djvu-rs 0.20.0

//! Backwards-compatible pixmap module.
//!
//! The implementation lives in the standalone `djvu-pixmap` crate. This module
//! preserves the historical `djvu_rs::pixmap::{Pixmap, GrayPixmap}` path and
//! hosts [`scale_lanczos3`], the context-free Lanczos-3 image resampler shared
//! by the render paths (it has no DjVu semantics, so it belongs with the pixmap
//! type rather than on the render interface).

pub use djvu_pixmap::{GrayPixmap, Pixmap};

/// Lanczos-3 kernel: `sinc(x) * sinc(x/3)` for `|x| < 3`, 0 otherwise.
///
/// Uses the normalised sinc: `sinc(x) = sin(π x) / (π x)`, `sinc(0) = 1`.
#[inline]
fn lanczos3_kernel(x: f32) -> f32 {
    let ax = x.abs();
    if ax >= 3.0 {
        return 0.0;
    }
    if ax < 1e-6 {
        return 1.0;
    }
    let pi_x = core::f32::consts::PI * ax;
    let sinc_x = pi_x.sin() / pi_x;
    let pi_x3 = pi_x / 3.0;
    let sinc_x3 = pi_x3.sin() / pi_x3;
    sinc_x * sinc_x3
}

/// Scale `src` to `dst_w × dst_h` using separable Lanczos-3 resampling.
///
/// Two-pass implementation:
/// 1. Horizontal pass: `src_w × src_h` → `dst_w × src_h` intermediate.
/// 2. Vertical pass: `dst_w × src_h` → `dst_w × dst_h` output.
///
/// Only RGBA pixmaps are handled (alpha is passed through unchanged at 255).
pub(crate) fn scale_lanczos3(src: &Pixmap, dst_w: u32, dst_h: u32) -> Pixmap {
    let src_w = src.width;
    let src_h = src.height;

    // Short-circuit: nothing to scale.
    if src_w == dst_w && src_h == dst_h {
        return src.clone();
    }
    if dst_w == 0 || dst_h == 0 {
        return Pixmap::white(dst_w.max(1), dst_h.max(1));
    }

    // ── Horizontal pass ───────────────────────────────────────────────────────
    // Map each output column `ox` (0..dst_w) to a source position, then sum
    // the Lanczos-3 kernel over the contributing source columns.
    let h_scale = src_w as f32 / dst_w as f32;
    let h_support = (3.0_f32 * h_scale.max(1.0)).ceil() as i32; // kernel half-width in src pixels

    let mut mid = Pixmap::new(dst_w, src_h, 255, 255, 255, 255);
    for oy in 0..src_h {
        for ox in 0..dst_w {
            // Centre of the output pixel in source coordinates.
            let cx = (ox as f32 + 0.5) * h_scale - 0.5;
            let x0 = (cx.floor() as i32 - h_support + 1).max(0);
            let x1 = (cx.floor() as i32 + h_support).min(src_w as i32 - 1);

            let mut r = 0.0_f32;
            let mut g = 0.0_f32;
            let mut b = 0.0_f32;
            let mut w_sum = 0.0_f32;

            for sx in x0..=x1 {
                let w = lanczos3_kernel((sx as f32 - cx) / h_scale.max(1.0));
                let (pr, pg, pb) = src.get_rgb(sx as u32, oy);
                r += pr as f32 * w;
                g += pg as f32 * w;
                b += pb as f32 * w;
                w_sum += w;
            }

            let norm = if w_sum.abs() > 1e-6 { 1.0 / w_sum } else { 1.0 };
            mid.set_rgb(
                ox,
                oy,
                (r * norm).round().clamp(0.0, 255.0) as u8,
                (g * norm).round().clamp(0.0, 255.0) as u8,
                (b * norm).round().clamp(0.0, 255.0) as u8,
            );
        }
    }

    // ── Vertical pass ─────────────────────────────────────────────────────────
    let v_scale = src_h as f32 / dst_h as f32;
    let v_support = (3.0_f32 * v_scale.max(1.0)).ceil() as i32;

    let mut out = Pixmap::new(dst_w, dst_h, 255, 255, 255, 255);
    for oy in 0..dst_h {
        let cy = (oy as f32 + 0.5) * v_scale - 0.5;
        let y0 = (cy.floor() as i32 - v_support + 1).max(0);
        let y1 = (cy.floor() as i32 + v_support).min(src_h as i32 - 1);

        for ox in 0..dst_w {
            let mut r = 0.0_f32;
            let mut g = 0.0_f32;
            let mut b = 0.0_f32;
            let mut w_sum = 0.0_f32;

            for sy in y0..=y1 {
                let w = lanczos3_kernel((sy as f32 - cy) / v_scale.max(1.0));
                let (pr, pg, pb) = mid.get_rgb(ox, sy as u32);
                r += pr as f32 * w;
                g += pg as f32 * w;
                b += pb as f32 * w;
                w_sum += w;
            }

            let norm = if w_sum.abs() > 1e-6 { 1.0 / w_sum } else { 1.0 };
            out.set_rgb(
                ox,
                oy,
                (r * norm).round().clamp(0.0, 255.0) as u8,
                (g * norm).round().clamp(0.0, 255.0) as u8,
                (b * norm).round().clamp(0.0, 255.0) as u8,
            );
        }
    }

    out
}

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

    /// `lanczos3_kernel(0)` == 1.0 (unity at origin).
    #[test]
    fn lanczos3_kernel_unity_at_zero() {
        assert!((lanczos3_kernel(0.0) - 1.0).abs() < 1e-5);
    }

    /// `lanczos3_kernel` is zero outside |x| ≥ 3.
    #[test]
    fn lanczos3_kernel_zero_outside_support() {
        assert_eq!(lanczos3_kernel(3.0), 0.0);
        assert_eq!(lanczos3_kernel(-3.5), 0.0);
        assert_eq!(lanczos3_kernel(10.0), 0.0);
    }

    /// `scale_lanczos3` preserves dimensions.
    #[test]
    fn scale_lanczos3_correct_dimensions() {
        let src = Pixmap::white(100, 80);
        let dst = scale_lanczos3(&src, 50, 40);
        assert_eq!(dst.width, 50);
        assert_eq!(dst.height, 40);
    }

    /// `scale_lanczos3` returns a clone when source and target match.
    #[test]
    fn scale_lanczos3_noop_when_same_size() {
        let src = Pixmap::new(4, 4, 200, 100, 50, 255);
        let dst = scale_lanczos3(&src, 4, 4);
        assert_eq!(dst.width, 4);
        assert_eq!(dst.height, 4);
        assert_eq!(dst.data, src.data);
    }

    /// Scaling a solid-color pixmap with Lanczos-3 preserves the color.
    #[test]
    fn scale_lanczos3_preserves_solid_color() {
        // Solid red 20×20 → 10×10
        let src = Pixmap::new(20, 20, 200, 0, 0, 255);
        let dst = scale_lanczos3(&src, 10, 10);
        assert_eq!(dst.width, 10);
        assert_eq!(dst.height, 10);
        // All output pixels should be close to red (200, 0, 0).
        for chunk in dst.data.chunks_exact(4) {
            let (r, g, b) = (chunk[0], chunk[1], chunk[2]);
            assert!(
                (r as i32 - 200).abs() <= 5 && g <= 5 && b <= 5,
                "expected near-red (200,0,0), got ({r},{g},{b})"
            );
        }
    }
}