feat(terrain): build BAS basement foundations

1 files changed, 809 insertions, 0 deletions

diff --git a/crates/fparkan-terrain/src/contour.rs b/crates/fparkan-terrain/src/contour.rs
new file mode 100644
index 0000000..de5077a
--- /dev/null
+++ b/crates/fparkan-terrain/src/contour.rs
@@ -0,0 +1,809 @@
+/// One triangle fragment retained after subtracting a building contour.
+/// `source_triangle` indexes the triangle array passed to
+/// [`subtract_contour`].
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub(crate) struct TerrainTriangleFragment {
+    /// Source triangle index, for preserving its material and vertex channels.
+    pub source_triangle: usize,
+    /// Fragment triangle vertices in the input coordinate space.
+    pub positions: [[f32; 3]; 3],
+}
+
+/// Error returned when a terrain contour cannot be subtracted safely.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub(crate) enum TerrainContourError {
+    /// The contour is non-finite, degenerate, or self-intersecting.
+    UnsupportedContour,
+    /// A source triangle has non-finite coordinates or a degenerate XY plane.
+    InvalidSourceTriangle {
+        /// Index of the source triangle.
+        triangle: usize,
+    },
+}
+
+impl std::fmt::Display for TerrainContourError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::UnsupportedContour => write!(
+                f,
+                "terrain contour must be finite, simple, and non-degenerate"
+            ),
+            Self::InvalidSourceTriangle { triangle } => {
+                write!(
+                    f,
+                    "terrain source triangle {triangle} is not a finite XY surface"
+                )
+            }
+        }
+    }
+}
+
+impl std::error::Error for TerrainContourError {}
+
+/// Subtracts a finite simple contour using the native per-source-face point set:
+/// the three source vertices plus the actual contour segments clipped to that
+/// source triangle. It does not extend cuts across the triangle or triangulate
+/// the contour into unrelated internal edges.
+///
+/// # Errors
+///
+/// Returns [`TerrainContourError::UnsupportedContour`] for malformed or
+/// self-intersecting contours, and
+/// [`TerrainContourError::InvalidSourceTriangle`] when a source face cannot be
+/// triangulated or lifted onto its original plane.
+pub(crate) fn subtract_contour(
+    triangles: &[[[f32; 3]; 3]],
+    contour: &[[f32; 2]],
+) -> Result<Vec<TerrainTriangleFragment>, TerrainContourError> {
+    let ring = normalize_ring(contour)?;
+    let contour_min: [f64; 2] = std::array::from_fn(|axis| {
+        ring.iter()
+            .map(|point| point[axis])
+            .fold(f64::INFINITY, f64::min)
+    });
+    let contour_max: [f64; 2] = std::array::from_fn(|axis| {
+        ring.iter()
+            .map(|point| point[axis])
+            .fold(f64::NEG_INFINITY, f64::max)
+    });
+    let mut output = Vec::new();
+
+    for (source_triangle, source) in triangles.iter().copied().enumerate() {
+        if !source.iter().flatten().all(|value| value.is_finite()) {
+            return Err(TerrainContourError::InvalidSourceTriangle {
+                triangle: source_triangle,
+            });
+        }
+        let source_xy = source.map(|point| [f64::from(point[0]), f64::from(point[1])]);
+        let source_area = cross2(source_xy[0], source_xy[1], source_xy[2]);
+        if source_area.abs() <= 1.0e-10 {
+            return Err(TerrainContourError::InvalidSourceTriangle {
+                triangle: source_triangle,
+            });
+        }
+        let source_min: [f64; 2] = std::array::from_fn(|axis| {
+            source_xy
+                .iter()
+                .map(|point| point[axis])
+                .fold(f64::INFINITY, f64::min)
+        });
+        let source_max: [f64; 2] = std::array::from_fn(|axis| {
+            source_xy
+                .iter()
+                .map(|point| point[axis])
+                .fold(f64::NEG_INFINITY, f64::max)
+        });
+        if (0..2).any(|axis| {
+            source_max[axis] < contour_min[axis] || contour_max[axis] < source_min[axis]
+        }) {
+            output.push(TerrainTriangleFragment {
+                source_triangle,
+                positions: source,
+            });
+            continue;
+        }
+
+        let mut vertices = source_xy.to_vec();
+        let initial = if source_area > 0.0 {
+            [0, 1, 2]
+        } else {
+            [0, 2, 1]
+        };
+        let mut triangulation = vec![initial];
+        let mut clipped_segments = Vec::new();
+        for index in 0..ring.len() {
+            let first = ring[index];
+            let second = ring[(index + 1) % ring.len()];
+            if let Some((start, end)) = clip_segment_to_triangle(first, second, source_xy) {
+                let start_id = insert_unique_point(&mut vertices, start);
+                let end_id = insert_unique_point(&mut vertices, end);
+                if start_id != end_id {
+                    clipped_segments.push((start_id, end_id));
+                }
+            }
+        }
+
+        if clipped_segments.is_empty() {
+            let center = [
+                (source_xy[0][0] + source_xy[1][0] + source_xy[2][0]) / 3.0,
+                (source_xy[0][1] + source_xy[1][1] + source_xy[2][1]) / 3.0,
+            ];
+            if !point_in_ring(center, &ring) {
+                output.push(TerrainTriangleFragment {
+                    source_triangle,
+                    positions: source,
+                });
+            }
+            continue;
+        }
+
+        for point_id in 3..vertices.len() {
+            insert_triangulation_point(point_id, &vertices, &mut triangulation).map_err(|()| {
+                TerrainContourError::InvalidSourceTriangle {
+                    triangle: source_triangle,
+                }
+            })?;
+        }
+
+        let mut constraints = source_boundary_constraints(&vertices, source_xy);
+        for (first, second) in clipped_segments {
+            let mut on_segment = vertices
+                .iter()
+                .copied()
+                .enumerate()
+                .filter_map(|(index, point)| {
+                    point_on_segment(point, vertices[first], vertices[second]).then_some((
+                        segment_parameter(point, vertices[first], vertices[second]),
+                        index,
+                    ))
+                })
+                .collect::<Vec<_>>();
+            on_segment.sort_by(|left, right| left.0.total_cmp(&right.0));
+            on_segment.dedup_by_key(|(_, index)| *index);
+            for pair in on_segment.windows(2) {
+                let edge = canonical_edge(pair[0].1, pair[1].1);
+                if edge.0 == edge.1 {
+                    continue;
+                }
+                recover_constraint(edge, &mut triangulation, &vertices, &constraints).map_err(
+                    |()| TerrainContourError::InvalidSourceTriangle {
+                        triangle: source_triangle,
+                    },
+                )?;
+                constraints.insert(edge);
+            }
+        }
+        legalize_constrained_delaunay(&mut triangulation, &vertices, &constraints).map_err(
+            |()| TerrainContourError::InvalidSourceTriangle {
+                triangle: source_triangle,
+            },
+        )?;
+
+        for triangle in triangulation {
+            let center = triangle.map(|index| vertices[index]);
+            let center = [
+                (center[0][0] + center[1][0] + center[2][0]) / 3.0,
+                (center[0][1] + center[1][1] + center[2][1]) / 3.0,
+            ];
+            if point_in_ring(center, &ring) {
+                continue;
+            }
+            let mut positions: [[f32; 3]; 3] = triangle
+                .map(|index| lift_to_source_plane(vertices[index], source))
+                .into_iter()
+                .collect::<Option<Vec<_>>>()
+                .and_then(|points| points.try_into().ok())
+                .ok_or(TerrainContourError::InvalidSourceTriangle {
+                    triangle: source_triangle,
+                })?;
+            if source_area < 0.0 {
+                positions.swap(1, 2);
+            }
+            output.push(TerrainTriangleFragment {
+                source_triangle,
+                positions,
+            });
+        }
+    }
+    Ok(output)
+}
+
+fn normalize_ring(contour: &[[f32; 2]]) -> Result<Vec<[f64; 2]>, TerrainContourError> {
+    let mut ring = contour
+        .iter()
+        .map(|point| [f64::from(point[0]), f64::from(point[1])])
+        .collect::<Vec<_>>();
+    if ring.iter().flatten().any(|value| !value.is_finite()) {
+        return Err(TerrainContourError::UnsupportedContour);
+    }
+    if ring.len() > 1 && distance_squared(ring[0], *ring.last().expect("nonempty")) <= 1.0e-16 {
+        ring.pop();
+    }
+    let mut normalized = Vec::with_capacity(ring.len());
+    for point in ring {
+        if normalized
+            .last()
+            .is_none_or(|previous| distance_squared(*previous, point) > 1.0e-16)
+        {
+            normalized.push(point);
+        }
+    }
+    if normalized.len() > 1
+        && distance_squared(normalized[0], *normalized.last().expect("nonempty")) <= 1.0e-16
+    {
+        normalized.pop();
+    }
+    if normalized.len() < 3 {
+        return Err(TerrainContourError::UnsupportedContour);
+    }
+    let signed_area = polygon_area(&normalized);
+    if !signed_area.is_finite() || signed_area.abs() <= 1.0e-10 {
+        return Err(TerrainContourError::UnsupportedContour);
+    }
+    for first in 0..normalized.len() {
+        let first_next = (first + 1) % normalized.len();
+        for second in (first + 1)..normalized.len() {
+            let second_next = (second + 1) % normalized.len();
+            if first_next == second || second_next == first {
+                continue;
+            }
+            if segments_intersect(
+                normalized[first],
+                normalized[first_next],
+                normalized[second],
+                normalized[second_next],
+            ) {
+                return Err(TerrainContourError::UnsupportedContour);
+            }
+        }
+    }
+    if signed_area < 0.0 {
+        normalized.reverse();
+    }
+    Ok(normalized)
+}
+
+fn clip_segment_to_triangle(
+    first: [f64; 2],
+    second: [f64; 2],
+    triangle: [[f64; 2]; 3],
+) -> Option<([f64; 2], [f64; 2])> {
+    let mut triangle = triangle;
+    if cross2(triangle[0], triangle[1], triangle[2]) < 0.0 {
+        triangle.swap(1, 2);
+    }
+    let mut start = 0.0_f64;
+    let mut end = 1.0_f64;
+    for edge in 0..3 {
+        let edge_first = triangle[edge];
+        let edge_second = triangle[(edge + 1) % 3];
+        let at_start = cross2(edge_first, edge_second, first);
+        let at_end = cross2(edge_first, edge_second, second);
+        let change = at_end - at_start;
+        if change.abs() <= 1.0e-14 {
+            if at_start < -1.0e-10 {
+                return None;
+            }
+            continue;
+        }
+        let crossing = -at_start / change;
+        if change > 0.0 {
+            start = start.max(crossing);
+        } else {
+            end = end.min(crossing);
+        }
+        if start > end + 1.0e-12 {
+            return None;
+        }
+    }
+    start = start.clamp(0.0, 1.0);
+    end = end.clamp(0.0, 1.0);
+    if end - start <= 1.0e-12 {
+        return None;
+    }
+    Some((
+        interpolate(first, second, start),
+        interpolate(first, second, end),
+    ))
+}
+
+fn insert_unique_point(vertices: &mut Vec<[f64; 2]>, point: [f64; 2]) -> usize {
+    // Terrain positions are f32 throughout the native data and renderer. Make
+    // clipped crossings use that same representable coordinate before building
+    // the local CDT, so distinct f64 intersections cannot collapse to one XY
+    // vertex only after fragment emission.
+    let point = [f64::from(point[0] as f32), f64::from(point[1] as f32)];
+    if let Some(index) = vertices
+        .iter()
+        .position(|candidate| distance_squared(*candidate, point) <= 1.0e-18)
+    {
+        return index;
+    }
+    let index = vertices.len();
+    vertices.push(point);
+    index
+}
+
+fn insert_triangulation_point(
+    point_id: usize,
+    vertices: &[[f64; 2]],
+    triangles: &mut Vec<[usize; 3]>,
+) -> Result<(), ()> {
+    let point = vertices[point_id];
+    let edge = triangulation_edges(triangles)
+        .keys()
+        .copied()
+        .find(|&(first, second)| point_on_open_segment(point, vertices[first], vertices[second]));
+    if let Some(edge) = edge {
+        let mut split = Vec::new();
+        triangles.retain(|triangle| {
+            if triangle_has_edge(*triangle, edge) {
+                let opposite = triangle
+                    .iter()
+                    .copied()
+                    .find(|index| *index != edge.0 && *index != edge.1)
+                    .expect("triangle has opposite vertex");
+                split.push(opposite);
+                false
+            } else {
+                true
+            }
+        });
+        if split.is_empty() {
+            return Err(());
+        }
+        for opposite in split {
+            let first = make_ccw([edge.0, point_id, opposite], vertices).ok_or(())?;
+            let second = make_ccw([point_id, edge.1, opposite], vertices).ok_or(())?;
+            triangles.extend([first, second]);
+        }
+        return Ok(());
+    }
+
+    let containing = triangles.iter().position(|triangle| {
+        point_in_triangle(
+            point,
+            vertices[triangle[0]],
+            vertices[triangle[1]],
+            vertices[triangle[2]],
+        )
+    });
+    let Some(containing) = containing else {
+        return Err(());
+    };
+    let triangle = triangles.swap_remove(containing);
+    for edge in 0..3 {
+        let split = make_ccw(
+            [triangle[edge], triangle[(edge + 1) % 3], point_id],
+            vertices,
+        )
+        .ok_or(())?;
+        triangles.push(split);
+    }
+    Ok(())
+}
+
+fn source_boundary_constraints(
+    vertices: &[[f64; 2]],
+    triangle: [[f64; 2]; 3],
+) -> std::collections::BTreeSet<(usize, usize)> {
+    let mut constraints = std::collections::BTreeSet::new();
+    for edge in 0..3 {
+        let first = triangle[edge];
+        let second = triangle[(edge + 1) % 3];
+        let mut points = vertices
+            .iter()
+            .copied()
+            .enumerate()
+            .filter_map(|(index, point)| {
+                point_on_segment(point, first, second)
+                    .then_some((segment_parameter(point, first, second), index))
+            })
+            .collect::<Vec<_>>();
+        points.sort_by(|left, right| left.0.total_cmp(&right.0));
+        points.dedup_by_key(|(_, index)| *index);
+        for pair in points.windows(2) {
+            constraints.insert(canonical_edge(pair[0].1, pair[1].1));
+        }
+    }
+    constraints
+}
+
+fn recover_constraint(
+    target: (usize, usize),
+    triangles: &mut [[usize; 3]],
+    vertices: &[[f64; 2]],
+    constraints: &std::collections::BTreeSet<(usize, usize)>,
+) -> Result<(), ()> {
+    let limit = vertices
+        .len()
+        .saturating_mul(vertices.len())
+        .saturating_mul(8)
+        .max(1);
+    for _ in 0..limit {
+        if edge_exists(triangles, target) {
+            return Ok(());
+        }
+        let mut crossed = triangulation_edges(triangles)
+            .into_iter()
+            .filter_map(|(edge, adjacent)| {
+                if adjacent.len() != 2
+                    || edge.0 == target.0
+                    || edge.0 == target.1
+                    || edge.1 == target.0
+                    || edge.1 == target.1
+                {
+                    return None;
+                }
+                proper_intersection_parameter(
+                    vertices[target.0],
+                    vertices[target.1],
+                    vertices[edge.0],
+                    vertices[edge.1],
+                )
+                .map(|parameter| (parameter, edge, adjacent))
+            })
+            .collect::<Vec<_>>();
+        crossed.sort_by(|left, right| left.0.total_cmp(&right.0));
+        let mut flipped = false;
+        for (_, edge, adjacent) in crossed {
+            if constraints.contains(&edge) {
+                return Err(());
+            }
+            let first_index = adjacent[0].0;
+            let second_index = adjacent[1].0;
+            let first_opposite = adjacent[0].1;
+            let second_opposite = adjacent[1].1;
+            if !convex_quadrilateral(
+                vertices[edge.0],
+                vertices[edge.1],
+                vertices[first_opposite],
+                vertices[second_opposite],
+            ) {
+                continue;
+            }
+            let replacement = canonical_edge(first_opposite, second_opposite);
+            if constraints.contains(&replacement)
+                || proper_intersection_parameter(
+                    vertices[target.0],
+                    vertices[target.1],
+                    vertices[replacement.0],
+                    vertices[replacement.1],
+                )
+                .is_some()
+            {
+                continue;
+            }
+            let (a, b) = directed_edge(triangles[first_index], edge).ok_or(())?;
+            triangles[first_index] =
+                make_ccw([first_opposite, second_opposite, b], vertices).ok_or(())?;
+            triangles[second_index] =
+                make_ccw([second_opposite, first_opposite, a], vertices).ok_or(())?;
+            flipped = true;
+            break;
+        }
+        if !flipped {
+            return Err(());
+        }
+    }
+    Err(())
+}
+
+fn legalize_constrained_delaunay(
+    triangles: &mut [[usize; 3]],
+    vertices: &[[f64; 2]],
+    constraints: &std::collections::BTreeSet<(usize, usize)>,
+) -> Result<(), ()> {
+    let limit = triangles
+        .len()
+        .saturating_mul(triangles.len())
+        .saturating_mul(8)
+        .max(1);
+    for _ in 0..limit {
+        let edges = triangulation_edges(triangles);
+        let mut flipped = false;
+        for (edge, adjacent) in edges {
+            if adjacent.len() != 2 || constraints.contains(&edge) {
+                continue;
+            }
+            let first_index = adjacent[0].0;
+            let second_index = adjacent[1].0;
+            let c = adjacent[0].1;
+            let d = adjacent[1].1;
+            if !convex_quadrilateral(vertices[edge.0], vertices[edge.1], vertices[c], vertices[d]) {
+                continue;
+            }
+            let (a, b) = directed_edge(triangles[first_index], edge).ok_or(())?;
+            if in_circle(vertices[a], vertices[b], vertices[c], vertices[d]) <= 1.0e-10 {
+                continue;
+            }
+            let replacement = canonical_edge(c, d);
+            if constraints.iter().any(|constraint| {
+                proper_intersection_parameter(
+                    vertices[replacement.0],
+                    vertices[replacement.1],
+                    vertices[constraint.0],
+                    vertices[constraint.1],
+                )
+                .is_some()
+            }) {
+                continue;
+            }
+            triangles[first_index] = make_ccw([c, d, b], vertices).ok_or(())?;
+            triangles[second_index] = make_ccw([d, c, a], vertices).ok_or(())?;
+            flipped = true;
+            break;
+        }
+        if !flipped {
+            return Ok(());
+        }
+    }
+    Err(())
+}
+
+fn triangulation_edges(
+    triangles: &[[usize; 3]],
+) -> std::collections::BTreeMap<(usize, usize), Vec<(usize, usize)>> {
+    let mut edges = std::collections::BTreeMap::new();
+    for (triangle_index, triangle) in triangles.iter().copied().enumerate() {
+        for edge in 0..3 {
+            let first = triangle[edge];
+            let second = triangle[(edge + 1) % 3];
+            let opposite = triangle[(edge + 2) % 3];
+            edges
+                .entry(canonical_edge(first, second))
+                .or_insert_with(Vec::new)
+                .push((triangle_index, opposite));
+        }
+    }
+    edges
+}
+
+fn triangle_has_edge(triangle: [usize; 3], edge: (usize, usize)) -> bool {
+    triangle.contains(&edge.0) && triangle.contains(&edge.1)
+}
+
+fn edge_exists(triangles: &[[usize; 3]], edge: (usize, usize)) -> bool {
+    triangles
+        .iter()
+        .copied()
+        .any(|triangle| triangle_has_edge(triangle, edge))
+}
+
+fn directed_edge(triangle: [usize; 3], edge: (usize, usize)) -> Option<(usize, usize)> {
+    (0..3).find_map(|index| {
+        let first = triangle[index];
+        let second = triangle[(index + 1) % 3];
+        (canonical_edge(first, second) == edge).then_some((first, second))
+    })
+}
+
+fn canonical_edge(first: usize, second: usize) -> (usize, usize) {
+    if first < second {
+        (first, second)
+    } else {
+        (second, first)
+    }
+}
+
+fn make_ccw(triangle: [usize; 3], vertices: &[[f64; 2]]) -> Option<[usize; 3]> {
+    let area = cross2(
+        vertices[triangle[0]],
+        vertices[triangle[1]],
+        vertices[triangle[2]],
+    );
+    if !area.is_finite() || area.abs() <= 1.0e-12 {
+        return None;
+    }
+    Some(if area > 0.0 {
+        triangle
+    } else {
+        [triangle[0], triangle[2], triangle[1]]
+    })
+}
+
+fn point_in_triangle(point: [f64; 2], a: [f64; 2], b: [f64; 2], c: [f64; 2]) -> bool {
+    let edges = [(a, b), (b, c), (c, a)];
+    edges.into_iter().all(|(first, second)| {
+        let scale = distance_squared(first, second).sqrt();
+        cross2(first, second, point) >= -linear_tolerance(first, second, point) * scale
+    })
+}
+
+fn point_on_open_segment(point: [f64; 2], first: [f64; 2], second: [f64; 2]) -> bool {
+    if !point_on_segment(point, first, second) {
+        return false;
+    }
+    let t = segment_parameter(point, first, second);
+    t > 1.0e-9 && t < 1.0 - 1.0e-9
+}
+
+fn point_on_segment(point: [f64; 2], first: [f64; 2], second: [f64; 2]) -> bool {
+    let length_squared = distance_squared(first, second);
+    if length_squared <= 1.0e-20 {
+        return distance_squared(point, first) <= 1.0e-18;
+    }
+    let cross = cross2(first, second, point).abs();
+    let dot = (point[0] - first[0]) * (second[0] - first[0])
+        + (point[1] - first[1]) * (second[1] - first[1]);
+    let tolerance = linear_tolerance(first, second, point);
+    let length = length_squared.sqrt();
+    cross <= tolerance * length
+        && dot >= -tolerance * length
+        && dot <= length_squared + tolerance * length
+}
+
+fn linear_tolerance(first: [f64; 2], second: [f64; 2], point: [f64; 2]) -> f64 {
+    let coordinate_scale = first
+        .into_iter()
+        .chain(second)
+        .chain(point)
+        .map(f64::abs)
+        .fold(1.0_f64, f64::max);
+    2.0 * f64::from(f32::EPSILON) * coordinate_scale
+}
+
+fn segment_parameter(point: [f64; 2], first: [f64; 2], second: [f64; 2]) -> f64 {
+    let delta = [second[0] - first[0], second[1] - first[1]];
+    let length_squared = delta[0] * delta[0] + delta[1] * delta[1];
+    if length_squared <= 1.0e-20 {
+        0.0
+    } else {
+        ((point[0] - first[0]) * delta[0] + (point[1] - first[1]) * delta[1]) / length_squared
+    }
+}
+
+fn proper_intersection_parameter(
+    first: [f64; 2],
+    second: [f64; 2],
+    edge_first: [f64; 2],
+    edge_second: [f64; 2],
+) -> Option<f64> {
+    let first_delta = [second[0] - first[0], second[1] - first[1]];
+    let edge_delta = [
+        edge_second[0] - edge_first[0],
+        edge_second[1] - edge_first[1],
+    ];
+    let denominator = first_delta[0] * edge_delta[1] - first_delta[1] * edge_delta[0];
+    if denominator.abs() <= 1.0e-12 {
+        return None;
+    }
+    let offset = [edge_first[0] - first[0], edge_first[1] - first[1]];
+    let t = (offset[0] * edge_delta[1] - offset[1] * edge_delta[0]) / denominator;
+    let u = (offset[0] * first_delta[1] - offset[1] * first_delta[0]) / denominator;
+    (t > 1.0e-9 && t < 1.0 - 1.0e-9 && u > 1.0e-9 && u < 1.0 - 1.0e-9).then_some(t)
+}
+
+fn convex_quadrilateral(a: [f64; 2], b: [f64; 2], c: [f64; 2], d: [f64; 2]) -> bool {
+    cross2(a, b, c) * cross2(a, b, d) < -1.0e-12 && cross2(c, d, a) * cross2(c, d, b) < -1.0e-12
+}
+
+fn in_circle(a: [f64; 2], b: [f64; 2], c: [f64; 2], d: [f64; 2]) -> f64 {
+    let a = [a[0] - d[0], a[1] - d[1]];
+    let b = [b[0] - d[0], b[1] - d[1]];
+    let c = [c[0] - d[0], c[1] - d[1]];
+    let a2 = a[0] * a[0] + a[1] * a[1];
+    let b2 = b[0] * b[0] + b[1] * b[1];
+    let c2 = c[0] * c[0] + c[1] * c[1];
+    a2 * (b[0] * c[1] - b[1] * c[0]) - b2 * (a[0] * c[1] - a[1] * c[0])
+        + c2 * (a[0] * b[1] - a[1] * b[0])
+}
+
+fn segments_intersect(a: [f64; 2], b: [f64; 2], c: [f64; 2], d: [f64; 2]) -> bool {
+    let ab_c = cross2(a, b, c);
+    let ab_d = cross2(a, b, d);
+    let cd_a = cross2(c, d, a);
+    let cd_b = cross2(c, d, b);
+    let eps = 1.0e-8;
+    (ab_c.abs() <= eps && point_on_segment(c, a, b))
+        || (ab_d.abs() <= eps && point_on_segment(d, a, b))
+        || (cd_a.abs() <= eps && point_on_segment(a, c, d))
+        || (cd_b.abs() <= eps && point_on_segment(b, c, d))
+        || (ab_c * ab_d < 0.0 && cd_a * cd_b < 0.0)
+}
+
+fn point_in_ring(point: [f64; 2], ring: &[[f64; 2]]) -> bool {
+    let mut winding = 0_i32;
+    for index in 0..ring.len() {
+        let first = ring[index];
+        let second = ring[(index + 1) % ring.len()];
+        let side = cross2(first, second, point);
+        if first[1] <= point[1] {
+            if second[1] > point[1] && side > 0.0 {
+                winding += 1;
+            }
+        } else if second[1] <= point[1] && side < 0.0 {
+            winding -= 1;
+        }
+    }
+    winding != 0
+}
+
+fn polygon_area(points: &[[f64; 2]]) -> f64 {
+    (0..points.len())
+        .map(|index| {
+            let first = points[index];
+            let second = points[(index + 1) % points.len()];
+            first[0] * second[1] - second[0] * first[1]
+        })
+        .sum::<f64>()
+        * 0.5
+}
+
+fn lift_to_source_plane(point: [f64; 2], source: [[f32; 3]; 3]) -> Option<[f32; 3]> {
+    for vertex in source {
+        if distance_squared(point, [f64::from(vertex[0]), f64::from(vertex[1])]) <= 1.0e-18 {
+            return Some(vertex);
+        }
+    }
+    let a = source[0].map(f64::from);
+    let b = source[1].map(f64::from);
+    let c = source[2].map(f64::from);
+    let denominator = cross2([a[0], a[1]], [b[0], b[1]], [c[0], c[1]]);
+    if denominator.abs() <= 1.0e-10 {
+        return None;
+    }
+    let second_weight = cross2([a[0], a[1]], point, [c[0], c[1]]) / denominator;
+    let third_weight = cross2([a[0], a[1]], [b[0], b[1]], point) / denominator;
+    let first_weight = 1.0 - second_weight - third_weight;
+    let z = first_weight * a[2] + second_weight * b[2] + third_weight * c[2];
+    let position = [point[0] as f32, point[1] as f32, z as f32];
+    position
+        .iter()
+        .all(|value| value.is_finite())
+        .then_some(position)
+}
+
+fn cross2(first: [f64; 2], second: [f64; 2], third: [f64; 2]) -> f64 {
+    (second[0] - first[0]) * (third[1] - first[1]) - (second[1] - first[1]) * (third[0] - first[0])
+}
+
+fn interpolate(first: [f64; 2], second: [f64; 2], amount: f64) -> [f64; 2] {
+    [
+        first[0] + (second[0] - first[0]) * amount,
+        first[1] + (second[1] - first[1]) * amount,
+    ]
+}
+
+fn distance_squared(first: [f64; 2], second: [f64; 2]) -> f64 {
+    let delta_x = first[0] - second[0];
+    let delta_y = first[1] - second[1];
+    delta_x * delta_x + delta_y * delta_y
+}
+
+#[cfg(test)]
+mod tests {
+    use super::subtract_contour;
+
+    #[test]
+    fn translated_thin_outside_strip_keeps_its_area() {
+        let base = 10_000.0_f32;
+        let source = [[
+            [base, base, 0.0],
+            [base + 10.0, base, 0.0],
+            [base, base + 10.0, 0.0],
+        ]];
+        let outer = [
+            [base + 0.003, base + 0.003],
+            [base + 1.0, base + 0.003],
+            [base + 1.0, base + 0.997],
+            [base + 0.003, base + 0.997],
+        ];
+        let output = subtract_contour(&source, &outer).expect("simple contour");
+        let retained_area = output
+            .iter()
+            .map(|fragment| {
+                let [a, b, c] = fragment.positions;
+                let ab = [f64::from(b[0] - a[0]), f64::from(b[1] - a[1])];
+                let ac = [f64::from(c[0] - a[0]), f64::from(c[1] - a[1])];
+                (ab[0] * ac[1] - ab[1] * ac[0]).abs() * 0.5
+            })
+            .sum::<f64>();
+        let expected_area =
+            50.0 - (f64::from(outer[1][0] - outer[0][0]) * f64::from(outer[2][1] - outer[1][1]));
+        assert!(
+            (retained_area - expected_area).abs() < 0.001,
+            "retained area {retained_area}, expected {expected_area}"
+        );
+    }
+}