PDF export: transparent content pixels take the backdrop colour (fixes #1455)

The content raster's fully transparent pixels were RGB 0,0,0. A viewer that
smooths the image blends colour and mask separately, so the black showed as a
grey hairline around white pictures and white-filled shapes.

crates/pdf/src/lib.rs

@@ -451,6 +451,17 @@ fn opaque_px(rgba: &mut [u8]) {
     }
 }
 
+/// Gives every fully transparent content pixel the backdrop's colour. The renderer leaves them
+/// RGB 0,0,0; a viewer that smooths the image blends colour and mask separately, so the black
+/// shows as a grey hairline around white pictures and shapes (#1455).
+fn fill_clear(front: &mut [u8], back: &[u8]) {
+    for (f, b) in front.as_chunks_mut::<4>().0.iter_mut().zip(back.as_chunks::<4>().0) {
+        if f[3] == 0 {
+            f[..3].copy_from_slice(&b[..3]);
+        }
+    }
+}
+
 /// Most bytes of opaque backdrop rasters kept to recognise a backdrop seen before.
 const BACKDROP_CACHE_BYTES: usize = 32 << 20;
 
@@ -515,7 +526,8 @@ impl Exporter<'_> {
         // Two rasters per slide: the backdrop (background, master and layout graphics) is the same
         // on every slide that shares its layout, so the writer stores it once, and the slide's own
         // shapes draw over it (#845).
-        let back = deckcraft_render::render_slide_part(pres, i, deckcraft_render::SlidePart::Backdrop, &ropts);
+        let mut back = deckcraft_render::render_slide_part(pres, i, deckcraft_render::SlidePart::Backdrop, &ropts);
+        opaque(&mut back.pixels);
         let front = deckcraft_render::render_slide_part(pres, i, deckcraft_render::SlidePart::Content, &ropts);
         let (w, h) = (back.width, back.height);
         if w > 0
@@ -523,11 +535,15 @@ impl Exporter<'_> {
             && let Some(size) = Size::from_wh(b.width() as f32, b.height() as f32)
         {
             s.push_transform(&Transform::from_translate(b.x0 as f32, b.y0 as f32));
-            s.draw_image(self.backdrops.image(back), size);
             // Taken, not copied: a print-resolution slide is ~16 MB.
             let mut rgba = front.pixels;
-            if rgba.as_chunks::<4>().0.iter().any(|px| px[3] != 0) {
+            let content = rgba.as_chunks::<4>().0.iter().any(|px| px[3] != 0);
+            if content {
+                fill_clear(&mut rgba, &back.pixels);
                 straight(&mut rgba);
+            }
+            s.draw_image(self.backdrops.image(back), size);
+            if content {
                 s.draw_image(Image::from_rgba8(rgba, w, h), size);
             }
             s.pop();

crates/pdf/src/tests.rs

@@ -7,8 +7,8 @@ use deckcraft_render::RenderOpts;
 use kurbo::Rect;
 
 use super::{
-    MAX_RASTER_PIXELS, PAPERS, PageLayout, PdfError, PdfOptions, Presentation, export, glyph_text_ranges, handout_boxes, master_text_pieces,
-    master_texts, notes_frames, notes_slide_box, opaque, paper_size, raster_scale, straight,
+    MAX_RASTER_PIXELS, PAPERS, PageLayout, PdfError, PdfOptions, Presentation, export, fill_clear, glyph_text_ranges, handout_boxes,
+    master_text_pieces, master_texts, notes_frames, notes_slide_box, opaque, paper_size, raster_scale, straight,
 };
 use deckcraft_model::text::TextBody;
 
@@ -582,7 +582,20 @@ fn slides_share_one_backdrop_raster_and_keep_their_pixels() {
         deckcraft_render::SlidePart::Content,
         &RenderOpts { scale: scale(&p), threads: 4, ..Default::default() },
     );
-    let straight = expect.to_straight();
+    let mut straight = expect.to_straight();
+    // Fully transparent pixels carry the backdrop colour, not black (#1455).
+    let mut back = deckcraft_render::render_slide_part(
+        &p,
+        0,
+        deckcraft_render::SlidePart::Backdrop,
+        &RenderOpts { scale: scale(&p), threads: 4, ..Default::default() },
+    );
+    opaque(&mut back.pixels);
+    for (px, b) in straight.as_chunks_mut::<4>().0.iter_mut().zip(back.pixels.as_chunks::<4>().0) {
+        if px[3] == 0 {
+            px[..3].copy_from_slice(&b[..3]);
+        }
+    }
     let want_rgb: Vec<u8> = straight.as_chunks::<4>().0.iter().flat_map(|px| px[..3].iter().copied()).collect();
     let want_alpha: Vec<u8> = straight.as_chunks::<4>().0.iter().map(|px| px[3]).collect();
     assert_eq!(rgb, want_rgb, "content colors are straight, not premultiplied");
@@ -634,6 +647,15 @@ fn backdrop_and_content_pixels_convert_per_pixel() {
     }
 }
 
+/// Fully transparent content pixels take the backdrop colour (not black) so a smoothing viewer
+/// draws no hairline at the edge of a picture; every other pixel is untouched (#1455).
+#[test]
+fn transparent_content_pixels_take_the_backdrop_colour() {
+    let mut front = vec![0, 0, 0, 0, 10, 20, 30, 255, 5, 5, 5, 1, 0, 0, 0, 0];
+    fill_clear(&mut front, &[255, 250, 240, 255].repeat(4));
+    assert_eq!(front, [255, 250, 240, 0, 10, 20, 30, 255, 5, 5, 5, 1, 255, 250, 240, 0]);
+}
+
 #[test]
 fn glyph_ranges_follow_shaping_clusters() {
     // "fl" ligature: its glyph maps to both characters, the rest stay one to one (#164).