Find small objects in large documents with the Object Finder

Fixes #6576

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/algo/src/segment/grabcut.rs

@@ -144,6 +144,10 @@ pub const OBJECT_AT_WORK_PX: usize = 60_000;
 /// foreground, the cursor definite foreground, outside the box definite background), the part
 /// connected to the cursor is kept and the boundary is re-cut at full resolution, as the
 /// rectangle mode does.
+///
+/// When the whole-canvas pass finds nothing in a large document (an object only a few working
+/// pixels wide blurs into its surroundings), the search is repeated at finer steps in windows of
+/// the same working size around the point, down to full resolution.
 pub fn object_at(sampler: &dyn Sampler, canvas: Rect, x: f32, y: f32, work_px: usize) -> Option<Region> {
     if canvas.width() < 4 || canvas.height() < 4 || !x.is_finite() || !y.is_finite() {
         return None;
@@ -152,7 +156,30 @@ pub fn object_at(sampler: &dyn Sampler, canvas: Rect, x: f32, y: f32, work_px: u
     if !canvas.contains(px, py) {
         return None;
     }
-    let step = super::scale_for(canvas, work_px.max(1024));
+    let work_px = work_px.max(1024);
+    let mut step = super::scale_for(canvas, work_px);
+    if let Some(r) = object_in(sampler, canvas, px, py, step) {
+        return Some(r);
+    }
+    // Side (working pixels) of the square windows of the finer passes.
+    let side = (work_px as f64).sqrt() as usize;
+    while step > 1 {
+        step = (step / 4).max(1);
+        let half = (side.saturating_mul(step) / 2).min(i32::MAX as usize / 4) as i32;
+        let window = Rect::new(px.saturating_sub(half), py.saturating_sub(half), px.saturating_add(half), py.saturating_add(half)).intersect(&canvas);
+        if window.width() < 4 || window.height() < 4 {
+            continue;
+        }
+        if let Some(r) = object_in(sampler, window, px, py, step) {
+            return Some(r);
+        }
+    }
+    None
+}
+
+/// One [`object_at`] pass over `canvas` (the document or a window of it) at working step `step`;
+/// `(px, py)` lies inside `canvas`.
+fn object_in(sampler: &dyn Sampler, canvas: Rect, px: i32, py: i32, step: usize) -> Option<Region> {
     let img = sampler.rgb_scaled(canvas, step);
     let (w, h) = (img.w, img.h);
     if w < 2 || h < 2 {

crates/algo/src/segment/tests.rs

@@ -762,6 +762,28 @@ fn object_at_on_the_background_or_off_canvas_finds_nothing() {
     assert!(grabcut::object_at(&s, canvas, 400.0, 10.0, 0).is_none());
 }
 
+/// #6576: a 24 px object in a 4000×3000 document is found (a 1000×800 one still is).
+#[test]
+fn object_at_finds_small_object_in_large_canvas() {
+    let (w, h) = (4000usize, 3000usize);
+    let canvas = Rect::new(0, 0, w as i32, h as i32);
+    for (ox, oy, ow, oh) in [(188usize, 138usize, 24usize, 24usize), (1500, 1100, 1000, 800)] {
+        let inside = |x: usize, y: usize| x >= ox && x < ox + ow && y >= oy && y < oy + oh;
+        let img = RgbImage::from_fn(w, h, |x, y| if inside(x, y) { [0.8, 0.1, 0.1] } else { [0.9, 0.9, 0.9] });
+        let truth: Vec<bool> = (0..w * h).map(|i| inside(i % w, i / w)).collect();
+        let s = ImageSampler { img: &img, origin: (0, 0) };
+        let (cx, cy) = ((ox + ow / 2) as f32, (oy + oh / 2) as f32);
+        let reg = grabcut::object_at(&s, canvas, cx, cy, grabcut::OBJECT_AT_WORK_PX);
+        let reg = reg.unwrap_or_else(|| panic!("the {ow}×{oh} object is not found"));
+        let v = iou(&reg, &truth, w, h);
+        assert!(v > 0.8, "{ow}×{oh}: IoU {v}");
+    }
+    // The flat background around the small object is still nothing.
+    let img = RgbImage::from_fn(w, h, |x, y| if (188..212).contains(&x) && (138..162).contains(&y) { [0.8, 0.1, 0.1] } else { [0.9, 0.9, 0.9] });
+    let s = ImageSampler { img: &img, origin: (0, 0) };
+    assert!(grabcut::object_at(&s, canvas, 400.0, 300.0, grabcut::OBJECT_AT_WORK_PX).is_none());
+}
+
 // ---------- Select Subject on the cases reported in #2788 ----------
 
 /// A figure whose upper part is close to the background colour (a cream shirt on a near-white