrix = transformationMatrix(linearDest);
 
       // (matrix * [linear0, linear1, linear2]).map(linearDest.fromLinear)
-      transformed0 = linearDest.fromLinear(
-          matrix[0] * linear0 + matrix[1] * linear1 + matrix[2] * linear2);
-      transformed1 = linearDest.fromLinear(
-          matrix[3] * linear0 + matrix[4] * linear1 + matrix[5] * linear2);
-      transformed2 = linearDest.fromLinear(
-          matrix[6] * linear0 + matrix[7] * linear1 + matrix[8] * linear2);
+      transformed0 = channel0 == null
+          ? null
+          : linearDest.fromLinear(
+              matrix[0] * linear0 + matrix[1] * linear1 + matrix[2] * linear2);
+      transformed1 = channel1 == null
+          ? null
+          : linearDest.fromLinear(
+              matrix[3] * linear0 + matrix[4] * linear1 + matrix[5] * linear2);
+      transformed2 = channel2 == null
+          ? null
+          : linearDest.fromLinear(
+              matrix[6] * linear0 + matrix[7] * linear1 + matrix[8] * linear2);
     }
 
     switch (dest) {

lib/src/value/color/space/hsl.dart

@@ -26,12 +26,12 @@ class HslColorSpace extends ColorSpace {
           LinearChannel('lightness', 0, 100, requiresPercent: true)
         ]);
 
-  SassColor convert(ColorSpace dest, double hue, double saturation,
-      double lightness, double alpha) {
+  SassColor convert(ColorSpace dest, double? hue, double? saturation,
+      double? lightness, double alpha) {
     // Algorithm from the CSS3 spec: https://www.w3.org/TR/css3-color/#hsl-color.
-    var scaledHue = (hue / 360) % 1;
-    var scaledSaturation = saturation / 100;
-    var scaledLightness = lightness / 100;
+    var scaledHue = ((hue ?? 0) / 360) % 1;
+    var scaledSaturation = (saturation ?? 0) / 100;
+    var scaledLightness = (lightness ?? 0) / 100;
 
     var m2 = scaledLightness <= 0.5
         ? scaledLightness * (scaledSaturation + 1)
@@ -40,11 +40,15 @@ class HslColorSpace extends ColorSpace {
             scaledLightness * scaledSaturation;
     var m1 = scaledLightness * 2 - m2;
 
-    return ColorSpace.srgb.convert(
-        dest,
-        hueToRgb(m1, m2, scaledHue + 1 / 3),
-        hueToRgb(m1, m2, scaledHue),
-        hueToRgb(m1, m2, scaledHue - 1 / 3),
-        alpha);
+    return forwardMissingChannels(
+        ColorSpace.srgb.convert(
+            dest,
+            hueToRgb(m1, m2, scaledHue + 1 / 3),
+            hueToRgb(m1, m2, scaledHue),
+            hueToRgb(m1, m2, scaledHue - 1 / 3),
+            alpha),
+        missingLightness: lightness == null,
+        missingColorfulness: saturation == null,
+        missingHue: hue == null);
   }
 }

lib/src/value/color/space/hwb.dart

@@ -26,12 +26,12 @@ class HwbColorSpace extends ColorSpace {
           LinearChannel('blackness', 0, 100, requiresPercent: true)
         ]);
 
-  SassColor convert(ColorSpace dest, double hue, double whiteness,
-      double blackness, double alpha) {
+  SassColor convert(ColorSpace dest, double? hue, double? whiteness,
+      double? blackness, double alpha) {
     // From https://www.w3.org/TR/css-color-4/#hwb-to-rgb
-    var scaledHue = hue % 360 / 360;
-    var scaledWhiteness = whiteness / 100;
-    var scaledBlackness = blackness / 100;
+    var scaledHue = (hue ?? 0) % 360 / 360;
+    var scaledWhiteness = (whiteness ?? 0) / 100;
+    var scaledBlackness = (blackness ?? 0) / 100;
 
     var sum = scaledWhiteness + scaledBlackness;
     if (sum > 1) {
@@ -42,9 +42,9 @@ class HwbColorSpace extends ColorSpace {
     var factor = 1 - scaledWhiteness - scaledBlackness;
     double toRgb(double hue) => hueToRgb(0, 1, hue) * factor + scaledWhiteness;
 
-    // Non-null because an in-gamut HSL color is guaranteed to be in-gamut for
-    // HWB as well.
-    return ColorSpace.srgb.convert(dest, toRgb(scaledHue + 1 / 3),
-        toRgb(scaledHue), toRgb(scaledHue - 1 / 3), alpha);
+    return forwardMissingChannels(
+        ColorSpace.srgb.convert(dest, toRgb(scaledHue + 1 / 3),
+            toRgb(scaledHue), toRgb(scaledHue - 1 / 3), alpha),
+        missingHue: hue == null);
   }
 }

lib/src/value/color/space/lab.dart

@@ -30,10 +30,10 @@ class LabColorSpace extends ColorSpace {
         ]);
 
   SassColor convert(
-      ColorSpace dest, double lightness, double a, double b, double alpha) {
+      ColorSpace dest, double? lightness, double? a, double? b, double alpha) {
     switch (dest) {
       case ColorSpace.lab:
-        var powerlessAB = fuzzyEquals(lightness, 0);
+        var powerlessAB = lightness == null || fuzzyEquals(lightness, 0);
         return SassColor.lab(
             lightness, powerlessAB ? null : a, powerlessAB ? null : b, alpha);
 
@@ -43,17 +43,20 @@ class LabColorSpace extends ColorSpace {
       default:
         // Algorithm from https://www.w3.org/TR/css-color-4/#color-conversion-code
         // and http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
-        var f1 = (lightness + 16) / 116;
+        var lightnessOr0 = lightness ?? 0;
+        var f1 = (lightnessOr0 + 16) / 116;
 
-        return ColorSpace.xyzD50.convert(
-            dest,
-            _convertFToXorZ(a / 500 + f1) * d50[0],
-            (lightness > labKappa * labEpsilon
-                    ? math.pow((lightness + 16) / 116, 3) * 1.0
-                    : lightness / labKappa) *
-                d50[1],
-            _convertFToXorZ(f1 - b / 200) * d50[2],
-            alpha);
+        return forwardMissingChannels(
+            ColorSpace.xyzD50.convert(
+                dest,
+                _convertFToXorZ((a??0) / 500 + f1) * d50[0],
+                (lightnessOr0 > labKappa * labEpsilon
+                        ? math.pow(f1, 3) * 1.0
+                        : lightnessOr0 / labKappa) *
+                    d50[1],
+                _convertFToXorZ(f1 - (b??0) / 200) * d50[2],
+                alpha),
+            missingLightness: lightness == null);
     }
   }
 

lib/src/value/color/space/lch.dart

@@ -28,10 +28,18 @@ class LchColorSpace extends ColorSpace {
           hueChannel
         ]);
 
-  SassColor convert(ColorSpace dest, double lightness, double chroma,
-      double hue, double alpha) {
-    var hueRadians = hue * math.pi / 180;
-    return ColorSpace.lab.convert(dest, lightness,
-        chroma * math.cos(hueRadians), chroma * math.sin(hueRadians), alpha);
+  SassColor convert(ColorSpace dest, double? lightness, double? chroma,
+      double? hue, double alpha) {
+    var hueRadians = (hue ?? 0) * math.pi / 180;
+    return forwardMissingChannels(
+        ColorSpace.lab.convert(
+            dest,
+            lightness ?? 0,
+            (chroma ?? 0) * math.cos(hueRadians),
+            (chroma ?? 0) * math.sin(hueRadians),
+            alpha),
+        missingLightness: lightness == null,
+        missingColorfulness: chroma == null,
+        missingHue: hue == null);
   }
 }

lib/src/value/color/space/lms.dart

@@ -33,13 +33,13 @@ class LmsColorSpace extends ColorSpace {
         ]);
 
   SassColor convert(
-      ColorSpace dest, double long, double medium, double short, double alpha) {
+      ColorSpace dest, double? long, double? medium, double? short, double alpha) {
     switch (dest) {
       case ColorSpace.oklab:
         // Algorithm from https://drafts.csswg.org/css-color-4/#color-conversion-code
-        var longScaled = math.pow(long, 1 / 3);
-        var mediumScaled = math.pow(medium, 1 / 3);
-        var shortScaled = math.pow(short, 1 / 3);
+        var longScaled = math.pow(long ?? 0, 1 / 3);
+        var mediumScaled = math.pow(medium ?? 0, 1 / 3);
+        var shortScaled = math.pow(short ?? 0, 1 / 3);
         var lightness = lmsToOklab[0] * longScaled +
             lmsToOklab[1] * mediumScaled +
             lmsToOklab[2] * shortScaled;
@@ -62,9 +62,9 @@ class LmsColorSpace extends ColorSpace {
         // This is equivalent to converting to OKLab and then to OKLCH, but we
         // do it inline to avoid extra list allocations since we expect
         // conversions to and from OKLCH to be very common.
-        var longScaled = math.pow(long, 1 / 3);
-        var mediumScaled = math.pow(medium, 1 / 3);
-        var shortScaled = math.pow(short, 1 / 3);
+        var longScaled = math.pow(long ?? 0, 1 / 3);
+        var mediumScaled = math.pow(medium ?? 0, 1 / 3);
+        var shortScaled = math.pow(short ?? 0, 1 / 3);
         return labToLch(
             dest,
             lmsToOklab[0] * longScaled +

lib/src/value/color/space/oklab.dart

@@ -29,32 +29,37 @@ class OklabColorSpace extends ColorSpace {
         ]);
 
   SassColor convert(
-      ColorSpace dest, double lightness, double a, double b, double alpha) {
+      ColorSpace dest, double? lightness, double? a, double? b, double alpha) {
     if (dest == ColorSpace.oklch) {
       return labToLch(dest, lightness, a, b, alpha);
     }
 
     // Algorithm from https://www.w3.org/TR/css-color-4/#color-conversion-code
-    return ColorSpace.lms.convert(
-        dest,
-        math.pow(
-                oklabToLms[0] * lightness +
-                    oklabToLms[1] * a +
-                    oklabToLms[2] * b,
-                3) +
-            0.0,
-        math.pow(
-                oklabToLms[3] * lightness +
-                    oklabToLms[4] * a +
-                    oklabToLms[5] * b,
-                3) +
-            0.0,
-        math.pow(
-                oklabToLms[6] * lightness +
-                    oklabToLms[7] * a +
-                    oklabToLms[8] * b,
-                3) +
-            0.0,
-        alpha);
+    lightness ??= 0;
+    a ??= 0;
+    b ??= 0;
+    return forwardMissingChannels(
+        ColorSpace.lms.convert(
+            dest,
+            math.pow(
+                    oklabToLms[0] * lightness +
+                        oklabToLms[1] * a +
+                        oklabToLms[2] * b,
+                    3) +
+                0.0,
+            math.pow(
+                    oklabToLms[3] * lightness +
+                        oklabToLms[4] * a +
+                        oklabToLms[5] * b,
+                    3) +
+                0.0,
+            math.pow(
+                    oklabToLms[6] * lightness +
+                        oklabToLms[7] * a +
+                        oklabToLms[8] * b,
+                    3) +
+                0.0,
+            alpha),
+        missingLightness: lightness == null);
   }
 }

lib/src/value/color/space/oklch.dart

@@ -28,10 +28,18 @@ class OklchColorSpace extends ColorSpace {
           hueChannel
         ]);
 
-  SassColor convert(ColorSpace dest, double lightness, double chroma,
-      double hue, double alpha) {
+  SassColor convert(ColorSpace dest, double? lightness, double? chroma,
+      double? hue, double alpha) {
     var hueRadians = hue * math.pi / 180;
-    return ColorSpace.oklab.convert(dest, lightness,
-        chroma * math.cos(hueRadians), chroma * math.sin(hueRadians), alpha);
+    return forwardMissingChannels(
+        ColorSpace.oklab.convert(
+            dest,
+            lightness ?? 0,
+            (chroma ?? 0) * math.cos(hueRadians),
+            (chroma ?? 0) * math.sin(hueRadians),
+            alpha),
+        missingLightness: lightness == null,
+        missingColorfulness: chroma == null,
+        missingHue: hue == null);
   }
 }