Bug 1899011 - In Lab/Oklab, very small a and b values makes the color achromatic r=layout-reviewers,emilio

Using an dynamic epsilon to check for small a and b values.

Differential Revision: https://phabricator.services.mozilla.com/D211665

servo/components/style/color/convert.rs

@@ -141,17 +141,29 @@ pub fn rgb_to_hwb(from: &ColorComponents) -> ColorComponents {
     ColorComponents(hue, whiteness * 100.0, blackness * 100.0)
 }
 
+/// Calculate an epsilon for a specified range.
+#[inline]
+pub fn epsilon_for_range(min: f32, max: f32) -> f32 {
+    (max - min) / 1.0e5
+}
+
 /// Convert from the rectangular orthogonal to the cylindrical polar coordinate
 /// system. This is used to convert (ok)lab to (ok)lch.
 /// <https://drafts.csswg.org/css-color-4/#lab-to-lch>
 #[inline]
-pub fn orthogonal_to_polar(from: &ColorComponents) -> ColorComponents {
+pub fn orthogonal_to_polar(from: &ColorComponents, e: f32) -> ColorComponents {
     let ColorComponents(lightness, a, b) = *from;
 
     let chroma = (a * a + b * b).sqrt();
 
-    // Very small chroma values make the hue component powerless.
-    let hue = if chroma.abs() < 1.0e-6 {
+    let hue = if a.abs() < e && b.abs() < e {
+        // For extremely small values of a and b ... the reported hue angle
+        // swinging about wildly and being essentially random ... this means
+        // the hue is powerless, and treated as missing when converted into LCH
+        // or Oklch.
+        f32::NAN
+    } else if chroma.abs() < e {
+        // Very small chroma values make the hue component powerless.
         f32::NAN
     } else {
         normalize_hue(b.atan2(a).to_degrees())
@@ -794,7 +806,7 @@ impl ColorSpaceConversion for Lch {
         let lab = Lab::from_xyz(&from);
 
         // Then convert the Lab to LCH.
-        orthogonal_to_polar(&lab)
+        orthogonal_to_polar(&lab, epsilon_for_range(0.0, 100.0))
     }
 
     fn to_gamma_encoded(from: &ColorComponents) -> ColorComponents {
@@ -892,7 +904,7 @@ impl ColorSpaceConversion for Oklch {
         let lab = Oklab::from_xyz(&from);
 
         // Then convert Oklab to OkLCH.
-        orthogonal_to_polar(&lab)
+        orthogonal_to_polar(&lab, epsilon_for_range(0.0, 1.0))
     }
 
     fn to_gamma_encoded(from: &ColorComponents) -> ColorComponents {

servo/components/style/color/mod.rs

@@ -555,7 +555,10 @@ impl AbsoluteColor {
             (Srgb, Hwb) => convert::rgb_to_hwb(&components),
             (Hsl, Srgb) => convert::hsl_to_rgb(&components),
             (Hwb, Srgb) => convert::hwb_to_rgb(&components),
-            (Lab, Lch) | (Oklab, Oklch) => convert::orthogonal_to_polar(&components),
+            (Lab, Lch) | (Oklab, Oklch) => convert::orthogonal_to_polar(
+                &components,
+                convert::epsilon_for_range(0.0, if color_space == Lch { 100.0 } else { 1.0 }),
+            ),
             (Lch, Lab) | (Oklch, Oklab) => convert::polar_to_orthogonal(&components),
 
             // All other conversions need to convert to XYZ first.

testing/web-platform/meta/css/css-color/parsing/color-computed-color-mix-function.html.ini

@@ -1,6 +0,0 @@
-[color-computed-color-mix-function.html]
-  [Property color value 'color-mix(in lch, white, blue)']
-    expected: FAIL
-
-  [Property color value 'color-mix(in lch, white 10%, blue)']
-    expected: FAIL