Save and restore graphics state for every frame (#4496)

Co-authored-by: Laurenz <laurmaedje@gmail.com>

3 files changed, 35 insertions, 25 deletions

diff --git a/crates/typst-pdf/src/content.rs b/crates/typst-pdf/src/content.rs
index 8ae2c424..da9e4ed4 100644
--- a/crates/typst-pdf/src/content.rs
+++ b/crates/typst-pdf/src/content.rs
@@ -92,7 +92,7 @@ pub struct Builder<'a, R = ()> {
     state: State,
     /// Stack of saved graphic states.
     saves: Vec<State>,
-    /// Wheter any stroke or fill was not totally opaque.
+    /// Whether any stroke or fill was not totally opaque.
     uses_opacities: bool,
     /// All clickable links that are present in this content.
     links: Vec<(Destination, Rect)>,
@@ -129,7 +129,7 @@ struct State {
     /// The color space of the current fill paint.
     fill_space: Option<Name<'static>>,
     /// The current external graphic state.
-    external_graphics_state: Option<ExtGState>,
+    external_graphics_state: ExtGState,
     /// The current stroke paint.
     stroke: Option<FixedStroke>,
     /// The color space of the current stroke paint.
@@ -148,7 +148,7 @@ impl State {
             font: None,
             fill: None,
             fill_space: None,
-            external_graphics_state: None,
+            external_graphics_state: ExtGState::default(),
             stroke: None,
             stroke_space: None,
             text_rendering_mode: TextRenderingMode::Fill,
@@ -191,12 +191,13 @@ impl Builder<'_, ()> {
     }
 
     fn set_external_graphics_state(&mut self, graphics_state: &ExtGState) {
-        let current_state = self.state.external_graphics_state.as_ref();
-        if current_state != Some(graphics_state) {
+        let current_state = &self.state.external_graphics_state;
+        if current_state != graphics_state {
             let index = self.resources.ext_gs.insert(*graphics_state);
             let name = eco_format!("Gs{index}");
             self.content.set_parameters(Name(name.as_bytes()));
 
+            self.state.external_graphics_state = *graphics_state;
             if graphics_state.uses_opacities() {
                 self.uses_opacities = true;
             }
@@ -204,29 +205,27 @@ impl Builder<'_, ()> {
     }
 
     fn set_opacities(&mut self, stroke: Option<&FixedStroke>, fill: Option<&Paint>) {
-        let stroke_opacity = stroke
-            .map(|stroke| {
-                let color = match &stroke.paint {
-                    Paint::Solid(color) => *color,
-                    Paint::Gradient(_) | Paint::Pattern(_) => return 255,
-                };
-
-                color.alpha().map_or(255, |v| (v * 255.0).round() as u8)
-            })
-            .unwrap_or(255);
-        let fill_opacity = fill
-            .map(|paint| {
-                let color = match paint {
-                    Paint::Solid(color) => *color,
-                    Paint::Gradient(_) | Paint::Pattern(_) => return 255,
-                };
-
-                color.alpha().map_or(255, |v| (v * 255.0).round() as u8)
-            })
-            .unwrap_or(255);
+        let get_opacity = |paint: &Paint| {
+            let color = match paint {
+                Paint::Solid(color) => *color,
+                Paint::Gradient(_) | Paint::Pattern(_) => return 255,
+            };
+
+            color.alpha().map_or(255, |v| (v * 255.0).round() as u8)
+        };
+
+        let stroke_opacity = stroke.map_or(255, |stroke| get_opacity(&stroke.paint));
+        let fill_opacity = fill.map_or(255, get_opacity);
         self.set_external_graphics_state(&ExtGState { stroke_opacity, fill_opacity });
     }
 
+    fn reset_opacities(&mut self) {
+        self.set_external_graphics_state(&ExtGState {
+            stroke_opacity: 255,
+            fill_opacity: 255,
+        });
+    }
+
     pub fn transform(&mut self, transform: Transform) {
         let Transform { sx, ky, kx, sy, tx, ty } = transform;
         self.state.transform = self.state.transform.pre_concat(transform);
@@ -542,6 +541,8 @@ fn write_color_glyphs(ctx: &mut Builder, pos: Point, text: TextItemView) {
 
     let mut last_font = None;
 
+    ctx.reset_opacities();
+
     ctx.content.begin_text();
     ctx.content.set_text_matrix([1.0, 0.0, 0.0, -1.0, x, y]);
     // So that the next call to ctx.set_font() will change the font to one that
@@ -671,6 +672,8 @@ fn write_image(ctx: &mut Builder, x: f32, y: f32, image: &Image, size: Size) {
         image
     });
 
+    ctx.reset_opacities();
+
     let name = eco_format!("Im{index}");
     let w = size.x.to_f32();
     let h = size.y.to_f32();
diff --git a/tests/ref/issue-4361-transparency-leak.png b/tests/ref/issue-4361-transparency-leak.png
new file mode 100644
index 00000000..4060d43a
--- /dev/null
+++ b/tests/ref/issue-4361-transparency-leak.png
diff --git a/tests/suite/visualize/color.typ b/tests/suite/visualize/color.typ
index 45000ab2..bc8f8be5 100644
--- a/tests/suite/visualize/color.typ
+++ b/tests/suite/visualize/color.typ
@@ -333,3 +333,10 @@
 --- issue-color-mix-luma ---
 // When mixing luma colors, we accidentally used the wrong component.
 #rect(fill: gradient.linear(black, silver, space: luma))
+
+--- issue-4361-transparency-leak ---
+// Ensure that transparency doesn't leak from shapes to images in PDF. The PNG
+// test doesn't validate it, but at least we can discover regressions on the PDF
+// output with a PDF comparison script.
+#rect(fill: red.transparentize(50%))
+#image("/assets/images/tiger.jpg", width: 45pt)