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Utilize const generics to pass settings to VW algorithm. #942

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merged 4 commits into from
Dec 6, 2022
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Utilize const generics to pass settings to VW algorithm. #942

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Utilize const generics to pass settings to VW algorithm.
frewsxcv Nov 29, 2022
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frewsxcv Nov 29, 2022
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81 changes: 37 additions & 44 deletions geo/src/algorithm/simplifyvw.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -56,18 +56,6 @@ where
}
}

/// Settings for Ring and Line geometries
// initial min: if we ever have fewer than these, stop immediately
// min_points: if we detect a self-intersection before point removal, and we only
// have min_points left, stop: since a self-intersection causes removal of the spatially previous
// point, THAT could lead to a further self-intersection without the possibility of removing
// more points, potentially leaving the geometry in an invalid state.
#[derive(Debug, Clone, Copy)]
struct GeomSettings {
initial_min: usize,
min_points: usize,
}

/// Simplify a line using the [Visvalingam-Whyatt](http://www.tandfonline.com/doi/abs/10.1179/000870493786962263) algorithm
//
// This method returns the **indices** of the simplified line
Expand Down Expand Up @@ -197,11 +185,20 @@ where
.collect()
}

/// Wrap the actual VW function so the R* Tree can be shared.
// Wrap the actual VW function so the R* Tree can be shared.
// this ensures that shell and rings have access to all segments, so
// intersections between outer and inner rings are detected
fn vwp_wrapper<T>(
geomtype: &GeomSettings,
//
// Constants:
//
// * `INITIAL_MIN`
// * If we ever have fewer than these, stop immediately
// * `MIN_POINTS`
// * If we detect a self-intersection before point removal, and we only have `MIN_POINTS` left,
// stop: since a self-intersection causes removal of the spatially previous point, THAT could
// lead to a further self-intersection without the possibility of removing more points,
// potentially leaving the geometry in an invalid state.
fn vwp_wrapper<T, const INITIAL_MIN: usize, const MIN_POINTS: usize>(
exterior: &LineString<T>,
interiors: Option<&[LineString<T>]>,
epsilon: &T,
Expand All @@ -224,20 +221,33 @@ where
);

// Simplify shell
rings.push(visvalingam_preserve(geomtype, exterior, epsilon, &mut tree));
rings.push(visvalingam_preserve::<T, INITIAL_MIN, MIN_POINTS>(
exterior, epsilon, &mut tree,
));
// Simplify interior rings, if any
if let Some(interior_rings) = interiors {
for ring in interior_rings {
rings.push(visvalingam_preserve(geomtype, ring, epsilon, &mut tree))
rings.push(visvalingam_preserve::<T, INITIAL_MIN, MIN_POINTS>(
ring, epsilon, &mut tree,
))
}
}
rings
}

/// Visvalingam-Whyatt with self-intersection detection to preserve topologies
/// this is a port of the technique at https://www.jasondavies.com/simplify/
fn visvalingam_preserve<T>(
geomtype: &GeomSettings,
//
// Constants:
//
// * `INITIAL_MIN`
// * If we ever have fewer than these, stop immediately
// * `MIN_POINTS`
// * If we detect a self-intersection before point removal, and we only have `MIN_POINTS` left,
// stop: since a self-intersection causes removal of the spatially previous point, THAT could
// lead to a further self-intersection without the possibility of removing more points,
// potentially leaving the geometry in an invalid state.
fn visvalingam_preserve<T, const INITIAL_MIN: usize, const MIN_POINTS: usize>(
orig: &LineString<T>,
epsilon: &T,
tree: &mut RTree<Line<T>>,
Expand Down Expand Up @@ -288,7 +298,7 @@ where
if smallest.area > *epsilon {
continue;
}
if counter <= geomtype.initial_min {
if counter <= INITIAL_MIN {
// we can't remove any more points no matter what
break;
}
Expand All @@ -304,7 +314,7 @@ where
// because we could then no longer form a valid geometry if removal of next also caused an intersection.
// The simplification process is thus over.
smallest.intersector = tree_intersect(tree, &smallest, &orig.0);
if smallest.intersector && counter <= geomtype.min_points {
if smallest.intersector && counter <= MIN_POINTS {
break;
}
// We've got a valid triangle, and its area is smaller than epsilon, so
Expand Down Expand Up @@ -560,11 +570,7 @@ where
T: CoordFloat + RTreeNum + HasKernel,
{
fn simplifyvw_preserve(&self, epsilon: &T) -> LineString<T> {
let gt = GeomSettings {
initial_min: 2,
min_points: 4,
};
let mut simplified = vwp_wrapper(&gt, self, None, epsilon);
let mut simplified = vwp_wrapper::<_, 2, 4>(self, None, epsilon);
LineString::from(simplified.pop().unwrap())
}
}
Expand All @@ -588,11 +594,8 @@ where
T: CoordFloat + RTreeNum + HasKernel,
{
fn simplifyvw_preserve(&self, epsilon: &T) -> Polygon<T> {
let gt = GeomSettings {
initial_min: 4,
min_points: 6,
};
let mut simplified = vwp_wrapper(&gt, self.exterior(), Some(self.interiors()), epsilon);
let mut simplified =
vwp_wrapper::<_, 4, 6>(self.exterior(), Some(self.interiors()), epsilon);
let exterior = LineString::from(simplified.remove(0));
let interiors = simplified.into_iter().map(LineString::from).collect();
Polygon::new(exterior, interiors)
Expand Down Expand Up @@ -666,9 +669,7 @@ where

#[cfg(test)]
mod test {
use super::{
cartesian_intersect, visvalingam, vwp_wrapper, GeomSettings, SimplifyVW, SimplifyVWPreserve,
};
use super::{cartesian_intersect, visvalingam, vwp_wrapper, SimplifyVW, SimplifyVWPreserve};
use crate::{
line_string, point, polygon, Coord, LineString, MultiLineString, MultiPolygon, Point,
Polygon,
Expand Down Expand Up @@ -725,11 +726,7 @@ mod test {
(x: 300., y: 40.),
(x: 301., y: 10.)
];
let gt = &GeomSettings {
initial_min: 2,
min_points: 4,
};
let simplified = vwp_wrapper(gt, &ls, None, &668.6);
let simplified = vwp_wrapper::<_, 2, 4>(&ls, None, &668.6);
// this is the correct, non-intersecting LineString
let correct = vec![
(10., 60.),
Expand Down Expand Up @@ -802,11 +799,7 @@ mod test {
fn very_long_vwp_test() {
// simplify an 8k-point LineString, eliminating self-intersections
let points_ls = geo_test_fixtures::norway_main::<f64>();
let gt = &GeomSettings {
initial_min: 2,
min_points: 4,
};
let simplified = vwp_wrapper(gt, &points_ls, None, &0.0005);
let simplified = vwp_wrapper::<_, 2, 4>(&points_ls, None, &0.0005);
assert_eq!(simplified[0].len(), 3278);
}

Expand Down