Find hot photosites without repairing them, and measure a sensor's aging

The hot-pixel pass could only repair: it returned how many photosites it
changed and threw away which. find_hot_pixels runs the same pass and
returns them as sensor coordinates, leaving the frame alone, so a sensor's
defects can be tracked across frames.

sensor_scan prints each frame's candidates, and with --probe reads a list
of coordinates back out of every frame. Run over 53 6D raws from 2015 to
2026, it found 32 persistent defects, 2 in 2015 and 32 by 2026, and showed
what a defect map has to account for: a frame that does not flag a
photosite proves nothing unless its neighbourhood is dark, and the 6D
hides some of its defects itself above ISO 5000. docs/dev/sensor-health.md
records the findings and the design they argue for.

core/dr-gpu/examples/sensor_scan.rs

@@ -0,0 +1,117 @@
+//! List each RAW file's hot and dead photosite candidates (docs/dev/sensor-health.md).
+//!
+//! Reads paths on stdin and prints one JSON line per file: its capture
+//! conditions and every photosite [`Demosaicer::find_hot_pixels`] flags, as
+//! `[x, y, value, hot]` in sensor coordinates. Which candidates are defects
+//! is a question across frames, so this answers nothing on its own.
+//!
+//! ```sh
+//! find ~/Pictures -name '*.CR2' | cargo run --release -p dr-gpu --example sensor_scan
+//! ```
+
+use std::io::{BufRead, Write};
+
+use dr_gpu::{Demosaicer, GpuContext};
+
+fn main() {
+    if let Some(list) = std::env::args().skip_while(|a| a != "--probe").nth(1) {
+        return probe(&list);
+    }
+    let ctx = pollster::block_on(GpuContext::new_headless()).expect("a GPU for the hot-pixel pass");
+    let demosaicer = Demosaicer::new(&ctx).expect("demosaicer");
+    for line in std::io::stdin().lock().lines() {
+        let path = line.expect("stdin");
+        match scan(&path, &demosaicer) {
+            Ok(json) => println!("{json}"),
+            Err(e) => eprintln!("fail\t{path}\t{e}"),
+        }
+        std::io::stdout().flush().ok();
+    }
+}
+
+fn scan(path: &str, demosaicer: &Demosaicer) -> Result<String, String> {
+    let bytes = std::fs::read(path).map_err(|e| e.to_string())?;
+    let raw = dr_decode::decode(&bytes).map_err(|e| e.to_string())?;
+    let meta = dr_decode::metadata(&bytes).map_err(|e| e.to_string())?;
+    let sites = demosaicer
+        .find_hot_pixels(&raw)
+        .map_err(|e| e.to_string())?;
+    let opt = |v: Option<f64>| v.map_or("null".to_string(), |v| v.to_string());
+    let list: Vec<String> = sites
+        .iter()
+        .map(|s| {
+            let v = raw.data[(s.y * raw.width + s.x) as usize];
+            format!("[{},{},{},{}]", s.x, s.y, v, u8::from(s.hot))
+        })
+        .collect();
+    Ok(format!(
+        "{{\"path\":{:?},\"model\":{:?},\"captured\":{},\"iso\":{},\"shutter\":{},\"white\":{},\"black\":{:?},\"crop\":[{},{},{},{}],\"sites\":[{}]}}",
+        path,
+        format!("{} {}", raw.make, raw.model),
+        meta.captured_at.map_or("null".to_string(), |t| t.to_string()),
+        opt(meta.iso.map(f64::from)),
+        opt(meta.shutter.map(f64::from)),
+        raw.white_level,
+        raw.black_level,
+        raw.crop.x,
+        raw.crop.y,
+        raw.crop.width,
+        raw.crop.height,
+        list.join(","),
+    ))
+}
+
+/// `--probe COORDS`: for each path on stdin, each `x y` line of COORDS as
+/// `[value, same-colour neighbour max, median]` over black, on the CPU. A
+/// probe asks whether a photosite stood out in a frame where it would have
+/// been visible, which the scan's verdict cannot say: a frame that does not
+/// flag a defect may only have been too bright around it.
+fn probe(list: &str) {
+    let coords: Vec<(u32, u32)> = std::fs::read_to_string(list)
+        .expect("coords")
+        .lines()
+        .filter_map(|l| {
+            let mut it = l.split_whitespace().map(|v| v.parse().ok());
+            Some((it.next()??, it.next()??))
+        })
+        .collect();
+    for line in std::io::stdin().lock().lines() {
+        let path = line.expect("stdin");
+        let Ok(bytes) = std::fs::read(&path) else {
+            continue;
+        };
+        let (Ok(raw), Ok(meta)) = (dr_decode::decode(&bytes), dr_decode::metadata(&bytes)) else {
+            continue;
+        };
+        let w = raw.width as i64;
+        let at = |x: i64, y: i64| {
+            let cell = (((y - raw.crop.y as i64) & 1) * 2 + ((x - raw.crop.x as i64) & 1)) as usize;
+            raw.data[(y * w + x) as usize].saturating_sub(raw.black_level[cell])
+        };
+        let rows: Vec<String> = coords
+            .iter()
+            .map(|&(x, y)| {
+                let (x, y) = (x as i64, y as i64);
+                let mut n: Vec<u16> = Vec::new();
+                for dy in [-2i64, 0, 2] {
+                    for dx in [-2i64, 0, 2] {
+                        if (dx, dy) != (0, 0) {
+                            n.push(at(x + dx, y + dy));
+                        }
+                    }
+                }
+                n.sort_unstable();
+                format!("[{},{},{}]", at(x, y), n[n.len() - 1], n[n.len() / 2])
+            })
+            .collect();
+        println!(
+            "{{\"path\":{:?},\"captured\":{},\"iso\":{},\"shutter\":{},\"range\":{},\"p\":[{}]}}",
+            path,
+            meta.captured_at.unwrap_or(0),
+            meta.iso.unwrap_or(0),
+            meta.shutter.unwrap_or(0.0),
+            raw.white_level - raw.black_level[0],
+            rows.join(","),
+        );
+    }
+}