Render: decode a cropped picture for the whole image's size, not its box's (#762)

A picture was decoded at the size of its box on screen. A cropped picture
shows only part of the image, magnified, so it was decoded at a fraction of
the resolution it needs and stretched back up: Merge Shapes > Intersect,
which crops the picture to the new smaller box, left it visibly blurred.
Size the decode from the rectangle the whole image occupies instead.

crates/render/src/paint.rs

@@ -188,16 +188,6 @@ pub fn picture(ctx: &mut RenderContext, pres: &Presentation, pf: &PictureFill, b
         return;
     };
     let px_scale = crate::scale_of(m);
-    let want = (bounds.width().max(bounds.height()) * px_scale).max(1.0);
-    let frame = gif_time.and_then(|t| images::gif_pixmap(&item.data, &pf.adjust, t));
-    let Some(pm) = frame.or_else(|| images::decode_for(&item.data, want, &pf.adjust)) else {
-        missing(ctx, bounds);
-        return;
-    };
-    let (iw, ih) = (pm.width() as f64, pm.height() as f64);
-    if iw < 1.0 || ih < 1.0 {
-        return;
-    }
     let dest = match &pf.mode {
         PictureMode::Stretch { fill_rect } => {
             let [l, t, r, b] = *fill_rect;
@@ -211,6 +201,18 @@ pub fn picture(ctx: &mut RenderContext, pres: &Presentation, pf: &PictureFill, b
         }
         PictureMode::Tile { .. } => bounds,
     };
+    // Decode for the whole image's size on screen, not the box's: a cropped picture shows only part
+    // of it, magnified (#762).
+    let want = (dest.width().abs().max(dest.height().abs()) * px_scale).max(1.0);
+    let frame = gif_time.and_then(|t| images::gif_pixmap(&item.data, &pf.adjust, t));
+    let Some(pm) = frame.or_else(|| images::decode_for(&item.data, want, &pf.adjust)) else {
+        missing(ctx, bounds);
+        return;
+    };
+    let (iw, ih) = (pm.width() as f64, pm.height() as f64);
+    if iw < 1.0 || ih < 1.0 {
+        return;
+    }
     let alpha = pf.alpha.unwrap_or(1.0).clamp(0.0, 1.0);
     let image = vello_cpu::Image {
         image: vello_cpu::ImageSource::Pixmap(pm.clone()),

crates/render/src/tests.rs

@@ -2509,3 +2509,35 @@ fn single_series_column_chart_varies_colours_by_point() {
     };
     assert_ne!(render(true), render(false));
 }
+
+/// A 1024×1024 PNG of 2 px wide black and white vertical stripes.
+fn fine_stripes_png() -> Vec<u8> {
+    let img =
+        image::RgbaImage::from_fn(1024, 1024, |x, _| if (x / 2) % 2 == 0 { image::Rgba([0, 0, 0, 255]) } else { image::Rgba([255, 255, 255, 255]) });
+    let mut out = std::io::Cursor::new(Vec::new());
+    image::DynamicImage::ImageRgba8(img).write_to(&mut out, image::ImageFormat::Png).expect("png");
+    out.into_inner()
+}
+
+#[test]
+fn a_cropped_picture_is_decoded_at_the_resolution_of_the_whole_image() {
+    // The top-left sixteenth of the image in a 64 pt box: 64 image pixels over 64 output pixels.
+    // Decoding at the box's size instead of the whole image's blurs the stripes to grey (#762).
+    let mut p = Presentation::default();
+    let media = p.add_media("stripes.png", "image/png", fine_stripes_png()).expect("media");
+    let fill = deckcraft_model::style::PictureFill { media, crop: [0.0, 0.0, 15.0 / 16.0, 15.0 / 16.0], ..Default::default() };
+    let sh = Shape { id: ShapeId(600), xfrm: Some(Xfrm::new(100.0, 100.0, 64.0, 64.0)), kind: ShapeKind::Picture { fill }, ..Default::default() };
+    std::sync::Arc::make_mut(&mut p.slides[0]).shapes = vec![sh];
+    let img = render_slide(&p, 0, &RenderOpts { scale: 1.0, ..Default::default() });
+    let (mut sharp, mut total) = (0, 0);
+    for y in 110..154 {
+        for x in 110..154 {
+            let v = img.pixel(x, y)[0];
+            total += 1;
+            if !(48..=208).contains(&v) {
+                sharp += 1;
+            }
+        }
+    }
+    assert!(sharp * 10 >= total * 9, "the stripes stay sharp: {sharp} of {total} pixels black or white");
+}