projective-grid 0.10.0

Image-free, target-agnostic projective grid recovery: label 2D feature points with (i, j) lattice coordinates under perspective
Documentation 
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//! Delaunator wrapper used by the axis-driven topological pipeline.
//!
//! Thin wrapper around the [`delaunator`] crate. The wrapper converts a slice
//! of `nalgebra::Point2<f32>` into `delaunator`'s `f64` `Point` format and
//! exposes the half-edge structure needed by the topological pipeline. The
//! conversion cost is negligible compared to triangulation, and the `f64`
//! internal type buys numerical robustness for near-degenerate inputs.
//!
//! All items here are crate-internal; the `topological` module's
//! [`super::detect_square_oriented2_topological_all`] orchestrator owns the
//! public surface.

use nalgebra::Point2;

/// Result of triangulating a feature cloud.
pub(super) struct Triangulation {
    /// Flat list of triangle vertex indices: triangle `t` occupies
    /// `triangles[3t..3t+3]`. Length is always a multiple of 3.
    pub(super) triangles: Vec<usize>,
    /// Half-edge buddies. `halfedges[e]` is the matching half-edge in
    /// the neighbour triangle, or [`delaunator::EMPTY`] (== `usize::MAX`)
    /// if `e` is on the convex hull.
    pub(super) halfedges: Vec<usize>,
}

impl Triangulation {
    /// Number of triangles.
    #[inline]
    pub(super) fn num_tri(&self) -> usize {
        self.triangles.len() / 3
    }

    /// Convenience: half-edges of triangle `t` are at offsets `3t..3t+3`.
    /// `next_edge(e)` walks to the next half-edge inside the same triangle.
    #[inline]
    pub(super) fn next_edge(e: usize) -> usize {
        if e % 3 == 2 {
            e - 2
        } else {
            e + 1
        }
    }

    /// Triangle index containing half-edge `e`.
    #[inline]
    pub(super) fn tri_of(e: usize) -> usize {
        e / 3
    }
}

/// Convert `Point2<f32>` to `delaunator::Point`. The `f64` internal type
/// buys numerical robustness for near-degenerate inputs.
#[inline]
fn point_to_f64(p: Point2<f32>) -> delaunator::Point {
    delaunator::Point {
        x: p.x as f64,
        y: p.y as f64,
    }
}

#[cfg_attr(
    feature = "tracing",
    tracing::instrument(
        name = "delaunay_triangulate",
        level = "debug",
        skip_all,
        fields(num_points = positions.len()),
    )
)]
pub(super) fn triangulate(positions: &[Point2<f32>]) -> Triangulation {
    let pts: Vec<delaunator::Point> = positions.iter().copied().map(point_to_f64).collect();
    let t = delaunator::triangulate(&pts);
    Triangulation {
        triangles: t.triangles,
        halfedges: t.halfedges,
    }
}

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

    fn pt(x: f32, y: f32) -> Point2<f32> {
        Point2::new(x, y)
    }

    #[test]
    fn triangulates_square() {
        let positions = vec![pt(0.0, 0.0), pt(1.0, 0.0), pt(1.0, 1.0), pt(0.0, 1.0)];
        let t = triangulate(&positions);
        assert_eq!(t.num_tri(), 2);
        for (e, &buddy) in t.halfedges.iter().enumerate() {
            if buddy != delaunator::EMPTY {
                assert_eq!(t.halfedges[buddy], e);
            }
        }
    }

    #[test]
    fn next_edge_walks_triangle() {
        assert_eq!(Triangulation::next_edge(0), 1);
        assert_eq!(Triangulation::next_edge(1), 2);
        assert_eq!(Triangulation::next_edge(2), 0);
        assert_eq!(Triangulation::next_edge(3), 4);
        assert_eq!(Triangulation::next_edge(5), 3);
        assert_eq!(Triangulation::tri_of(2), 0);
        assert_eq!(Triangulation::tri_of(5), 1);
    }
}