Fix HWB round trip of out of gamut colors (#427)

* Fix HWB round trip of out of gamut colors

* Fix mypy

coloraide/spaces/hsl/__init__.py

@@ -7,7 +7,7 @@
 from ...types import Vector
 
 
-def srgb_to_hsl(rgb: Vector) -> Vector:
+def srgb_to_hsl(rgb: Vector, correct_neg_sat: bool = True) -> Vector:
     """Convert sRGB to HSL."""
 
     r, g, b = rgb
@@ -29,7 +29,7 @@ def srgb_to_hsl(rgb: Vector) -> Vector:
         h *= 60.0
 
     # Adjust for negative saturation
-    if s < 0:
+    if correct_neg_sat and s < 0:
         s *= -1.0
         h += 180.0
 

coloraide/spaces/hwb/__init__.py

@@ -10,7 +10,8 @@
 def srgb_to_hwb(srgb: Vector) -> Vector:
     """HWB to sRGB."""
 
-    return [srgb_to_hsl(srgb)[0], min(srgb), 1 - max(srgb)]
+    # Do not use a corrected hue due to negative saturation as HWB ignores the context of negative saturation.
+    return [srgb_to_hsl(srgb, correct_neg_sat=False)[0], min(srgb), 1 - max(srgb)]
 
 
 def hwb_to_srgb(hwb: Vector) -> Vector:

docs/src/markdown/about/changelog.md

@@ -15,6 +15,10 @@
         spaces if desired via the `pspace` parameter. Additionally, documentation has been added so users can easily
         recreate the aforementioned pre-configured methods themselves or their own desired variants.
 
+-   **FIX**: HWB should not use a corrected HSL hue when the color yields a negative saturation as it does not calculate
+    the whiteness and blackness with the negative saturation. Using a corrected hue without the negative saturation
+    context leads to bad round trip with out of gamut colors.
+
 ## 3.3.1
 
 -   **FIX**: Ray trace gamut mapping algorithm will better handle perceptual spaces like CAM16 and HCT which have

tests/test_hwb.py

@@ -36,7 +36,9 @@ class TestHWB(util.ColorAssertsPyTest):
         # Test percent ranges
         ('color(--hwb 0 0% 0%)', 'color(--hwb 0 0 none)'),
         ('color(--hwb 360 100% 100%)', 'color(--hwb 360 1 1 / 1)'),
-        ('color(--hwb -360 -100% -100%)', 'color(--hwb -360 -1 -1 / 1)')
+        ('color(--hwb -360 -100% -100%)', 'color(--hwb -360 -1 -1 / 1)'),
+        # Test out of gamut conversion
+        ('lab(100 104.3 -50.9)', 'color(--hwb 311.21 0.58776 -0.5935 / 1)')
     ]
 
     @pytest.mark.parametrize('color1,color2', COLORS)