chess_corners_core/detect/radon/

primitives.rs

//! Shared primitives for the Duda-Frese (2018) localized Radon
//! response, used by the whole-image
//! [`radon_response_u8`](super::radon_response_u8) detection path.
//!
//! The module exists so the angular basis, the peak-fit, and the
//! response-map box blur live in exactly one place.

use serde::{Deserialize, Serialize};

#[cfg(feature = "rayon")]
use rayon::prelude::*;

/// Subpixel peak-fitting mode.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
#[non_exhaustive]
pub enum PeakFitMode {
    /// Classic parabolic fit on the raw response values.
    Parabolic,
    /// Parabolic fit on `log(response)` — equivalent to fitting a
    /// Gaussian through three samples. This is the paper default and
    /// the facade preset.
    #[default]
    Gaussian,
}

/// Fit the peak of three samples along one axis. Returns a fractional
/// offset in `[-0.5, 0.5]` grid-steps from the middle sample.
///
/// The parabolic mode is `y = a + b·x + c·x²`; the Gaussian mode is the
/// same fit applied to `log(y)`, provided all three samples are
/// strictly positive. Negative or zero samples trigger a parabolic
/// fallback. A denominator near zero (flat or rising slope at the
/// "peak") returns 0.0 rather than diverging.
#[inline]
pub(crate) fn fit_peak_frac(y_minus: f32, y_c: f32, y_plus: f32, mode: PeakFitMode) -> f32 {
    let (ym, y0, yp) = match mode {
        PeakFitMode::Gaussian if y_minus > 0.0 && y_c > 0.0 && y_plus > 0.0 => {
            (y_minus.ln(), y_c.ln(), y_plus.ln())
        }
        _ => (y_minus, y_c, y_plus),
    };
    let denom = ym - 2.0 * y0 + yp;
    // A true maximum has denom < 0. If denom ≥ 0 the neighbours aren't
    // strictly below the centre; fall back to "no subpixel shift"
    // rather than producing a divergent extrapolation.
    if denom > -1e-12 {
        return 0.0;
    }
    let frac = 0.5 * (ym - yp) / denom;
    frac.clamp(-0.5, 0.5)
}

/// Separable `(2·radius+1)²` box blur applied in place to a flat
/// row-major `w × h` grid. `scratch` must match `resp` in length and
/// is used as temporary storage. `radius = 0` is a no-op.
///
/// The grid does not need to be square: `w` and `h` are taken
/// independently so whole-image response maps (typically rectangular
/// at camera resolution) and the refiner's square local response
/// patch share the same implementation.
pub(crate) fn box_blur_inplace(
    resp: &mut [f32],
    scratch: &mut [f32],
    w: usize,
    h: usize,
    radius: usize,
) {
    debug_assert_eq!(resp.len(), w * h);
    debug_assert_eq!(scratch.len(), w * h);
    if radius == 0 {
        return;
    }
    // `par_chunks_mut(w)` / `chunks_mut(w)` panic when `w == 0`; the
    // original loop-based implementation returned a silent no-op on
    // zero-extent grids, so preserve that contract here too.
    if w == 0 || h == 0 {
        return;
    }

    // Horizontal pass: resp[y, x] -> scratch[y, x].
    //
    // Each row is independent, so this trivially parallelizes
    // row-wise under the `rayon` feature.
    let horiz_kernel = |y: usize, scratch_row: &mut [f32], resp_full: &[f32]| {
        let row_start = y * w;
        for (x, dst) in scratch_row.iter_mut().enumerate() {
            let x0 = x.saturating_sub(radius);
            let x1 = (x + radius + 1).min(w);
            let mut acc = 0.0f32;
            let mut n = 0.0f32;
            for xx in x0..x1 {
                acc += resp_full[row_start + xx];
                n += 1.0;
            }
            *dst = acc / n;
        }
    };

    #[cfg(feature = "rayon")]
    {
        // Need to read from `resp` while writing to `scratch`, so
        // borrow `resp` immutably for the read view first.
        let resp_view: &[f32] = resp;
        scratch
            .par_chunks_mut(w)
            .enumerate()
            .for_each(|(y, scratch_row)| {
                horiz_kernel(y, scratch_row, resp_view);
            });
    }
    #[cfg(not(feature = "rayon"))]
    {
        let resp_view: &[f32] = resp;
        for (y, scratch_row) in scratch.chunks_mut(w).enumerate() {
            horiz_kernel(y, scratch_row, resp_view);
        }
    }

    // Vertical pass: scratch[y, x] -> resp[y, x].
    //
    // Rewritten to row-major: for each output row `y`, accumulate the
    // contributions from scratch rows `y0..y1` into the output. This
    // gives unit-stride reads/writes (vs. the earlier column-strided
    // `for x { for y { ... } }`), and lets each output row be filled
    // independently of every other output row.
    let vert_kernel = |y: usize, dst: &mut [f32], scratch_full: &[f32]| {
        let y0 = y.saturating_sub(radius);
        let y1 = (y + radius + 1).min(h);
        let n = (y1 - y0) as f32;
        for v in dst.iter_mut() {
            *v = 0.0;
        }
        for yy in y0..y1 {
            let src_row = &scratch_full[yy * w..(yy + 1) * w];
            for (d, s) in dst.iter_mut().zip(src_row.iter()) {
                *d += *s;
            }
        }
        let inv_n = 1.0 / n;
        for v in dst.iter_mut() {
            *v *= inv_n;
        }
    };

    #[cfg(feature = "rayon")]
    {
        let scratch_view: &[f32] = scratch;
        resp.par_chunks_mut(w)
            .enumerate()
            .for_each(|(y, dst)| vert_kernel(y, dst, scratch_view));
    }
    #[cfg(not(feature = "rayon"))]
    {
        let scratch_view: &[f32] = scratch;
        for (y, dst) in resp.chunks_mut(w).enumerate() {
            vert_kernel(y, dst, scratch_view);
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn fit_peak_frac_symmetric_parabola_is_zero() {
        // y = -x² + 1 sampled at ±1 and 0 → peak at 0.
        let f = fit_peak_frac(0.0, 1.0, 0.0, PeakFitMode::Parabolic);
        assert!(f.abs() < 1e-6, "expected 0.0, got {f}");
    }

    #[test]
    fn fit_peak_frac_shifted_parabola_recovers_offset() {
        // y = -(x - 0.25)² + 1 sampled at x = -1, 0, 1.
        let y_of = |x: f32| -(x - 0.25).powi(2) + 1.0;
        let f = fit_peak_frac(y_of(-1.0), y_of(0.0), y_of(1.0), PeakFitMode::Parabolic);
        assert!((f - 0.25).abs() < 1e-5, "expected 0.25, got {f}");
    }

    #[test]
    fn fit_peak_frac_gaussian_mode_handles_log() {
        // Pure Gaussian: y = exp(-(x-0.2)²/(2·0.5²)).
        let g = |x: f32| (-((x - 0.2f32).powi(2)) / 0.5).exp();
        let f = fit_peak_frac(g(-1.0), g(0.0), g(1.0), PeakFitMode::Gaussian);
        assert!((f - 0.2).abs() < 1e-5, "Gaussian-log fit off: {f}");
    }

    #[test]
    fn fit_peak_frac_rejects_non_maximum() {
        // Non-maximum (denominator ≥ 0) → 0.0 fallback.
        let f = fit_peak_frac(1.0, 0.5, 1.0, PeakFitMode::Parabolic);
        assert_eq!(f, 0.0);
    }

    #[test]
    fn fit_peak_frac_gaussian_falls_back_on_nonpositive() {
        // Any ≤0 sample → parabolic branch.
        let parab = fit_peak_frac(-0.5, 2.0, 1.5, PeakFitMode::Parabolic);
        let gauss = fit_peak_frac(-0.5, 2.0, 1.5, PeakFitMode::Gaussian);
        assert_eq!(parab, gauss);
    }

    #[test]
    fn box_blur_zero_radius_is_identity() {
        let side = 5usize;
        let mut resp: Vec<f32> = (0..(side * side)).map(|i| i as f32).collect();
        let before = resp.clone();
        let mut scratch = vec![0.0; side * side];
        box_blur_inplace(&mut resp, &mut scratch, side, side, 0);
        assert_eq!(resp, before);
    }

    #[test]
    fn box_blur_smooths_impulse() {
        let side = 5usize;
        let mut resp = vec![0.0f32; side * side];
        let mut scratch = vec![0.0f32; side * side];
        let mid = side / 2;
        resp[mid * side + mid] = 9.0;
        box_blur_inplace(&mut resp, &mut scratch, side, side, 1);
        // 3×3 blur of an impulse = 9/9 = 1 at the center.
        assert!(
            (resp[mid * side + mid] - 1.0).abs() < 1e-6,
            "center after blur = {}",
            resp[mid * side + mid]
        );
    }

    #[test]
    fn box_blur_preserves_constant_field() {
        let side = 7usize;
        let mut resp = vec![3.5f32; side * side];
        let mut scratch = vec![0.0f32; side * side];
        box_blur_inplace(&mut resp, &mut scratch, side, side, 1);
        for v in &resp {
            assert!((v - 3.5).abs() < 1e-6);
        }
    }

    #[test]
    fn box_blur_handles_rectangular_grid() {
        // Regression: the detector calls the blur on a w×h response
        // map where w != h. Previous signature took a single `side`
        // parameter and panicked out-of-bounds on non-square inputs.
        let w = 8usize;
        let h = 5usize;
        let mut resp = vec![2.0f32; w * h];
        let mut scratch = vec![0.0f32; w * h];
        box_blur_inplace(&mut resp, &mut scratch, w, h, 1);
        for v in &resp {
            assert!((v - 2.0).abs() < 1e-6);
        }
    }
}