Read a DNG's NoiseProfile

The converter's measured noise for the body at that ISO, (S, O) per CFA
plane: the learned denoise's best source for a body with no table of its
own (denoise.md §3.3). Read from the header beside the colour tags, and
printed by rawinfo. Checked against tifffile on a 6D DNG at ISO 5000: all
six values agree.

core/dr-decode/src/profile.rs

@@ -740,6 +740,35 @@ pub fn read_dng_matrices(decoder: &dyn rawler::decoders::Decoder) -> DngMatrices
     }
 }
 
+/// TRACES: FR-DEV-3g
+/// The DNG `NoiseProfile` tag (51041): the converter's measured noise for
+/// this body at this ISO, as `(S, O)` per CFA colour plane, so that a
+/// photosite's variance is `S·x + O` with `x` normalised black-to-white.
+///
+/// One pair means all planes share it. `None` where the file has no such
+/// tag — every proprietary raw, and DNGs from converters that do not measure
+/// — or where a value is not a finite non-negative number. The learned
+/// denoise's second-best noise source (denoise.md §3.3), after a measured
+/// table for the body.
+pub fn read_noise_profile(decoder: &dyn rawler::decoders::Decoder) -> Option<Vec<(f32, f32)>> {
+    use rawler::decoders::WellKnownIFD;
+    use rawler::tags::DngTag;
+
+    let ifd = decoder.ifd(WellKnownIFD::Root).ok()??;
+    let entry = ifd.get_entry_recursive(DngTag::NoiseProfile)?;
+    let n = entry.count() as usize;
+    if n < 2 || n % 2 != 0 {
+        return None;
+    }
+    let pairs: Vec<(f32, f32)> = (0..n / 2)
+        .map(|i| (entry.force_f32(2 * i), entry.force_f32(2 * i + 1)))
+        .collect();
+    pairs
+        .iter()
+        .all(|(s, o)| s.is_finite() && o.is_finite() && *s >= 0.0 && *o >= 0.0)
+        .then_some(pairs)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;