Render: slide backgrounds are laid over white

PowerPoint shows white around a background picture stretched into part of
the slide and under a see-through background colour. DeckCraft left those
areas transparent, which exports and the slide show draw black.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/fonts/src/fontdb.rs

@@ -79,6 +79,10 @@ pub struct FontFace {
     /// values).
     pub cap_height: f64,
     pub x_height: f64,
+    /// Where the baseline sits in a line, as a share of the line's height from its top: the
+    /// Windows ascent over the Windows ascent plus descent (OS/2 `usWinAscent`/`usWinDescent`),
+    /// the split PowerPoint uses (computed by `line_split`).
+    pub line_split: f64,
     pub shaper: harfrust::ShaperData,
     /// Variable fonts: the named instance's axis settings (user units; empty = default instance),
     /// the normalised location and the shaper's view of it.
@@ -436,6 +440,23 @@ pub(crate) fn last_resort_face() -> Arc<FontFace> {
     .clone()
 }
 
+/// The share of a line above the baseline (`FontFace::line_split`): OS/2 `usWinAscent` over
+/// `usWinAscent + usWinDescent`, else the ascent over ascent plus descent. Fonts whose Windows
+/// extent is huge (Gabriola, Cambria Math: over 1.5 em) get 0.81, where PowerPoint puts their
+/// baseline (observed).
+fn line_split(f: &skrifa::FontRef, ascent: f64, descent: f64, upem: f64) -> f64 {
+    use skrifa::raw::TableProvider;
+    let (a, d) = match f.os2() {
+        Ok(os2) if os2.us_win_ascent() > 0 => (os2.us_win_ascent() as f64, os2.us_win_descent() as f64),
+        _ => (ascent, descent),
+    };
+    let total = a + d;
+    if !total.is_finite() || total <= 0.0 || a < 0.0 || d < 0.0 {
+        return 0.8;
+    }
+    if total > upem * 1.5 { 0.81 } else { (a / total).clamp(0.5, 1.0) }
+}
+
 fn make_face(bytes: FontBytes, index: u32, family: String, style: String, coords: Vec<([u8; 4], f32)>) -> Option<FontFace> {
     let data: &[u8] = match &bytes {
         FontBytes::Static(b) => b,
@@ -467,6 +488,7 @@ fn make_face(bytes: FontBytes, index: u32, family: String, style: String, coords
         descent: -(m.descent as f64),
         cap_height: m.cap_height.map(|v| v as f64).filter(|v| *v > 0.0).unwrap_or(m.ascent as f64 * 0.72),
         x_height: m.x_height.map(|v| v as f64).filter(|v| *v > 0.0).unwrap_or(m.ascent as f64 * 0.5),
+        line_split: line_split(&f, m.ascent as f64, -(m.descent as f64), m.units_per_em.max(1) as f64),
         shaper,
         coords,
         location,

crates/fonts/src/lib.rs

@@ -143,6 +143,17 @@ mod tests {
         assert!(f.upem > 0.0);
     }
 
+    #[test]
+    fn line_split_follows_the_windows_ascent_and_descent() {
+        use skrifa::raw::TableProvider;
+        let f = fontdb::last_resort_face();
+        let font = skrifa::FontRef::new(f.data()).unwrap();
+        let os2 = font.os2().unwrap();
+        let (a, d) = (os2.us_win_ascent() as f64, os2.us_win_descent() as f64);
+        assert!((f.line_split - a / (a + d)).abs() < 1e-9, "{} vs {}", f.line_split, a / (a + d));
+        assert!(f.line_split > 0.5 && f.line_split < 1.0);
+    }
+
     #[test]
     fn some_face_always_resolves() {
         let db = FontDb::with_font_dirs(Vec::new());

crates/render/src/lib.rs

@@ -222,11 +222,14 @@ pub struct Renderer {
     resources: Resources,
     /// Outlines morphing in the frame being drawn by `blend`.
     paths: Vec<PathMorph>,
+    /// The background of the slide (layout, master) being drawn, its placeholder colour and the
+    /// slide's view transform: shapes filled with the background (`useBgFill`) show it through.
+    bg: Option<(Fill, Option<Rgba>, Affine)>,
 }
 
 impl Default for Renderer {
     fn default() -> Self {
-        Renderer { resources: Resources::new(), paths: Vec::new() }
+        Renderer { resources: Resources::new(), paths: Vec::new(), bg: None }
     }
 }
 
@@ -589,7 +592,12 @@ impl Renderer {
         let r = Rect::new(0.0, 0.0, f.pres.slide_size.width, f.pres.slide_size.height);
         ctx.set_transform(view);
         let path = r.to_path(0.1);
+        // PowerPoint lays every background over white: a picture stretched into part of the
+        // slide, or a see-through colour, leaves white around and under it.
+        ctx.set_paint(color(Rgba::WHITE, 1.0));
+        ctx.fill_path(&path);
         paint::fill_path(ctx, rctx, &fill, ph, &path, r, view, FillMode::Norm, 1.0, f.pres);
+        self.bg = Some((fill, ph, view));
     }
 
     fn shape(&mut self, ctx: &mut RenderContext, f: &Frame, rctx: &Ctx, s: &Shape, parent: Affine, depth: usize) {
@@ -711,7 +719,16 @@ impl Renderer {
                         }
                         ctx.set_transform(m);
                         ctx.set_fill_rule(if sp.even_odd { peniko::Fill::EvenOdd } else { peniko::Fill::NonZero });
-                        paint::fill_path(ctx, rctx, fl, fill_ph, &sp.path, Rect::new(0.0, 0.0, w, h), m, sp.fill, fade(i).fill, f.pres);
+                        match (fl, &self.bg) {
+                            // `useBgFill`: the slide's background, laid out on the slide (not the
+                            // shape), shows through the shape.
+                            (Fill::Background, Some((bg, ph, view))) => {
+                                let slide = Rect::new(0.0, 0.0, f.pres.slide_size.width, f.pres.slide_size.height);
+                                let path = (view.inverse() * m) * sp.path.clone();
+                                paint::fill_path(ctx, rctx, bg, *ph, &path, slide, *view, sp.fill, fade(i).fill, f.pres);
+                            }
+                            _ => paint::fill_path(ctx, rctx, fl, fill_ph, &sp.path, Rect::new(0.0, 0.0, w, h), m, sp.fill, fade(i).fill, f.pres),
+                        }
                         ctx.set_fill_rule(peniko::Fill::NonZero);
                     }
                 }

crates/render/src/tests.rs

@@ -572,3 +572,63 @@ fn a_gradient_with_a_stop_off_the_end_stays_linear() {
         assert!((got - want).abs() <= 4, "x {x}: {got} vs {want}");
     }
 }
+
+#[test]
+fn a_shape_filled_with_the_background_shows_the_slide_background() {
+    // `useBgFill`: PowerPoint paints the slide's background into the shape, laid out on the
+    // slide. A white panel covers a black-to-grey background; a shape on it with the background
+    // fill is a window onto that background.
+    use deckcraft_model::style::{Fill, Gradient, GradientShape, GradientStop};
+    let none = || Some(deckcraft_model::style::Line { fill: Some(Fill::None), ..Default::default() });
+    let panel = Shape {
+        id: ShapeId(710),
+        xfrm: Some(Xfrm::new(0.0, 0.0, 960.0, 540.0)),
+        fill: Some(Fill::solid(ColorRef::rgb(deckcraft_color::Rgba::rgb(255, 255, 255)))),
+        line: none(),
+        ..Default::default()
+    };
+    let window = Shape {
+        id: ShapeId(711),
+        xfrm: Some(Xfrm::new(100.0, 100.0, 760.0, 340.0)),
+        fill: Some(Fill::Background),
+        line: none(),
+        ..Default::default()
+    };
+    let mut p = deck_with(vec![panel, window]);
+    let stop = |pos, v| GradientStop { pos, color: ColorRef::rgb(deckcraft_color::Rgba::rgb(v, v, v)) };
+    std::sync::Arc::make_mut(&mut p.slides[0]).background = Some(deckcraft_model::Background::Fill {
+        fill: Fill::Gradient(Gradient {
+            stops: vec![stop(0.0, 0), stop(0.5, 0), stop(1.0, 200)],
+            shape: GradientShape::Linear { angle: 0.0, scaled: false },
+            rotate_with_shape: true,
+        }),
+    });
+    let img = render_slide(&p, 0, &RenderOpts { scale: 1.0, ..Default::default() });
+    assert_eq!(img.pixel(50, 50)[0], 255, "the panel stays white outside the window");
+    assert!(img.pixel(300, 270)[0] < 8, "the window shows the background's black half: {:?}", img.pixel(300, 270));
+    // Laid out on the slide: three quarters across the slide is half way from black to grey.
+    let v = img.pixel(720, 270)[0] as i32;
+    assert!((v - 100).abs() <= 6, "{v}");
+}
+
+#[test]
+fn a_background_is_laid_over_white() {
+    // A background picture stretched into the bottom-right quarter (fillRect l=50% t=50%), and a
+    // half-transparent colour: PowerPoint shows white around and under them, not transparency.
+    use deckcraft_model::style::{Fill, PictureFill, PictureMode};
+    let mut png = std::io::Cursor::new(Vec::new());
+    image::DynamicImage::ImageRgb8(image::RgbImage::from_pixel(4, 4, image::Rgb([0, 0, 200]))).write_to(&mut png, image::ImageFormat::Png).unwrap();
+    let mut p = deck_with(vec![]);
+    let media = p.add_media("bg.png", "image/png", png.into_inner());
+    let pf = PictureFill { media, mode: PictureMode::Stretch { fill_rect: [0.5, 0.5, 0.0, 0.0] }, ..Default::default() };
+    std::sync::Arc::make_mut(&mut p.slides[0]).background = Some(deckcraft_model::Background::Fill { fill: Fill::Picture(pf) });
+    let img = render_slide(&p, 0, &RenderOpts { scale: 0.5, ..Default::default() });
+    assert_eq!(img.pixel(20, 20), [255, 255, 255, 255], "white where the picture doesn't reach");
+    let q = img.pixel(400, 230);
+    assert!(q[2] > 190 && q[0] < 10 && q[3] == 255, "{q:?}");
+    let see_through = ColorRef::rgb(deckcraft_color::Rgba { r: 0, g: 0, b: 0, a: 128 });
+    std::sync::Arc::make_mut(&mut p.slides[0]).background = Some(deckcraft_model::Background::Fill { fill: Fill::solid(see_through) });
+    let img = render_slide(&p, 0, &RenderOpts { scale: 0.5, ..Default::default() });
+    let g = img.pixel(20, 20);
+    assert!(g[3] == 255 && (g[0] as i32 - 127).abs() <= 3, "half-transparent black over white is grey: {g:?}");
+}

crates/text/src/lib.rs

@@ -702,9 +702,20 @@ fn measure_str(face: &FontFace, size: f64, s: &str) -> (f64, Vec<(u32, f64)>) {
     (x, out)
 }
 
+/// Single line spacing as a multiple of the font size. PowerPoint spaces single-spaced lines 1.2
+/// times the size apart in every font (observed on twenty Latin, CJK, Thai and Indic fonts at 18
+/// and 40 pt), whatever the font's own ascent and descent.
+///
+/// Caveat: this was measured on one Windows install of desktop PowerPoint. It may be a Windows or
+/// machine setting; PowerPoint on macOS or the web may space lines by the font's own metrics.
+/// Keep it a single tunable value, and re-measure on the other platform before changing it.
+const LINE_PITCH: f64 = 1.2;
+
+/// A line's height above and below the baseline at `size`: [`LINE_PITCH`] × size, split where
+/// the font's Windows ascent and descent put the baseline.
 fn line_metrics(face: &FontFace, size: f64) -> (f64, f64) {
-    let k = size / face.upem.max(1.0);
-    (face.ascent * k, face.descent * k)
+    let h = size * LINE_PITCH;
+    (h * face.line_split, h * (1.0 - face.line_split))
 }
 
 /// Break opportunities: allowed[i] = a line may start at char i.

crates/text/src/tests.rs

@@ -489,3 +489,34 @@ fn an_ordinary_hang_still_puts_the_text_at_the_margin() {
     assert!((first - rest).abs() < 0.5, "first {first} rest {rest}");
     assert!((rest - bx.unwrap() - 36.0).abs() < 0.5);
 }
+
+#[test]
+fn single_spaced_lines_are_1_2_times_the_size_apart_whatever_the_font() {
+    // PowerPoint spaces single-spaced lines 1.2 × the font size apart in every font, with the
+    // baseline where the font's Windows ascent/descent split puts it (observed on twenty fonts).
+    for size in [18.0, 40.0] {
+        let mut run = Run::new("HHH");
+        run.props.size = Some(size);
+        let (p, mut sh) = bidi_setup(vec![run], 2000.0, false, Align::Left);
+        let t = sh.text.as_mut().unwrap();
+        t.body.autofit = Some(AutoFit::None);
+        let second = t.paragraphs[0].clone();
+        t.paragraphs.push(second.clone());
+        t.paragraphs.push(second);
+        for para in &mut t.paragraphs {
+            para.props.line_spacing = Some(Spacing::Pct(1.0));
+            para.props.space_before = Some(Spacing::Pts(0.0));
+            para.props.space_after = Some(Spacing::Pts(0.0));
+        }
+        let l = lay(&p, &sh, 2000.0, 600.0);
+        assert_eq!(l.lines.len(), 3);
+        // Each line is 1.2 × the size tall (the face is not compared across lines: another
+        // test's thread can load the family between them, see #252).
+        for line in &l.lines {
+            assert!((line.bottom - line.top - size * LINE_PITCH).abs() < 1e-6, "{size}: {}", line.bottom - line.top);
+        }
+        let face = &l.runs[0].face;
+        let first = &l.lines[0];
+        assert!((first.baseline - first.top - size * LINE_PITCH * face.line_split).abs() < 1e-6);
+    }
+}