pdf: CMYK blending without an output intent for plain print PDFs

print::cmyk_blending(profile) blends transparency in CMYK and writes no
output intent or /Trapped, as a non-PDF/X print PDF does. print::settings
builds on it and adds the GTS_PDFX output intent and /Trapped /False.

crates/pdf/README.md

@@ -122,10 +122,10 @@ serialization uses the same backend version as before extraction. It provides:
 
 ### Print color
 
-`write::print::settings` returns `SerializeSettings` for CMYK print: DeviceCMYK color,
-transparency blending in CMYK (`GroupColorSpace::Cmyk`), the document's CMYK profile as a
-`GTS_PDFX` output intent and `/Trapped /False`. The profile is the document's working space;
-print export always carries it. craft-pdf takes the profile bytes and does not depend on
+`write::print::cmyk_blending(profile)` returns `SerializeSettings` for a plain CMYK print
+PDF: DeviceCMYK color and transparency blending in CMYK (`GroupColorSpace::Cmyk`), with no
+output intent. `write::print::settings` adds the document's CMYK profile as a `GTS_PDFX`
+output intent and `/Trapped /False`, for PDF/X. The profile is the document's working space. craft-pdf takes the profile bytes and does not depend on
 craft-cms. Its CC0 `builtin::COATED_FOGRA39_ICC` is the default working space, described by
 `PrintColor::fogra39`:
 

crates/pdf/src/write/print.rs

@@ -1,8 +1,9 @@
 //! Serialization settings for CMYK print output.
 //!
-//! The document's CMYK profile, its working space, describes the printing condition: the
-//! settings write it as the `GTS_PDFX` output intent, and DeviceCMYK content and the CMYK
-//! blending groups are interpreted through it. This module takes the profile bytes and has no
+//! The document's CMYK profile, its working space, describes the printing condition.
+//! [`cmyk_blending`] blends transparency in CMYK and writes no output intent, like a plain
+//! print PDF; [`settings`] also writes the profile as the `GTS_PDFX` output intent, through
+//! which DeviceCMYK content and the CMYK blending groups are interpreted, as PDF/X needs. This module takes the profile bytes and has no
 //! color-management dependency; craft-cms's `builtin::COATED_FOGRA39_ICC` is a suitable
 //! default profile, described by [`PrintColor::fogra39`].
 //!
@@ -42,28 +43,41 @@ impl<'a> PrintColor<'a> {
     }
 }
 
-/// Settings for CMYK print output: DeviceCMYK color, CMYK blending groups, the profile as
-/// the `GTS_PDFX` output intent (and as `cmyk_profile`) and `/Trapped /False`. Other fields
-/// keep their defaults; override them with struct update syntax.
+/// Settings for a CMYK print PDF without an output intent: DeviceCMYK color and CMYK
+/// blending groups, with `profile` as `cmyk_profile`. Other fields keep their defaults;
+/// override them with struct update syntax.
 ///
 /// Fails with [`WriteError::InvalidProfile`] when `profile` is not a readable CMYK ICC
 /// profile.
-pub fn settings(color: PrintColor<'_>) -> Result<SerializeSettings, WriteError> {
+pub fn cmyk_blending(profile: &[u8]) -> Result<SerializeSettings, WriteError> {
     guard(|| {
-        let profile = ICCProfile::<4>::new(color.profile).ok_or(WriteError::InvalidProfile)?;
+        let profile = ICCProfile::<4>::new(profile).ok_or(WriteError::InvalidProfile)?;
         Ok(SerializeSettings {
             no_device_cs: false,
-            cmyk_profile: Some(profile.clone()),
+            cmyk_profile: Some(profile),
             group_color_space: GroupColorSpace::Cmyk,
-            output_intent: Some(OutputIntent {
-                profile,
-                condition_identifier: color.condition_identifier.to_owned(),
-                condition: color.condition.map(str::to_owned),
-                registry_name: color.registry_name.map(str::to_owned),
-                info: color.info.map(str::to_owned),
-            }),
-            trapped: Some(Trapped::False),
             ..SerializeSettings::default()
         })
     })
 }
+
+/// [`cmyk_blending`] plus the profile as the `GTS_PDFX` output intent and `/Trapped /False`,
+/// for PDF/X export.
+///
+/// Fails with [`WriteError::InvalidProfile`] when `profile` is not a readable CMYK ICC
+/// profile.
+pub fn settings(color: PrintColor<'_>) -> Result<SerializeSettings, WriteError> {
+    let settings = cmyk_blending(color.profile)?;
+    let profile = settings.cmyk_profile.clone().ok_or(WriteError::InvalidProfile)?;
+    Ok(SerializeSettings {
+        output_intent: Some(OutputIntent {
+            profile,
+            condition_identifier: color.condition_identifier.to_owned(),
+            condition: color.condition.map(str::to_owned),
+            registry_name: color.registry_name.map(str::to_owned),
+            info: color.info.map(str::to_owned),
+        }),
+        trapped: Some(Trapped::False),
+        ..settings
+    })
+}

crates/pdf/tests/print_color.rs

@@ -987,6 +987,21 @@ fn print_settings_blend_in_cmyk_and_name_the_profile_as_the_output_intent() {
     assert!(!text.contains("GTS_PDFXVersion"));
 }
 
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[cfg_attr(not(target_arch = "wasm32"), test)]
+fn cmyk_blending_writes_no_output_intent_or_trapped() {
+    let settings = print::cmyk_blending(FOGRA39).unwrap();
+    assert_eq!(settings.group_color_space, GroupColorSpace::Cmyk);
+    assert!(settings.output_intent.is_none() && settings.trapped.is_none());
+
+    let bytes = sample(settings.clone(), placed(&settings).as_deref()).unwrap();
+    let cmyk_group = Some(Some("DeviceCMYK".to_owned()));
+    assert_eq!(groups(&bytes).pages, vec![cmyk_group.clone(), None, cmyk_group]);
+    assert!(output_intents(&bytes).is_empty());
+    assert!(!String::from_utf8_lossy(&bytes).contains("Trapped"));
+    assert!(matches!(print::cmyk_blending(b"not a profile"), Err(WriteError::InvalidProfile)));
+}
+
 #[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
 #[cfg_attr(not(target_arch = "wasm32"), test)]
 fn print_settings_reject_a_profile_that_is_not_cmyk() {