PDF/X-4: warn when a transparency group still blends in RGB

The CMYK rewrite matches krilla's exact output; if that ever changes,
the export says so instead of silently writing RGB groups.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/engine/src/cmd/export.rs

@@ -888,12 +888,14 @@ mod print_group_tests {
         let t = text(&mut s);
         assert!(t.contains("/S/Transparency/I true/CS/DeviceCMYK>>"), "CMYK group");
         assert!(!t.contains("DeviceRGB"), "no RGB left");
+        assert!(!designcraft_pdf::has_rgb_groups(t.as_bytes()));
         // The file still reads, with the same page.
         let r = s.execute("file.exportPdf", &json!({})).unwrap();
         let bytes = base64_decode(r["base64"].as_str().unwrap());
         assert_eq!(hayro_syntax::Pdf::new(bytes).unwrap().pages().len(), 1);
         // An RGB blend space keeps RGB groups.
         s.execute("edit.transparencyBlendSpace", &json!({"space": "rgb"})).unwrap();
-        assert!(text(&mut s).contains("/CS/DeviceRGB"));
+        let t = text(&mut s);
+        assert!(t.contains("/CS/DeviceRGB") && designcraft_pdf::has_rgb_groups(t.as_bytes()));
     }
 }

crates/pdf/src/export.rs

@@ -205,10 +205,14 @@ pub fn export_pdf_with_report(doc: &Document, cache: &Cache, opts: &PdfOptions)
     ex.check_placed_versions()?;
     let mut bytes = pdf.finish().map_err(|e| PdfError::Write(format!("{e:?}")))?;
     // Transparency blends in the document's blend space: CMYK for print (and always in PDF/X).
+    let mut warnings = ex.warnings;
     if !ex.rgb_only && (doc.settings.blend_space == designcraft_doc::BlendSpace::Cmyk || opts.standard == Standard::PdfX4) {
         crate::pdfx::cmyk_group_spaces(&mut bytes);
+        // The rewrite matches krilla's exact output; say so if a group got past it.
+        if opts.standard == Standard::PdfX4 && crate::pdfx::has_rgb_groups(&bytes) {
+            warnings.push("PDF/X-4: some transparency groups still blend in RGB".into());
+        }
     }
-    let mut warnings = ex.warnings;
     // Form fields (Buttons and Forms) and, for interactive PDF, video and sound; not in PDF/X.
     let fields = crate::forms::collect(doc, &sheets);
     let media = if opts.media { crate::forms::collect_media(doc, &sheets) } else { Vec::new() };

crates/pdf/src/lib.rs

@@ -35,7 +35,7 @@ mod forms;
 mod pdfx;
 mod transitions;
 pub use export::{BookletKind, BookletOptions, booklet_pairs, export_booklet, export_pdf, export_pdf_with_report, merge_pdfs, sheet_spreads};
-pub use pdfx::{check_pdfx4, cmyk_group_spaces, make_pdfx4};
+pub use pdfx::{check_pdfx4, cmyk_group_spaces, has_rgb_groups, make_pdfx4};
 pub use transitions::{add_blend_space, add_catalog_entries, add_page_entries, add_transitions};
 
 /// PDF standard to target.

crates/pdf/src/pdfx.rs

@@ -125,6 +125,13 @@ pub fn make_pdfx4(pdf: &[u8], title: &str) -> Option<Vec<u8>> {
 /// DeviceCMYK in place and at the same length (the optional `/Type/Group` gives the room), so
 /// every cross-reference offset stays valid. Only object dictionaries are touched, never stream
 /// data. Returns how many groups changed.
+/// Whether a transparency group dictionary still names DeviceRGB as its blending space.
+pub fn has_rgb_groups(pdf: &[u8]) -> bool {
+    pdf.windows(b"/CS/DeviceRGB".len()).enumerate().any(|(i, w)| {
+        w == b"/CS/DeviceRGB" && pdf.get(i.saturating_sub(64)..i).is_some_and(|before| before.windows(15).any(|b| b == b"/S/Transparency"))
+    })
+}
+
 pub fn cmyk_group_spaces(pdf: &mut [u8]) -> usize {
     const FORMS: [(&[u8], &[u8]); 2] = [
         (b"/Group<</Type/Group/S/Transparency/I true/CS/DeviceRGB>>", b"/Group<</S/Transparency/I true/CS/DeviceCMYK>>"),