[feat](trx-wefax): add continuous slant correction via line cross-correlation

Track sample-clock drift between transmitter and receiver by
cross-correlating each new scan line against the previous one at
shifts of ±6 samples. The best-matching shift nudges the slicer's
extraction cursor, keeping adjacent lines aligned and removing the
diagonal skew that would otherwise accumulate over an 800-line image.

A small correlation-peak deadband prefers d=0 on quiet lines, and a
minimum-variance guard skips flat reference lines where drift
estimation is meaningless. Enabled by default via
WefaxConfig::slant_correction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/decoders/trx-wefax/src/config.rs

@@ -19,6 +19,9 @@ pub struct WefaxConfig {
     pub output_dir: Option<String>,
     /// Whether to emit line-by-line progress events.
     pub emit_progress: bool,
+    /// Whether to continuously track and correct sample-clock drift
+    /// (line-to-line cross-correlation) to remove image slant.
+    pub slant_correction: bool,
 }
 
 impl Default for WefaxConfig {
@@ -30,6 +33,7 @@ impl Default for WefaxConfig {
             deviation_hz: 400.0,
             output_dir: None,
             emit_progress: true,
+            slant_correction: true,
         }
     }
 }

src/decoders/trx-wefax/src/decoder.rs

@@ -534,7 +534,13 @@ impl WefaxDecoder {
             lpm, phase_offset, verified, "WEFAX: entering receiving"
         );
         let ppl = WefaxConfig::pixels_per_line(ioc) as usize;
-        self.slicer = Some(LineSlicer::new(lpm, ioc, INTERNAL_RATE, phase_offset));
+        self.slicer = Some(LineSlicer::with_slant(
+            lpm,
+            ioc,
+            INTERNAL_RATE,
+            phase_offset,
+            self.config.slant_correction,
+        ));
         self.image = Some(ImageAssembler::new(ppl));
         self.tone_detector.reset();
         self.low_corr_lines = 0;

src/decoders/trx-wefax/src/line_slicer.rs

@@ -7,9 +7,21 @@
 //! Once the phasing detector has established a line-start phase offset,
 //! the line slicer accumulates demodulated luminance samples and extracts
 //! complete image lines at the configured LPM rate.
+//!
+//! When `slant_correction` is enabled, the slicer tracks line-to-line
+//! drift via cross-correlation with the previous line and nudges the
+//! extraction cursor by ±`MAX_DRIFT_SAMPLES` per line. This compensates
+//! for the small mismatch between the transmitter's and receiver's
+//! sample clocks that would otherwise skew the assembled image.
 
 use crate::config::WefaxConfig;
 
+/// Maximum per-line drift (in samples at the internal rate) searched for
+/// when slant correction is enabled. At 120 LPM / 11025 Hz there are
+/// ~5512 samples per line, so ±6 samples is ~0.1% drift per line — more
+/// than enough for any real-world sample-clock mismatch.
+const MAX_DRIFT_SAMPLES: usize = 6;
+
 /// Line slicer for WEFAX image assembly.
 pub struct LineSlicer {
     /// Samples per line at the internal sample rate.
@@ -18,16 +30,34 @@ pub struct LineSlicer {
     pixels_per_line: usize,
     /// Phase offset in samples from the phasing detector.
     phase_offset: usize,
-    /// Accumulated luminance samples.
+    /// Accumulated luminance samples. While `slant_correction` is on,
+    /// the buffer anchor is the *start of the previous line* (so the
+    /// first `samples_per_line` samples are the reference for drift
+    /// tracking). Without slant correction the anchor is simply the
+    /// start of the next line to extract.
     buffer: Vec<f32>,
-    /// Number of samples consumed since the last phase alignment point.
-    consumed: usize,
     /// Whether we have aligned to the phase offset yet.
     aligned: bool,
+    /// Whether a reference (previous) line is held at the buffer anchor.
+    has_reference: bool,
+    /// Enable line-to-line drift tracking.
+    slant_correction: bool,
+    /// Cumulative drift applied so far (samples). Diagnostic.
+    pub(crate) total_drift: i64,
 }
 
 impl LineSlicer {
     pub fn new(lpm: u16, ioc: u16, sample_rate: u32, phase_offset: usize) -> Self {
+        Self::with_slant(lpm, ioc, sample_rate, phase_offset, true)
+    }
+
+    pub fn with_slant(
+        lpm: u16,
+        ioc: u16,
+        sample_rate: u32,
+        phase_offset: usize,
+        slant_correction: bool,
+    ) -> Self {
         let samples_per_line = WefaxConfig::samples_per_line(lpm, sample_rate);
         let pixels_per_line = WefaxConfig::pixels_per_line(ioc) as usize;
 
@@ -35,9 +65,11 @@ impl LineSlicer {
             samples_per_line,
             pixels_per_line,
             phase_offset,
-            buffer: Vec::with_capacity(samples_per_line * 2),
-            consumed: 0,
+            buffer: Vec::with_capacity(samples_per_line * 3),
             aligned: false,
+            has_reference: false,
+            slant_correction,
+            total_drift: 0,
         }
     }
 
@@ -58,17 +90,57 @@ impl LineSlicer {
             self.aligned = true;
         }
 
-        // Extract complete lines (single drain at the end to avoid O(n²)).
-        let mut offset = 0;
-        while offset + self.samples_per_line <= self.buffer.len() {
-            let line_samples = &self.buffer[offset..offset + self.samples_per_line];
-            let pixels = self.resample_line(line_samples);
-            lines.push(pixels);
-            offset += self.samples_per_line;
-            self.consumed += self.samples_per_line;
+        let spl = self.samples_per_line;
+
+        if !self.slant_correction {
+            // Simple fixed-period extraction.
+            let mut offset = 0;
+            while offset + spl <= self.buffer.len() {
+                let line_samples = &self.buffer[offset..offset + spl];
+                let pixels = self.resample_line(line_samples);
+                lines.push(pixels);
+                offset += spl;
+            }
+            if offset > 0 {
+                self.buffer.drain(..offset);
+            }
+            return lines;
         }
-        if offset > 0 {
-            self.buffer.drain(..offset);
+
+        // Slant-corrected extraction.
+        let max_shift = MAX_DRIFT_SAMPLES;
+
+        // Bootstrap: the very first line has no previous reference.
+        // Extract it naively and keep it in the buffer as the reference.
+        if !self.has_reference {
+            if self.buffer.len() < spl {
+                return lines;
+            }
+            let first = self.buffer[0..spl].to_vec();
+            let pixels = self.resample_line(&first);
+            lines.push(pixels);
+            self.has_reference = true;
+            // Do NOT drain: the first `spl` samples remain as the
+            // reference for the next line's drift search.
+        }
+
+        // Subsequent lines: for each iteration, buffer[0..spl] is the
+        // reference line, and we search for the best starting position
+        // of the NEXT line in the range [spl - max_shift, spl + max_shift].
+        while self.buffer.len() >= 2 * spl + max_shift {
+            let prev = &self.buffer[0..spl];
+            let (best_d, _best_r) =
+                search_best_shift(prev, &self.buffer, spl, max_shift);
+
+            let start = (spl as i32 + best_d) as usize;
+            let next_line = self.buffer[start..start + spl].to_vec();
+            let pixels = self.resample_line(&next_line);
+            lines.push(pixels);
+
+            // Advance the anchor to the start of the line we just
+            // emitted — it becomes the reference for the next iteration.
+            self.buffer.drain(..start);
+            self.total_drift += best_d as i64;
         }
 
         lines
@@ -78,10 +150,16 @@ impl LineSlicer {
         self.pixels_per_line
     }
 
+    /// Samples per line at the internal rate (for diagnostics).
+    pub fn samples_per_line(&self) -> usize {
+        self.samples_per_line
+    }
+
     pub fn reset(&mut self) {
         self.buffer.clear();
-        self.consumed = 0;
         self.aligned = false;
+        self.has_reference = false;
+        self.total_drift = 0;
     }
 
     /// Resample a line's worth of luminance samples to the target pixel count
@@ -112,6 +190,82 @@ impl LineSlicer {
     }
 }
 
+/// Search for the drift `d ∈ [-max_shift, +max_shift]` that maximises
+/// the Pearson correlation between `reference` and
+/// `buffer[spl+d .. spl+d+spl]`.
+///
+/// Returns `(best_d, best_r)`. A correlation-peak deadband prefers
+/// `d = 0` when the peak is only marginally better than at zero, which
+/// keeps tracking stable on quiet lines.
+fn search_best_shift(
+    reference: &[f32],
+    buffer: &[f32],
+    spl: usize,
+    max_shift: usize,
+) -> (i32, f32) {
+    debug_assert!(buffer.len() >= 2 * spl + max_shift);
+    debug_assert_eq!(reference.len(), spl);
+
+    // Pre-compute reference mean + variance.
+    let n = spl as f32;
+    let mean_r = reference.iter().sum::<f32>() / n;
+    let mut var_r = 0.0f32;
+    for &v in reference {
+        let d = v - mean_r;
+        var_r += d * d;
+    }
+
+    // Guard against a flat reference line — drift tracking is useless.
+    const MIN_VAR: f32 = 32.0;
+    if var_r < MIN_VAR {
+        return (0, 0.0);
+    }
+
+    let ms = max_shift as i32;
+    let mut best_d = 0i32;
+    let mut best_r = f32::NEG_INFINITY;
+    let mut r_at_zero = 0.0f32;
+
+    for d in -ms..=ms {
+        let start = (spl as i32 + d) as usize;
+        let candidate = &buffer[start..start + spl];
+
+        let mean_c = candidate.iter().sum::<f32>() / n;
+        let mut var_c = 0.0f32;
+        let mut cov = 0.0f32;
+        for (i, &v) in candidate.iter().enumerate() {
+            let dr = reference[i] - mean_r;
+            let dc = v - mean_c;
+            cov += dr * dc;
+            var_c += dc * dc;
+        }
+
+        let r = if var_c < MIN_VAR {
+            // Skip flat candidate slices.
+            f32::NEG_INFINITY
+        } else {
+            cov / (var_r.sqrt() * var_c.sqrt())
+        };
+
+        if d == 0 {
+            r_at_zero = r;
+        }
+        if r > best_r {
+            best_r = r;
+            best_d = d;
+        }
+    }
+
+    // Deadband: if the peak is only marginally better than `d = 0`,
+    // stick with zero. This avoids per-line jitter when drift is small.
+    const DEADBAND: f32 = 0.01;
+    if r_at_zero.is_finite() && best_r - r_at_zero < DEADBAND {
+        return (0, r_at_zero);
+    }
+
+    (best_d, best_r)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -124,7 +278,8 @@ mod tests {
         let spl = WefaxConfig::samples_per_line(lpm, sr);
         let ppl = WefaxConfig::pixels_per_line(ioc) as usize;
 
-        let mut slicer = LineSlicer::new(lpm, ioc, sr, 0);
+        // Slant correction off for deterministic line count.
+        let mut slicer = LineSlicer::with_slant(lpm, ioc, sr, 0, false);
         // Feed exactly 3 lines worth of white.
         let samples = vec![1.0f32; spl * 3];
         let lines = slicer.process(&samples);
@@ -141,7 +296,7 @@ mod tests {
         let sr = 11025;
         let spl = WefaxConfig::samples_per_line(lpm, sr);
 
-        let mut slicer = LineSlicer::new(lpm, ioc, sr, 0);
+        let mut slicer = LineSlicer::with_slant(lpm, ioc, sr, 0, false);
         // Feed a linear ramp from 0.0 to 1.0.
         let samples: Vec<f32> = (0..spl).map(|i| i as f32 / spl as f32).collect();
         let lines = slicer.process(&samples);
@@ -150,4 +305,92 @@ mod tests {
         assert!(lines[0][0] < 5);
         assert!(lines[0].last().copied().unwrap_or(0) > 250);
     }
+
+    /// Synthesise a noisy-ish gradient line that repeats with a small
+    /// per-line offset, simulating a sample-clock mismatch. The slant
+    /// tracker should follow the drift.
+    #[test]
+    fn slant_tracker_follows_drift() {
+        let lpm = 120;
+        let ioc = 576;
+        let sr = 11025;
+        let spl = WefaxConfig::samples_per_line(lpm, sr);
+
+        // Build a signal where each real line is `spl + 3` samples long
+        // (i.e. transmitter clock is slower than expected → positive drift
+        // of +3 samples per line). The content needs high-frequency
+        // structure for a few-sample shift to be detectable against the
+        // deadband.
+        let true_line_len = spl + 3;
+        let mut signal: Vec<f32> = Vec::new();
+        let base: Vec<f32> = (0..true_line_len)
+            .map(|i| {
+                // Pseudo-random-but-repeatable content with a narrow
+                // bright stripe — sharp features make sub-line shifts
+                // easy to localise.
+                let x = ((i as u32).wrapping_mul(2654435761)) >> 16;
+                let noise = (x & 0xff) as f32 / 255.0;
+                let stripe = if i == true_line_len / 3 { 1.0 } else { 0.0 };
+                0.3 + 0.4 * noise + stripe
+            })
+            .collect();
+        // 20 lines, each identical.
+        for _ in 0..20 {
+            signal.extend_from_slice(&base);
+        }
+
+        let mut slicer = LineSlicer::with_slant(lpm, ioc, sr, 0, true);
+        let lines = slicer.process(&signal);
+
+        // Expect ~ (20*true_line_len - spl) / (spl+drift) lines with
+        // drift absorbing the extra 2 samples per line.
+        assert!(
+            lines.len() >= 15,
+            "slant-corrected slicer produced only {} lines",
+            lines.len()
+        );
+        // Should have tracked positive drift.
+        assert!(
+            slicer.total_drift > 0,
+            "expected positive drift, got {}",
+            slicer.total_drift
+        );
+        // Roughly +3 per line (after the first bootstrap line); allow wide tolerance.
+        let per_line = slicer.total_drift as f32 / (lines.len() - 1) as f32;
+        assert!(
+            per_line > 1.5 && per_line < 4.0,
+            "per-line drift {:.2} out of range (total {}, lines {})",
+            per_line,
+            slicer.total_drift,
+            lines.len()
+        );
+    }
+
+    #[test]
+    fn slant_tracker_deadband_on_no_drift() {
+        let lpm = 120;
+        let ioc = 576;
+        let sr = 11025;
+        let spl = WefaxConfig::samples_per_line(lpm, sr);
+
+        // Perfectly aligned lines → drift should stay at zero.
+        let line: Vec<f32> = (0..spl)
+            .map(|i| {
+                let t = i as f32 / spl as f32;
+                0.5 + 0.4 * (t * 9.0 * std::f32::consts::PI).sin()
+            })
+            .collect();
+        let mut signal = Vec::new();
+        for _ in 0..10 {
+            signal.extend_from_slice(&line);
+        }
+
+        let mut slicer = LineSlicer::with_slant(lpm, ioc, sr, 0, true);
+        let _ = slicer.process(&signal);
+        // Deadband should keep drift at 0.
+        assert_eq!(
+            slicer.total_drift, 0,
+            "no drift expected for identical lines"
+        );
+    }
 }