Add support for opacities (#1844)

4 files changed, 113 insertions, 4 deletions

diff --git a/crates/typst-library/src/compute/construct.rs b/crates/typst-library/src/compute/construct.rs
index 4329f259..841bf406 100644
--- a/crates/typst-library/src/compute/construct.rs
+++ b/crates/typst-library/src/compute/construct.rs
@@ -105,10 +105,6 @@ pub fn luma(
 ///
 /// The color is specified in the sRGB color space.
 ///
-/// _Note:_ While you can specify transparent colors and Typst's preview will
-/// render them correctly, the PDF export does not handle them properly at the
-/// moment. This will be fixed in the future.
-///
 /// ## Example { #example }
 /// ```example
 /// #square(fill: rgb("#b1f2eb"))
diff --git a/crates/typst/src/export/pdf/external_graphics_state.rs b/crates/typst/src/export/pdf/external_graphics_state.rs
new file mode 100644
index 00000000..164de1b6
--- /dev/null
+++ b/crates/typst/src/export/pdf/external_graphics_state.rs
@@ -0,0 +1,37 @@
+use crate::export::pdf::{PdfContext, RefExt};
+use pdf_writer::Finish;
+
+/// A PDF external graphics state.
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
+pub struct ExternalGraphicsState {
+    // In the range 0-255, needs to be divided before being written into the graphics state!
+    pub stroke_opacity: u8,
+    // In the range 0-255, needs to be divided before being written into the graphics state!
+    pub fill_opacity: u8,
+}
+
+impl Default for ExternalGraphicsState {
+    fn default() -> Self {
+        Self { stroke_opacity: 255, fill_opacity: 255 }
+    }
+}
+
+impl ExternalGraphicsState {
+    pub fn uses_opacities(&self) -> bool {
+        self.stroke_opacity != 255 || self.fill_opacity != 255
+    }
+}
+
+/// Embed all used external graphics states into the PDF.
+#[tracing::instrument(skip_all)]
+pub fn write_external_graphics_states(ctx: &mut PdfContext) {
+    for external_gs in ctx.ext_gs_map.items() {
+        let gs_ref = ctx.alloc.bump();
+        ctx.ext_gs_refs.push(gs_ref);
+
+        let mut gs = ctx.writer.ext_graphics(gs_ref);
+        gs.non_stroking_alpha(external_gs.fill_opacity as f32 / 255.0)
+            .stroking_alpha(external_gs.stroke_opacity as f32 / 255.0);
+        gs.finish();
+    }
+}
diff --git a/crates/typst/src/export/pdf/mod.rs b/crates/typst/src/export/pdf/mod.rs
index 48485862..b650d5b9 100644
--- a/crates/typst/src/export/pdf/mod.rs
+++ b/crates/typst/src/export/pdf/mod.rs
@@ -1,5 +1,6 @@
 //! Exporting into PDF documents.
 
+mod external_graphics_state;
 mod font;
 mod image;
 mod outline;
@@ -21,6 +22,8 @@ use crate::geom::{Abs, Dir, Em};
 use crate::image::Image;
 use crate::model::Introspector;
 
+use external_graphics_state::ExternalGraphicsState;
+
 /// Export a document into a PDF file.
 ///
 /// Returns the raw bytes making up the PDF file.
@@ -30,6 +33,7 @@ pub fn pdf(document: &Document) -> Vec<u8> {
     page::construct_pages(&mut ctx, &document.pages);
     font::write_fonts(&mut ctx);
     image::write_images(&mut ctx);
+    external_graphics_state::write_external_graphics_states(&mut ctx);
     page::write_page_tree(&mut ctx);
     write_catalog(&mut ctx);
     ctx.writer.finish()
@@ -50,9 +54,11 @@ pub struct PdfContext<'a> {
     page_tree_ref: Ref,
     font_refs: Vec<Ref>,
     image_refs: Vec<Ref>,
+    ext_gs_refs: Vec<Ref>,
     page_refs: Vec<Ref>,
     font_map: Remapper<Font>,
     image_map: Remapper<Image>,
+    ext_gs_map: Remapper<ExternalGraphicsState>,
     /// For each font a mapping from used glyphs to their text representation.
     /// May contain multiple chars in case of ligatures or similar things. The
     /// same glyph can have a different text representation within one document,
@@ -78,8 +84,10 @@ impl<'a> PdfContext<'a> {
             page_refs: vec![],
             font_refs: vec![],
             image_refs: vec![],
+            ext_gs_refs: vec![],
             font_map: Remapper::new(),
             image_map: Remapper::new(),
+            ext_gs_map: Remapper::new(),
             glyph_sets: HashMap::new(),
             languages: HashMap::new(),
         }
diff --git a/crates/typst/src/export/pdf/page.rs b/crates/typst/src/export/pdf/page.rs
index 22e590d5..0c0d2957 100644
--- a/crates/typst/src/export/pdf/page.rs
+++ b/crates/typst/src/export/pdf/page.rs
@@ -5,6 +5,7 @@ use pdf_writer::types::{
 use pdf_writer::writers::ColorSpace;
 use pdf_writer::{Content, Filter, Finish, Name, Rect, Ref, Str};
 
+use super::external_graphics_state::ExternalGraphicsState;
 use super::{deflate, AbsExt, EmExt, PdfContext, RefExt, D65_GRAY, SRGB};
 use crate::doc::{Destination, Frame, FrameItem, GroupItem, Meta, TextItem};
 use crate::font::Font;
@@ -32,6 +33,7 @@ pub fn construct_page(ctx: &mut PdfContext, frame: &Frame) {
     let mut ctx = PageContext {
         parent: ctx,
         page_ref,
+        uses_opacities: false,
         content: Content::new(),
         state: State::default(),
         saves: vec![],
@@ -59,6 +61,7 @@ pub fn construct_page(ctx: &mut PdfContext, frame: &Frame) {
         size,
         content: ctx.content,
         id: ctx.page_ref,
+        uses_opacities: ctx.uses_opacities,
         links: ctx.links,
     };
 
@@ -98,6 +101,14 @@ pub fn write_page_tree(ctx: &mut PdfContext) {
     }
 
     images.finish();
+
+    let mut ext_gs_states = resources.ext_g_states();
+    for (gs_ref, gs) in ctx.ext_gs_map.pdf_indices(&ctx.ext_gs_refs) {
+        let name = eco_format!("Gs{}", gs);
+        ext_gs_states.pair(Name(name.as_bytes()), gs_ref);
+    }
+    ext_gs_states.finish();
+
     resources.finish();
     pages.finish();
 }
@@ -115,6 +126,16 @@ fn write_page(ctx: &mut PdfContext, page: Page) {
     page_writer.media_box(Rect::new(0.0, 0.0, w, h));
     page_writer.contents(content_id);
 
+    if page.uses_opacities {
+        page_writer
+            .group()
+            .transparency()
+            .isolated(false)
+            .knockout(false)
+            .color_space()
+            .srgb();
+    }
+
     let mut annotations = page_writer.annotations();
     for (dest, rect) in page.links {
         let mut annotation = annotations.push();
@@ -161,6 +182,8 @@ pub struct Page {
     pub size: Size,
     /// The page's content stream.
     pub content: Content,
+    /// Whether the page uses opacities.
+    pub uses_opacities: bool,
     /// Links in the PDF coordinate system.
     pub links: Vec<(Destination, Rect)>,
 }
@@ -173,6 +196,7 @@ struct PageContext<'a, 'b> {
     state: State,
     saves: Vec<State>,
     bottom: f32,
+    uses_opacities: bool,
     links: Vec<(Destination, Rect)>,
 }
 
@@ -184,6 +208,7 @@ struct State {
     font: Option<(Font, Abs)>,
     fill: Option<Paint>,
     fill_space: Option<Name<'static>>,
+    external_graphics_state: Option<ExternalGraphicsState>,
     stroke: Option<Stroke>,
     stroke_space: Option<Name<'static>>,
 }
@@ -199,6 +224,46 @@ impl PageContext<'_, '_> {
         self.state = self.saves.pop().expect("missing state save");
     }
 
+    fn set_external_graphics_state(&mut self, graphics_state: &ExternalGraphicsState) {
+        let current_state = self.state.external_graphics_state.as_ref();
+        if current_state != Some(graphics_state) {
+            self.parent.ext_gs_map.insert(*graphics_state);
+            let name = eco_format!("Gs{}", self.parent.ext_gs_map.map(*graphics_state));
+            self.content.set_parameters(Name(name.as_bytes()));
+
+            if graphics_state.uses_opacities() {
+                self.uses_opacities = true;
+            }
+        }
+    }
+
+    fn set_opacities(&mut self, stroke: Option<&Stroke>, fill: Option<&Paint>) {
+        let stroke_opacity = stroke
+            .map(|stroke| {
+                let Paint::Solid(color) = stroke.paint;
+                if let Color::Rgba(rgba_color) = color {
+                    rgba_color.a
+                } else {
+                    255
+                }
+            })
+            .unwrap_or(255);
+        let fill_opacity = fill
+            .map(|paint| {
+                let Paint::Solid(color) = paint;
+                if let Color::Rgba(rgba_color) = color {
+                    rgba_color.a
+                } else {
+                    255
+                }
+            })
+            .unwrap_or(255);
+        self.set_external_graphics_state(&ExternalGraphicsState {
+            stroke_opacity,
+            fill_opacity,
+        });
+    }
+
     fn transform(&mut self, transform: Transform) {
         let Transform { sx, ky, kx, sy, tx, ty } = transform;
         self.state.transform = self.state.transform.pre_concat(transform);
@@ -373,6 +438,7 @@ fn write_text(ctx: &mut PageContext, x: f32, y: f32, text: &TextItem) {
 
     ctx.set_fill(&text.fill);
     ctx.set_font(&text.font, text.size);
+    ctx.set_opacities(None, Some(&text.fill));
     ctx.content.begin_text();
 
     // Positiosn the text.
@@ -438,6 +504,8 @@ fn write_shape(ctx: &mut PageContext, x: f32, y: f32, shape: &Shape) {
         ctx.set_stroke(stroke);
     }
 
+    ctx.set_opacities(stroke, shape.fill.as_ref());
+
     match shape.geometry {
         Geometry::Line(target) => {
             let dx = target.x.to_f32();