#323: a tight crop at its own pixels reads the source level its pixels need

An adversarial review found that sizing the loupe render from the cropped frame
(previous commit) left the source level chosen from the output edge alone: a
crop whose own long edge is under 2560 px was drawn from the 2560 px preview of
the whole photo, soft though sized right. The level now follows the pixels the
output needs along the source's long edge (output edge / crop fraction); grid
thumbnails keep their cheap level.

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

crates/engine/src/media.rs

@@ -104,6 +104,20 @@ impl SourceLevel {
     }
 }
 
+/// Pixels along the source's long edge that an output `out_long` pixels long needs: a crop shows
+/// only part of the photo, so its pixels come from a source that many times larger (the source
+/// level is chosen from this, or a tight crop shown at its own pixels would be a stretched preview).
+fn source_edge_needed(p: &Photo, settings: &DevelopSettings, apply_crop: bool, out_long: usize) -> usize {
+    let (w, h) = (f64::from(p.width.max(1)), f64::from(p.height.max(1)));
+    let r = settings.crop.geometry.rect;
+    let shown = if apply_crop { (r.width() * w).max(r.height() * h) } else { w.max(h) };
+    if !shown.is_finite() || shown < 1.0 {
+        return out_long;
+    }
+    let needed = out_long as f64 * w.max(h) / shown;
+    if needed.is_finite() { needed.min(usize::MAX as f64 / 2.0) as usize } else { out_long }
+}
+
 /// Decodes a file into a linear Rec.2020 image no larger than `max_edge` (set by the app).
 pub type FileLoader = Arc<dyn Fn(&str, usize) -> Result<(Rgb32f, SourceInfo), String> + Send + Sync>;
 
@@ -718,9 +732,13 @@ impl crate::Session {
         thumb_bucket: Option<usize>,
     ) -> Option<RenderJob> {
         let p = self.catalog.photo(id)?.clone();
-        let level = SourceLevel::for_size(max_w.max(max_h));
-        let source = self.media.source_ref(&p, level);
         let settings = if before { Arc::new(self.before_settings(&p)) } else { p.develop.clone() };
+        let level = SourceLevel::for_size(if thumb_bucket.is_some() {
+            max_w.max(max_h) // (grid thumbnails of tight crops stay cheap)
+        } else {
+            source_edge_needed(&p, &settings, apply_crop, max_w.max(max_h))
+        });
+        let source = self.media.source_ref(&p, level);
         let request = RenderRequest { apply_crop, ..RenderRequest::fit(max_w, max_h) };
         // the photo id is part of the key: two photos with the same settings and size must not
         // share a result (a view slot showing photo A would otherwise look current for photo B)
@@ -884,7 +902,7 @@ impl crate::Session {
     /// A render of one window of the loupe's frame, for a view zoomed past what one whole-frame
     /// render can hold: the frame is `full_w × full_h` (the size the loupe would draw it at) and
     /// the result is `window`'s pixels of it, at that scale, from the source level that size needs.
-    /// Nothing is cached here (a window is only worth keeping while it is on screen).
+    /// Nothing is cached here (a window is only worth keeping while it is on screen). 
     pub fn region_job(
         &mut self,
         id: PhotoId,
@@ -1269,6 +1287,27 @@ mod tests {
         assert!(again.view_cache.is_none() && again.cache.is_none());
     }
 
+    // Issue #323: a tight crop shown at its own pixels needs the original, not the 2560 px preview
+    #[test]
+    fn a_tight_crop_reads_the_source_level_its_pixels_need() {
+        let mut s = crate::Session::with_demo();
+        let p = s.catalog.photos().next().unwrap().clone();
+        let long = p.width.max(p.height) as usize;
+        assert!(long > 2560);
+        let id = p.id;
+        let mut d = (*s.develop_of(id).unwrap()).clone();
+        // the uncropped photo at 1600 px: the preview is plenty
+        assert_eq!(s.render_job(id, 1600, 1600, false, true).unwrap().level, SourceLevel::Preview);
+        // a crop to 30 % of each side shown 1600 px long needs 1600 / 0.3 px of the source
+        d.crop.geometry.rect = lightcraft_geom::Rect { x0: 0.2, y0: 0.2, x1: 0.5, y1: 0.5 };
+        s.set_develop(id, d, "Crop").unwrap();
+        assert_eq!(s.render_job(id, 1600, 1600, false, true).unwrap().level, SourceLevel::Full);
+        // …but with the crop tool open (the whole photo shown) it does not
+        assert_eq!(s.render_job(id, 1600, 1600, false, false).unwrap().level, SourceLevel::Preview);
+        // and a crop shown small enough still reads the preview
+        assert_eq!(s.render_job(id, 600, 600, false, true).unwrap().level, SourceLevel::Preview);
+    }
+
     #[test]
     fn variant_jobs_have_their_own_keys_and_hit_the_cache() {
         let mut s = crate::Session::with_demo();