lightcraft/crates/geom/src/crop.rs

//! The crop model.
//!
//! The image (w × h pixels after orientation) is rotated by `angle` about its centre. The crop is an
//! axis-aligned rectangle in that rotated frame, stored *normalized* to the image size
//! (`x / w`, `y / h`), so it is independent of preview resolution. The crop must stay inside the
//! rotated image (Lightroom's "Constrain to image").

use serde::{Deserialize, Serialize};

use crate::{Affine, Point, Rect, Vec2};

#[derive(Clone, Copy, Debug, PartialEq, Serialize, Deserialize)]
pub struct CropGeometry {
    /// Normalized crop rectangle in the rotated frame (0..1 when angle = 0).
    pub rect: Rect,
    /// Straighten angle in degrees, clockwise positive, −45..45.
    pub angle: f64,
}

impl Default for CropGeometry {
    fn default() -> Self {
        Self { rect: Rect::UNIT, angle: 0.0 }
    }
}

impl CropGeometry {
    pub fn is_identity(&self) -> bool {
        self.angle == 0.0 && self.rect == Rect::UNIT
    }

    /// Crop rectangle in pixels of the rotated frame for an image of `w × h`.
    pub fn rect_px(&self, w: f64, h: f64) -> Rect {
        self.rect.scale(w, h)
    }

    /// Transform from *output* (cropped) pixel coordinates to *source* (oriented image) pixel
    /// coordinates, for an output of `out_w × out_h` pixels rendered from a `w × h` source.
    pub fn output_to_source(&self, w: f64, h: f64, out_w: f64, out_h: f64) -> Affine {
        let r = self.rect_px(w, h);
        // output -> crop rect in rotated frame
        let to_rot = Affine::translate(Vec2::new(r.x0, r.y0)) * Affine::scale(r.width() / out_w, r.height() / out_h);
        // rotated frame -> source: inverse rotation about the image centre
        let c = Point::new(w / 2.0, h / 2.0);
        Affine::rotate_about(-self.angle.to_radians(), c) * to_rot
    }

    /// Whether every crop corner lies within the rotated image.
    pub fn is_within_image(&self, w: f64, h: f64) -> bool {
        let r = self.rect_px(w, h);
        let back = Affine::rotate_about(-self.angle.to_radians(), Point::new(w / 2.0, h / 2.0));
        r.corners().iter().all(|p| {
            let q = back.apply(*p);
            q.x >= -1e-6 * w && q.y >= -1e-6 * h && q.x <= w * (1.0 + 1e-6) && q.y <= h * (1.0 + 1e-6)
        })
    }

    /// Shrink the crop (about its centre, keeping aspect) and then shift it until it fits inside the
    /// rotated image.
    pub fn constrained(&self, w: f64, h: f64) -> CropGeometry {
        if self.is_within_image(w, h) {
            return *self;
        }
        let r = self.rect_px(w, h);
        // Try moving towards the image centre first.
        let c = Point::new(w / 2.0, h / 2.0);
        let mut best = *self;
        for i in 1..=32 {
            let t = i as f64 / 32.0;
            let nc = r.center().lerp(c, t);
            let cand = CropGeometry { rect: Rect::from_center(nc, r.width(), r.height()).scale(1.0 / w, 1.0 / h), angle: self.angle };
            if cand.is_within_image(w, h) {
                return cand;
            }
            best = cand;
        }
        // Then shrink around the centre.
        let s = max_inscribed_scale(w, h, self.angle, r.width(), r.height());
        let nr = Rect::from_center(c, r.width() * s, r.height() * s);
        let _ = best;
        CropGeometry { rect: nr.scale(1.0 / w, 1.0 / h), angle: self.angle }
    }
}

/// Largest scale `s` such that a centred `cw·s × ch·s` rectangle fits inside a `w × h` image rotated
/// by `angle_deg` about its centre.
pub fn max_inscribed_scale(w: f64, h: f64, angle_deg: f64, cw: f64, ch: f64) -> f64 {
    let a = angle_deg.to_radians().abs();
    let (s, c) = a.sin_cos();
    // Half extents of the crop must satisfy, in the image frame (rotated back):
    // |x| c + |y| s <= w/2 and |x| s + |y| c <= h/2 for the crop corner (x, y) = (cw/2, ch/2)·scale.
    let (hx, hy) = (cw / 2.0, ch / 2.0);
    let s1 = (w / 2.0) / (hx * c + hy * s);
    let s2 = (h / 2.0) / (hx * s + hy * c);
    s1.min(s2)
}

/// The biggest crop of the image's own aspect (or `aspect` if given) that fits at `angle_deg`.
pub fn crop_fit_angle(w: f64, h: f64, angle_deg: f64, aspect: Option<f64>) -> CropGeometry {
    let a = aspect.unwrap_or(w / h);
    let base = Rect::new(0.0, 0.0, w, h).fit_aspect(a);
    let s = max_inscribed_scale(w, h, angle_deg, base.width(), base.height()).min(1.0);
    let r = Rect::from_center(Point::new(w / 2.0, h / 2.0), base.width() * s, base.height() * s);
    CropGeometry { rect: r.scale(1.0 / w, 1.0 / h), angle: angle_deg }
}

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

    #[test]
    fn identity_maps_pixels() {
        let c = CropGeometry::default();
        let t = c.output_to_source(400.0, 300.0, 400.0, 300.0);
        let p = t.apply(Point::new(10.0, 20.0));
        assert!((p.x - 10.0).abs() < 1e-9 && (p.y - 20.0).abs() < 1e-9);
    }

    #[test]
    fn rotated_fit_is_inside() {
        for ang in [-45.0, -10.0, 0.0, 3.5, 30.0, 45.0] {
            let c = crop_fit_angle(600.0, 400.0, ang, None);
            assert!(c.is_within_image(600.0, 400.0), "{ang}");
            // and maximal: 1% bigger does not fit (except angle 0 where it equals the image)
            let big = CropGeometry { rect: Rect::from_center(c.rect.center(), c.rect.width() * 1.01, c.rect.height() * 1.01), angle: ang };
            assert!(!big.is_within_image(600.0, 400.0), "{ang}");
        }
    }

    #[test]
    fn constrain_pulls_crop_inside() {
        let c = CropGeometry { rect: Rect::new(0.5, 0.5, 1.4, 1.2), angle: 12.0 };
        let k = c.constrained(300.0, 200.0);
        assert!(k.is_within_image(300.0, 200.0));
    }

    #[test]
    fn half_crop_maps_to_centre_quadrant() {
        let c = CropGeometry { rect: Rect::new(0.25, 0.25, 0.75, 0.75), angle: 0.0 };
        let t = c.output_to_source(100.0, 100.0, 50.0, 50.0);
        let p = t.apply(Point::new(0.0, 0.0));
        assert!((p.x - 25.0).abs() < 1e-9 && (p.y - 25.0).abs() < 1e-9);
    }
}