diff --git a/crates/trace/src/raster.rs b/crates/trace/src/raster.rs
index 5e198e6fb7..51312c685a 100644
--- a/crates/trace/src/raster.rs
+++ b/crates/trace/src/raster.rs
@@ -48,6 +48,54 @@ pub(crate) fn fit_size(w: u32, h: u32, max: u64) -> (u32, u32) {
     (nw, nh)
 }
 
+/// The `w × h` RGBA8 `rgba` reduced to `nw × nh` (at most `w × h`): each pixel covers a block of
+/// whole source pixels, and every source pixel falls in exactly one block. A pixel's alpha is the
+/// mean of its block's alphas, and its color is the block's colors weighted by their alpha, so a
+/// clear pixel next to an opaque one does not darken or tint it. A wholly clear block (every alpha
+/// 0) keeps the plain mean of its colors.
+pub(crate) fn box_reduced(rgba: &[u8], w: u32, h: u32, nw: u32, nh: u32) -> Vec<u8> {
+    let span = |i: u32, n: u32, of: u32| {
+        let at = |i: u32| usize::try_from(u64::from(i) * u64::from(of) / u64::from(n.max(1))).unwrap_or(usize::MAX);
+        let start = at(i);
+        start..at(i + 1).max(start + 1)
+    };
+    let cols: Vec<_> = (0..nw).map(|x| span(x, nw, w)).collect();
+    let mut out = Vec::with_capacity(nw as usize * nh as usize * 4);
+    // Per output column: the alpha-weighted color sums, the alpha sum and the plain color sums.
+    let mut sums = vec![[0u64; 7]; cols.len()];
+    let row_len = w as usize * 4;
+    for ty in 0..nh {
+        let rows = span(ty, nh, h);
+        sums.fill([0; 7]);
+        for y in rows.clone() {
+            let Some(row) = rgba.get(y * row_len..(y + 1) * row_len) else { break };
+            for (s, c) in sums.iter_mut().zip(&cols) {
+                let px = row.get(c.start * 4..c.end * 4).unwrap_or_default();
+                for &[r, g, b, a] in px.as_chunks::<4>().0 {
+                    let a64 = u64::from(a);
+                    s[0] += u64::from(r) * a64;
+                    s[1] += u64::from(g) * a64;
+                    s[2] += u64::from(b) * a64;
+                    s[3] += a64;
+                    s[4] += u64::from(r);
+                    s[5] += u64::from(g);
+                    s[6] += u64::from(b);
+                }
+            }
+        }
+        for (s, c) in sums.iter().zip(&cols) {
+            let count = (rows.len() * c.len()).max(1) as u64;
+            let mean = |v: u64, n: u64| u8::try_from((v + n / 2) / n).unwrap_or(u8::MAX);
+            if s[3] == 0 {
+                out.extend_from_slice(&[mean(s[4], count), mean(s[5], count), mean(s[6], count), 0]);
+            } else {
+                out.extend_from_slice(&[mean(s[0], s[3]), mean(s[1], s[3]), mean(s[2], s[3]), mean(s[3], count)]);
+            }
+        }
+    }
+    out
+}
+
 /// An RGBA8 raster (row-major, top row first).
 #[derive(Clone, Debug, PartialEq)]
 pub struct Raster {
@@ -150,12 +198,14 @@ impl Raster {
         let mut limits = image::Limits::default();
         limits.max_alloc = Some(if cfg!(target_arch = "wasm32") { 512 << 20 } else { MAX_SOURCE_PIXELS * 4 });
         reader.limits(limits);
-        let img = reader.decode().map_err(decode_err)?.into_rgba8();
+        Ok(Self::fitted(&reader.decode().map_err(decode_err)?.into_rgba8(), MAX_PIXELS))
+    }
+    /// `img` scaled down, aspect kept, to the largest size of at most `max` pixels.
+    pub(crate) fn fitted(img: &image::RgbaImage, max: u64) -> Self {
         let (w, h) = img.dimensions();
-        let (nw, nh) = fit_size(w, h, MAX_PIXELS);
+        let (nw, nh) = fit_size(w, h, max);
         // Box-filtered: every source pixel counts, so thin lines and anti-aliased edges survive.
-        let small = image::imageops::thumbnail(&img, nw, nh);
-        Ok(Self { width: nw, height: nh, rgba: small.into_raw() })
+        Self { width: nw, height: nh, rgba: box_reduced(img.as_raw(), w, h, nw, nh) }
     }
     /// Encode as PNG.
     pub fn encode_png(&self) -> Vec<u8> {
diff --git a/crates/trace/src/tests.rs b/crates/trace/src/tests.rs
index 2bf9e1e2b6..80eb322bbc 100644
--- a/crates/trace/src/tests.rs
+++ b/crates/trace/src/tests.rs
@@ -1085,6 +1085,30 @@ fn decode_to_fit_scales_an_oversized_image_down() {
     assert!(matches!(Raster::decode_to_fit(&png_header(9000, 9000)), Err(TraceError::Decode(_))));
 }
 
+/// Scaling an oversized image down weighs each color by its alpha: a clear pixel next to an
+/// opaque one leaves the opaque color as it is and only lowers the alpha.
+#[test]
+fn scaling_down_weighs_color_by_alpha() {
+    use crate::raster::box_reduced;
+    const RED: [u8; 4] = [200, 30, 30, 255];
+    // Opaque red and clear black columns, decoded from a PNG and scaled to fit as `decode_to_fit`
+    // does: each scaled pixel covers a red and a clear one.
+    let half_clear = Raster::from_fn(4, 2, |x, _| if x % 2 == 0 { RED } else { [0; 4] });
+    let decoded = image::load_from_memory(&half_clear.encode_png()).unwrap().into_rgba8();
+    let fitted = Raster::fitted(&decoded, 2);
+    assert_eq!((fitted.width, fitted.height), (2, 1));
+    assert_eq!(fitted.rgba, [[200, 30, 30, 128]; 2].concat());
+    // A partly clear blue pulls an opaque red toward it by its alpha.
+    let mixed = Raster::from_fn(2, 1, |x, _| if x == 0 { [255, 0, 0, 255] } else { [0, 0, 255, 85] });
+    assert_eq!(box_reduced(&mixed.rgba, 2, 1, 1, 1), [191, 0, 64, 170]);
+    // A wholly clear block stays clear, with the plain mean of its colors.
+    let clear = Raster::from_fn(2, 1, |x, _| [100 * x as u8, 0, 0, 0]);
+    assert_eq!(box_reduced(&clear.rgba, 2, 1, 1, 1), [50, 0, 0, 0]);
+    // A 3 → 2 reduction counts every source pixel once.
+    let ramp = Raster::from_fn(3, 3, |x, y| [(x * 60 + y * 10) as u8, 0, 0, 255]);
+    assert_eq!(box_reduced(&ramp.rgba, 3, 3, 2, 2), [0, 0, 0, 255, 90, 0, 0, 255, 15, 0, 0, 255, 105, 0, 0, 255]);
+}
+
 /// Four flat colours with anti-aliased (blended) edges between the quadrants.
 fn flat_quads() -> Raster {
     const C: [[u8; 3]; 4] = [[220, 30, 30], [30, 200, 40], [30, 40, 210], [240, 220, 20]];