map_scatter 0.4.1

Rule-based object scattering library with field-graph evaluation and sampling
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140

//! Jittered-grid position sampling strategy.
use glam::Vec2;
use mint::Vector2;
use rand::RngCore;

use crate::sampling::{next_down, rand01, PositionSampling};

/// Jittered grid sampling.
#[derive(Debug, Clone)]
pub struct JitterGridSampling {
    /// Jitter amount in [0, 1], where 0 is grid centers and 1 is max jitter.
    pub jitter: f32,
    /// Cell size for the grid.
    pub cell_size: f32,
}

impl JitterGridSampling {
    /// Create a new JitterGridSampling with specified jitter (0.0 to 1.0).
    pub fn new(jitter: f32, cell_size: f32) -> Self {
        Self {
            jitter: jitter.clamp(0.0, 1.0),
            cell_size,
        }
    }
}

impl PositionSampling for JitterGridSampling {
    fn generate(&self, domain_extent: Vector2<f32>, rng: &mut dyn RngCore) -> Vec<Vector2<f32>> {
        let domain_extent = Vec2::from(domain_extent);
        if domain_extent.x <= 0.0 || domain_extent.y <= 0.0 {
            return Vec::new();
        }

        let w = domain_extent.x;
        let h = domain_extent.y;
        let eff = if self.cell_size.is_finite() && self.cell_size > 0.0 {
            self.cell_size
        } else {
            (w.min(h) / 10.0).max(1.0)
        };

        let mut cols = (w / eff).floor() as i32;
        let mut rows = (h / eff).floor() as i32;

        if cols < 1 {
            cols = 1;
        }
        if rows < 1 {
            rows = 1;
        }

        let cols = cols as usize;
        let rows = rows as usize;

        let cell_w = w / cols as f32;
        let cell_h = h / rows as f32;

        let half_w = w * 0.5;
        let half_h = h * 0.5;

        let max_x = next_down(half_w);
        let max_y = next_down(half_h);

        let jitter_x = self.jitter * (cell_w * 0.5);
        let jitter_y = self.jitter * (cell_h * 0.5);

        let mut points = Vec::with_capacity(cols * rows);

        for j in 0..rows {
            for i in 0..cols {
                let x0 = -half_w + i as f32 * cell_w;
                let y0 = -half_h + j as f32 * cell_h;
                let cx = x0 + cell_w * 0.5;
                let cy = y0 + cell_h * 0.5;
                let jx = if jitter_x > 0.0 {
                    let r = rand01(rng) * 2.0 - 1.0;
                    (r * jitter_x).clamp(-(cell_w * 0.5), cell_w * 0.5)
                } else {
                    0.0
                };
                let jy = if jitter_y > 0.0 {
                    let r = rand01(rng) * 2.0 - 1.0;
                    (r * jitter_y).clamp(-(cell_h * 0.5), cell_h * 0.5)
                } else {
                    0.0
                };
                let mut px = cx + jx;
                let mut py = cy + jy;
                px = px.clamp(-half_w, max_x);
                py = py.clamp(-half_h, max_y);

                points.push(Vec2::new(px, py));
            }
        }

        points.into_iter().map(Into::into).collect()
    }
}

#[cfg(test)]
mod tests {
    use rand::rngs::StdRng;
    use rand::SeedableRng;

    use super::*;

    #[test]
    fn new_clamps_jitter_value() {
        let sampler = JitterGridSampling::new(2.0, 1.0);
        assert_eq!(sampler.jitter, 1.0);
    }

    #[test]
    fn generate_returns_grid_centers_without_jitter() {
        let strategy = JitterGridSampling::new(0.0, 2.0);
        let mut rng = StdRng::seed_from_u64(1);
        let points = strategy.generate(Vec2::new(4.0, 4.0).into(), &mut rng);

        assert_eq!(points.len(), 4);
        let mut xs: Vec<_> = points.iter().map(|p| p.x).collect();
        xs.sort_by(|a, b| a.partial_cmp(b).unwrap());
        assert_eq!(xs, vec![-1.0, -1.0, 1.0, 1.0]);

        let mut ys: Vec<_> = points.iter().map(|p| p.y).collect();
        ys.sort_by(|a, b| a.partial_cmp(b).unwrap());
        assert_eq!(ys, vec![-1.0, -1.0, 1.0, 1.0]);
    }

    #[test]
    fn generate_returns_empty_for_non_positive_extent() {
        let strategy = JitterGridSampling::new(0.0, 1.0);
        let mut rng = StdRng::seed_from_u64(42);
        assert!(strategy
            .generate(Vec2::new(0.0, 5.0).into(), &mut rng)
            .is_empty());
        assert!(strategy
            .generate(Vec2::new(5.0, 0.0).into(), &mut rng)
            .is_empty());
    }
}