E: transparency blend space as the PDF page group

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ROADMAP.md

@@ -49,7 +49,7 @@ DesignCraft aims at full Adobe InDesign parity — and to be better: faster, ope
 - Endnotes (`endnote.insert/edit/delete/list/options`, Type ▸ Insert Endnote): references number through the document, the endnote frame on a new last page lists them under a heading; IDML keeps the listed text only.
 - Editorial notes (`note.new/edit/delete/convertToText/list`, Notes panel): anchored in text, flagged on screen, never printed or exported.
 - File ▸ Print Booklet (`file.printBooklet`): saddle-stitch or 2-up consecutive printer spreads as PDF, padded with blanks, with a gap between pages.
-- Edit ▸ Transparency Blend Space (`edit.transparencyBlendSpace`; RGB for web/mobile documents; IDML TransparencyPreference) — stored; compositing is still RGB on screen and in PDF.
+- Edit ▸ Transparency Blend Space (`edit.transparencyBlendSpace`; RGB for web/mobile documents; IDML TransparencyPreference) — exported PDFs give pages with transparency a DeviceCMYK or DeviceRGB page group, so viewers and RIPs blend in that space; on-screen compositing is still RGB.
 - High Contrast interface theme (fifth Interface Color Theme: black chrome, white text, yellow selection).
 - Place Excel workbooks (.xlsx): the first worksheet's used range becomes a table (shared/inline strings, numbers, booleans).
 - Object Library (`library.new/open/add/place/remove/list/json/close`, Library panel): `.dclib` files of snippets that keep stories, styles, swatches and images.

crates/engine/src/cmd/export.rs

@@ -247,11 +247,18 @@ fn export_pdf(s: &mut Session, p: &Value) -> Result<Value> {
     let doc: &designcraft_doc::Document = flattened.as_ref().map_or(&st.doc, |f| &f.0);
     let r = designcraft_pdf::export_pdf_with_report(doc, &s.cache, &opts).map_err(|e| EngineError::Other(e.to_string()))?;
     let mut r = r;
+    // Transparency Blend Space: the page group of pages with transparency.
+    let spreads = designcraft_pdf::sheet_spreads(doc, &opts);
+    let blended: Vec<bool> = spreads.iter().map(|si| doc.spreads.get(*si).is_some_and(|sp| sp.items.iter().any(|it| transparent(it)))).collect();
+    if blended.iter().any(|b| *b) {
+        match designcraft_pdf::add_blend_space(&r.bytes, &blended, doc.settings.blend_space == designcraft_doc::BlendSpace::Cmyk) {
+            Some(b) => r.bytes = b,
+            None => r.warnings.push("the transparency blend space couldn't be set".into()),
+        }
+    }
     // Page transitions (each spread's first page holds them).
-    let trans: Vec<Option<designcraft_doc::PageTransition>> = designcraft_pdf::sheet_spreads(doc, &opts)
-        .into_iter()
-        .map(|si| doc.spreads.get(si).and_then(|sp| sp.pages.first()).and_then(|pg| pg.transition))
-        .collect();
+    let trans: Vec<Option<designcraft_doc::PageTransition>> =
+        spreads.into_iter().map(|si| doc.spreads.get(si).and_then(|sp| sp.pages.first()).and_then(|pg| pg.transition)).collect();
     if trans.iter().any(Option::is_some) {
         match designcraft_pdf::add_transitions(&r.bytes, &trans) {
             Some(b) => r.bytes = b,
@@ -305,6 +312,10 @@ mod text_tests {
         // The document itself is untouched.
         assert!(s.doc().unwrap().doc.item(designcraft_doc::ItemId(id.as_u64().unwrap())).is_some());
         assert!(s.execute("file.exportPdf", &json!({"flatten": 5})).is_err());
+        // Unflattened, the page blends in the document's blend space (CMYK for print).
+        let r = s.execute("file.exportPdf", &json!({})).unwrap();
+        let text = String::from_utf8_lossy(&super::super::file::base64_decode(r["base64"].as_str().unwrap())).to_string();
+        assert_eq!(text.matches("/S/Transparency/CS/DeviceCMYK").count(), 1, "only the page with transparency");
     }
 
     #[test]

crates/engine/src/cmd/transitions.rs

@@ -99,6 +99,17 @@ mod tests {
         let r = s.execute("file.exportPdf", &json!({"spreads": true})).unwrap();
         let bytes = super::super::file::base64_decode(r["base64"].as_str().unwrap());
         assert_eq!(String::from_utf8_lossy(&bytes).matches("/Trans<<").count(), 1);
+        // With transparency too, the page keeps both its group and its transition.
+        let id = s.execute("frame.create", &json!({"rect": [100, 100, 200, 200]})).unwrap()["id"].clone();
+        s.execute("object.opacity", &json!({"ids": [id], "opacity": 0.5})).unwrap();
+        let r = s.execute("file.exportPdf", &json!({"pages": [1]})).unwrap();
+        let bytes = super::super::file::base64_decode(r["base64"].as_str().unwrap());
+        assert_eq!(designcraft_render::pdf_page_count(&bytes), Some(1));
+        s.execute("page.transition", &json!({"pages": [1], "kind": "fade"})).unwrap();
+        let r = s.execute("file.exportPdf", &json!({"pages": [1]})).unwrap();
+        let text = String::from_utf8_lossy(&super::super::file::base64_decode(r["base64"].as_str().unwrap())).to_string();
+        let last = &text[text.rfind("/Type/Page/").unwrap()..];
+        assert!(last.contains("/S/Fade") && last[..last.find("endobj").unwrap()].contains("/S/Transparency"), "{last}");
         s.execute("page.transition", &json!({"all": true, "kind": "none"})).unwrap();
         assert!(s.execute("page.transitions", &json!({})).unwrap().as_array().unwrap().is_empty());
     }

crates/pdf/src/lib.rs

@@ -34,7 +34,7 @@ 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, make_pdfx4};
-pub use transitions::add_transitions;
+pub use transitions::{add_blend_space, add_transitions};
 
 /// PDF standard to target.
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]

crates/pdf/src/transitions.rs

@@ -44,8 +44,9 @@ pub(crate) fn int_after(s: &str, key: &str) -> Option<usize> {
 
 /// Byte range of object `id`'s body (between `obj` and `endobj`).
 pub(crate) fn object(pdf: &[u8], id: usize) -> Option<std::ops::Range<usize>> {
+    // The latest copy (incremental updates re-state objects further on).
     let head = format!("\n{id} 0 obj");
-    let start = find(pdf, head.as_bytes(), 0)? + head.len();
+    let start = rfind(pdf, head.as_bytes())? + head.len();
     let end = find(pdf, b"endobj", start)?;
     Some(start..end)
 }
@@ -53,7 +54,19 @@ pub(crate) fn object(pdf: &[u8], id: usize) -> Option<std::ops::Range<usize>> {
 /// Add transitions to the pages of `pdf` (index = PDF page). `None` when the file's structure
 /// isn't the simple kind this understands (classic xref table, a flat page tree).
 pub fn add_transitions(pdf: &[u8], trans: &[Option<PageTransition>]) -> Option<Vec<u8>> {
-    if trans.iter().all(Option::is_none) {
+    add_page_entries(pdf, &trans.iter().map(|t| t.as_ref().map(trans_dict)).collect::<Vec<_>>())
+}
+
+/// Transparency blend space: pages with transparency get a page group in DeviceCMYK or DeviceRGB
+/// (`pages[i]` true = PDF page i).
+pub fn add_blend_space(pdf: &[u8], pages: &[bool], cmyk: bool) -> Option<Vec<u8>> {
+    let cs = if cmyk { "DeviceCMYK" } else { "DeviceRGB" };
+    add_page_entries(pdf, &pages.iter().map(|on| on.then(|| format!("/Group<</Type/Group/S/Transparency/CS/{cs}>>"))).collect::<Vec<_>>())
+}
+
+/// Append dictionary entries to pages (index = PDF page) as an incremental update.
+pub fn add_page_entries(pdf: &[u8], extra: &[Option<String>]) -> Option<Vec<u8>> {
+    if extra.iter().all(Option::is_none) {
         return Some(pdf.to_vec());
     }
     let t_at = rfind(pdf, b"trailer")?;
@@ -76,14 +89,14 @@ pub fn add_transitions(pdf: &[u8], trans: &[Option<PageTransition>]) -> Option<V
         out.push(b'\n');
     }
     let mut entries: Vec<(usize, usize)> = Vec::new();
-    for (i, t) in trans.iter().enumerate() {
+    for (i, t) in extra.iter().enumerate() {
         let (Some(t), Some(&kid)) = (t, kids.get(i)) else { continue };
         let body = String::from_utf8_lossy(&pdf[object(pdf, kid)?]).to_string();
         if !body.contains("/Type/Page") && !body.contains("/Type /Page") {
             return None;
         }
         let close = body.rfind(">>")?;
-        let new_body = format!("{}{}{}", &body[..close], trans_dict(t), &body[close..]);
+        let new_body = format!("{}{}{}", &body[..close], t, &body[close..]);
         entries.push((kid, out.len()));
         out.extend_from_slice(format!("{kid} 0 obj{new_body}endobj\n").as_bytes());
     }