Projection chips redraw the preview; auto-crop as the DNG default crop

Picking a chip stored the choice for the merge and changed nothing on
screen — the chip did not even highlight, since the selected property
was never written back. Now the pick is reflected, and the job, waiting
for its decision, takes a Preview request, draws the alignment on the
chosen surface at proxy cost and reports again; the drain puts the new
picture and its size up. Auto is the surface the field of view suggests.

Also:
The largest rectangle inside the frames' coverage is found a row at a
time — a histogram of consecutive covered rows and a stack pass per row —
so the composite is never held to be measured (FR-MRG-11). It is written
as DefaultCropOrigin/DefaultCropSize (FR-MRG-4): the file opens on the
picture, the border is still in it, and resetting the crop shows it.
rawler reports the crop as the picture, which the test checks.

FR-MRG-4 records the question raised the same day — fill the border
rather than crop it — as open: a non-generative fill through the heal,
or a generative inpainter with its licence and weights. Neither decided.

ui/dr-ui/src/merge.rs

@@ -110,6 +110,10 @@ impl MergeRequest {
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum Decision {
     Merge { projection: Option<Projection> },
+    /// Draw the alignment again on another surface, and report it again.
+    /// The page's projection chips: what a choice looks like before it is
+    /// confirmed, at proxy cost rather than the merge's.
+    Preview { projection: Option<Projection> },
     Abandon,
 }
 
@@ -279,8 +283,7 @@ fn run_inner(
     let projection = request
         .projection
         .unwrap_or_else(|| Projection::suggest(hfov, vfov));
-    let bounds = projection::bounds(projection, focal_full, &cameras, frame_size)
-        .ok_or("the frames project nowhere")?;
+    let suggested = projection;
 
     // 4. Gains, before the report so the preview shows them.
     let gains = if alignment.is_complete() {
@@ -288,23 +291,25 @@ fn run_inner(
     } else {
         vec![1.0; frames.len()]
     };
-    let preview = if alignment.is_complete() {
-        progress("Drawing the preview", 0, 1);
-        Some(preview(&colour, &proxies, &alignment, &gains, projection, &frames[0].raw))
-    } else {
-        None
+    let report = |projection: Projection| -> Result<AlignmentReport, String> {
+        let bounds = projection::bounds(projection, focal_full, &cameras, frame_size)
+            .ok_or("the frames project nowhere")?;
+        let preview = alignment
+            .is_complete()
+            .then(|| preview(&colour, &proxies, &alignment, &gains, projection, &frames[0].raw));
+        Ok(AlignmentReport {
+            focal_mm: focal_full * 36.0 / full_long,
+            rms_px: alignment.rms_px,
+            projection,
+            width: bounds.width().ceil() as u32,
+            height: bounds.height().ceil() as u32,
+            frames: describe(&alignment),
+            links: alignment.links.len(),
+            preview,
+        })
     };
-    let report = AlignmentReport {
-        focal_mm: focal_full * 36.0 / full_long,
-        rms_px: alignment.rms_px,
-        projection,
-        width: bounds.width().ceil() as u32,
-        height: bounds.height().ceil() as u32,
-        frames: describe(&alignment),
-        links: alignment.links.len(),
-        preview,
-    };
-    let _ = events.send(MergeEvent::Aligned(report));
+    progress("Drawing the preview", 0, 1);
+    let _ = events.send(MergeEvent::Aligned(report(suggested)?));
     if !alignment.is_complete() {
         let names: Vec<String> = alignment
             .unaligned
@@ -322,7 +327,10 @@ fn run_inner(
             return Ok(None);
         }
         match decision.recv_timeout(std::time::Duration::from_millis(100)) {
-            Ok(Decision::Merge { projection: p }) => break p.unwrap_or(projection),
+            Ok(Decision::Merge { projection: p }) => break p.unwrap_or(suggested),
+            Ok(Decision::Preview { projection: p }) => {
+                let _ = events.send(MergeEvent::Aligned(report(p.unwrap_or(suggested))?));
+            }
             Ok(Decision::Abandon) => return Ok(None),
             Err(std::sync::mpsc::RecvTimeoutError::Timeout) => continue,
             Err(std::sync::mpsc::RecvTimeoutError::Disconnected) => return Ok(None),
@@ -388,6 +396,10 @@ fn run_inner(
     // other's pace more than one band.
     let (band_tx, band_rx) = std::sync::mpsc::sync_channel::<Vec<u16>>(1);
     let rows_per_strip = request.chunk.1.max(1);
+    // The crop (FR-MRG-4), found as the bands go by; the writer reads it
+    // once the last strip is in, by which time every band has been seen.
+    let inscribed = Arc::new(std::sync::Mutex::new(dr_export::Inscribed::new(out_w)));
+    let inscribed_for_writer = inscribed.clone();
     let file = std::fs::File::create(&out_path).map_err(|e| format!("{}: {e}", out_path.display()))?;
     let writer = std::thread::spawn(move || -> Result<(), String> {
         let mut file = std::io::BufWriter::new(file);
@@ -405,6 +417,7 @@ fn run_inner(
                 buf.extend_from_slice(&band);
                 Ok(())
             },
+            || inscribed_for_writer.lock().ok().map(|i| i.best()),
         )
         .map_err(|e| e.to_string())
     });
@@ -435,6 +448,9 @@ fn run_inner(
         |band| {
             bands_done += 1;
             progress("Merging", bands_done, total_bands);
+            if let Ok(mut i) = inscribed.lock() {
+                i.push_rows(band.covered, band.rows);
+            }
             band_tx
                 .send(band.rgb.to_vec())
                 .map_err(|_| dr_gpu::GpuError::Readback("the writer stopped".into()))