krilla: a gray stop in a CMYK gradient becomes its exact black (patch 17)

A gray stop rewritten into a CMYK space took K as 1 - gray in f32, so gray 0.96 wrote K 0.04000002 and the 8-bit gray 128 wrote 0.4980392 rather than 127/255. K is now subtracted from the shortest decimal that writes the gray, and an 8-bit gray n gives (255 - n) / 255. Patch 17, SOURCE.json and the ledger are updated.

crates/pdf/tests/print_color.rs

@@ -1112,6 +1112,18 @@ fn f32_colors_keep_their_inks_in_shadings() {
     assert_eq!(mesh_shadings(&bytes), vec![(("DeviceCMYK".to_owned(), vec![]), 4, vec![corner.repeat(4)])]);
 }
 
+/// A gray stop in a CMYK gradient becomes the black ink `1 - gray`: gray 0.96 is K 0.04, and the
+/// 8-bit gray 128 is K 127/255, as `cmyk::Color::new` writes it.
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[cfg_attr(not(target_arch = "wasm32"), test)]
+fn gray_stops_keep_their_exact_black_in_cmyk_shadings() {
+    let cyan = || -> backend::color::Color { cmyk::Color::new_f32(0.04, 0.0, 0.0, 0.0).into() };
+    let bytes = page_with(|surface| fill_rect(surface, 0.0, linear([luma::Color::new_f32(0.96).into(), cyan()])));
+    assert_eq!(shading_ends(&bytes), (vec![0.0, 0.0, 0.0, 0.04], vec![0.04, 0.0, 0.0, 0.0]));
+    let bytes = page_with(|surface| fill_rect(surface, 0.0, linear([luma::Color::new(128).into(), cyan()])));
+    assert_eq!(shading_ends(&bytes).0, vec![0.0, 0.0, 0.0, 127.0 / 255.0]);
+}
+
 const FOGRA39: &[u8] = include_bytes!("../../cms/profiles/craft-coated-fogra39.icc");
 
 fn text(dict: &Dict<'_>, key: &[u8]) -> Option<String> {

vendor/krilla/README.craft.md

@@ -427,6 +427,10 @@ as a multiple of 1/255: C75 becomes 0.7490196 and C4 M4 Y4 K4 becomes 0.03921568
 - Gradient stops that are rewritten into one DeviceCMYK, Separation or DeviceN space keep
   their components, and a DeviceN color's CMYK alternate (annotations, tags) is no longer
   rounded.
+- A gray stop rewritten into a CMYK space becomes the black ink `1 - gray`, subtracted from the
+  shortest decimal that writes the gray, so gray 0.96 is K 0.04. An 8-bit gray `n` is K
+  `(255 - n) / 255`, as `cmyk::Color::new` writes it.
 - Mesh gradients write their corner colors and opacities at 16 bits a component instead of 8.
 
-Tests: `crates/pdf/tests/print_color.rs` (`f32_colors_*`, `mesh_*`, `rgb_mesh_is_a_device_rgb_shading`).
+Tests: `crates/pdf/tests/print_color.rs` (`f32_colors_*`, `gray_stops_keep_their_exact_black_in_cmyk_shadings`,
+`mesh_*`, `rgb_mesh_is_a_device_rgb_shading`).

vendor/krilla/SOURCE.json

@@ -152,7 +152,7 @@
     {
       "path": "src/graphics/color.rs",
       "upstream_sha256": "e5843d33ef184ab5c458025d1b1dc53df6ee6aaf085595ea14604d080b98f8a4",
-      "sha256": "5b0de85e0f3eac2905f975f676f1ef5d0eebb99a9a35670fefb419ed36fed6cf"
+      "sha256": "e052d835a4f113ec3730f32c1708e80ee4a284978110f58c77d88c48f3cd15ae"
     },
     {
       "path": "src/graphics/device_n.rs",
@@ -207,7 +207,7 @@
     {
       "path": "src/graphics/shading_function.rs",
       "upstream_sha256": "0e0fc8bb1a32272457ef560fa80933972d0f5ef5fb7046424c6de48093cfd978",
-      "sha256": "6da45a96a98d32d4ef878fa03ac1fddbccccea2e2d6333d3a0e90ca009061f81"
+      "sha256": "502571de1a0d1f1d32310d3801ae70250c7ac129506fdf4dd99e786c24268d12"
     },
     {
       "path": "src/graphics/shading_pattern.rs",

vendor/krilla/patches/17-color-precision.patch

@@ -1,5 +1,5 @@
 diff --git a/src/graphics/color.rs b/src/graphics/color.rs
-index 6fb0618..06a3098 100644
+index 6fb0618..581de00 100644
 --- a/src/graphics/color.rs
 +++ b/src/graphics/color.rs
 @@ -69,7 +69,7 @@
@@ -11,7 +11,7 @@ index 6fb0618..06a3098 100644
  
  use crate::configure::ValidationError;
  use crate::graphics::icc::ICCBasedColorSpace;
-@@ -82,6 +82,48 @@ pub(crate) const DEVICE_GRAY: &str = "DeviceGray";
+@@ -82,6 +82,60 @@ pub(crate) const DEVICE_GRAY: &str = "DeviceGray";
  /// The PDF name for the device CMYK color space.
  pub(crate) const DEVICE_CMYK: &str = "DeviceCMYK";
  
@@ -41,6 +41,18 @@ index 6fb0618..06a3098 100644
 +    pub(crate) fn to_u8(self) -> u8 {
 +        (self.0 * 255.0).round() as u8
 +    }
++
++    /// `1 - value`, the black ink of a gray. An 8-bit value `n / 255` gives `(255 - n) / 255`,
++    /// as the 8-bit constructors write it. Any other value is subtracted from 1 in f64, from
++    /// the shortest decimal that writes it, so gray 0.96 gives 0.04 rather than 0.04000002.
++    pub(crate) fn complement(self) -> f32 {
++        let byte = self.to_u8();
++        if Tint::from_u8(byte) == self {
++            return Tint::from_u8(255 - byte).0;
++        }
++        let written = self.0.to_string().parse::<f64>().unwrap_or(f64::from(self.0));
++        (1.0 - written) as f32
++    }
 +}
 +
 +impl PartialEq for Tint {
@@ -60,7 +72,7 @@ index 6fb0618..06a3098 100644
  /// A wrapper for storing colors from different color spaces.
  #[derive(Debug, Hash, Eq, PartialEq, Clone)]
  pub enum Color {
-@@ -160,7 +202,10 @@ impl RegularColor {
+@@ -160,7 +214,10 @@ impl RegularColor {
      pub(crate) fn as_rgb(self) -> Option<rgb::Color> {
          Some(match self {
              Self::Rgb(r) => r,
@@ -72,7 +84,7 @@ index 6fb0618..06a3098 100644
              Self::Cmyk(_) => return None,
          })
      }
-@@ -183,16 +228,24 @@ impl SpecialColor {
+@@ -183,16 +240,24 @@ impl SpecialColor {
  
  /// Gray-scale colors.
  pub mod luma {
@@ -100,7 +112,7 @@ index 6fb0618..06a3098 100644
          }
  
          /// Create a black luma color.
-@@ -206,7 +259,7 @@ pub mod luma {
+@@ -206,7 +271,7 @@ pub mod luma {
          }
  
          pub(crate) fn to_pdf_color(self) -> f32 {
@@ -109,7 +121,7 @@ index 6fb0618..06a3098 100644
          }
      }
  
-@@ -251,27 +304,43 @@ impl From<SpecialColor> for Color {
+@@ -251,27 +316,43 @@ impl From<SpecialColor> for Color {
  
  /// CMYK colors.
  pub mod cmyk {
@@ -162,7 +174,7 @@ index 6fb0618..06a3098 100644
          }
      }
  
-@@ -390,23 +459,35 @@ pub mod rgb {
+@@ -390,23 +471,35 @@ pub mod rgb {
  
  /// Separation (spot) colors.
  pub mod separation {
@@ -202,7 +214,7 @@ index 6fb0618..06a3098 100644
          }
  
          pub(crate) fn color_space(&self) -> SeparationSpace {
-@@ -498,7 +579,7 @@ pub mod separation {
+@@ -498,7 +591,7 @@ pub mod separation {
  pub mod device_n {
      use std::sync::Arc;
  
@@ -211,7 +223,7 @@ index 6fb0618..06a3098 100644
  
      /// A process colorant.
      #[derive(Debug, Eq, PartialEq, Hash, Clone, Copy)]
-@@ -656,7 +737,7 @@ pub mod device_n {
+@@ -656,7 +749,7 @@ pub mod device_n {
      /// A DeviceN color: one tint per colorant of its space.
      #[derive(Debug, Hash, Eq, PartialEq, Clone)]
      pub struct Color {
@@ -220,7 +232,7 @@ index 6fb0618..06a3098 100644
          pub(crate) space: DeviceNSpace,
      }
  
-@@ -666,20 +747,30 @@ pub mod device_n {
+@@ -666,20 +759,30 @@ pub mod device_n {
          ///
          /// Returns `None` unless there is one tint per colorant.
          pub fn new(tints: Vec<u8>, space: DeviceNSpace) -> Option<Self> {
@@ -258,7 +270,7 @@ index 6fb0618..06a3098 100644
      }
  
 diff --git a/src/graphics/shading_function.rs b/src/graphics/shading_function.rs
-index 74c56f3..8aae81a 100644
+index 74c56f3..47b55c5 100644
 --- a/src/graphics/shading_function.rs
 +++ b/src/graphics/shading_function.rs
 @@ -277,12 +277,12 @@ impl GradientProperties {
@@ -290,7 +302,7 @@ index 74c56f3..8aae81a 100644
 -            Color::Regular(RegularColor::Cmyk(c)) => inks.process = [c.0, c.1, c.2, c.3],
 -            Color::Regular(RegularColor::Luma(l)) => inks.process[3] = 255 - l.0,
 +            Color::Regular(RegularColor::Cmyk(c)) => inks.process = c.to_pdf_color(),
-+            Color::Regular(RegularColor::Luma(l)) => inks.process[3] = 1.0 - l.to_pdf_color(),
++            Color::Regular(RegularColor::Luma(l)) => inks.process[3] = l.0.complement(),
              Color::Regular(RegularColor::Rgb(_)) => return None,
              Color::Special(SpecialColor::Separation(s)) => {
                  let SeparationColorant::Custom(name) = &s.space.colorant else {

vendor/krilla/src/graphics/color.rs

@@ -108,6 +108,18 @@ impl Tint {
     pub(crate) fn to_u8(self) -> u8 {
         (self.0 * 255.0).round() as u8
     }
+
+    /// `1 - value`, the black ink of a gray. An 8-bit value `n / 255` gives `(255 - n) / 255`,
+    /// as the 8-bit constructors write it. Any other value is subtracted from 1 in f64, from
+    /// the shortest decimal that writes it, so gray 0.96 gives 0.04 rather than 0.04000002.
+    pub(crate) fn complement(self) -> f32 {
+        let byte = self.to_u8();
+        if Tint::from_u8(byte) == self {
+            return Tint::from_u8(255 - byte).0;
+        }
+        let written = self.0.to_string().parse::<f64>().unwrap_or(f64::from(self.0));
+        (1.0 - written) as f32
+    }
 }
 
 impl PartialEq for Tint {

vendor/krilla/src/graphics/shading_function.rs

@@ -359,7 +359,7 @@ pub(crate) fn common_color_space(colors: &[Color]) -> Option<Vec<Color>> {
         };
         match color {
             Color::Regular(RegularColor::Cmyk(c)) => inks.process = c.to_pdf_color(),
-            Color::Regular(RegularColor::Luma(l)) => inks.process[3] = 1.0 - l.to_pdf_color(),
+            Color::Regular(RegularColor::Luma(l)) => inks.process[3] = l.0.complement(),
             Color::Regular(RegularColor::Rgb(_)) => return None,
             Color::Special(SpecialColor::Separation(s)) => {
                 let SeparationColorant::Custom(name) = &s.space.colorant else {