Preview

//! Zero-gravity cloth simulation: a free-floating spring net wrapped by a ball.
//!
//! Iterative-only: spring forces, self-collision and ball-cloth collision have
//! no closed form; all state is carried across frames by `step`.
//!
//! The cloth is unpinned and weightless, so it floats perfectly flat (no
//! self-weight sag). A ball is driven straight down through it: the cloth
//! visibly wraps around the ball and is dragged down, then settles draped over
//! it. The cloth is rendered as a shaded `MeshItem` surface (smooth normals
//! each frame); the ball is a static sphere mesh moved by its transform.

use ranim::{
    color::palettes::manim,
    core::Extract,
    core::animation::eval::iterative::Iterative,
    core::components::rgba::Rgba,
    core::core_item::CoreItem,
    glam::{DVec3, dvec3},
    items::mesh::{MeshItem, Sphere, Surface},
    prelude::*,
};

const ROWS: usize = 16;
const COLS: usize = 16;
const SPACING: f64 = 0.22;
const CLOTH_Y: f64 = 2.2;
const DAMPING: f64 = 0.99;
const K_STRUCTURAL: f64 = 600.0;
const K_SHEAR: f64 = 420.0;
const K_BEND: f64 = 220.0;
const REPULSION_CUTOFF: f64 = 0.35 * SPACING;
const REPULSION_K: f64 = 300.0;
const BALL_RADIUS: f64 = 0.65;
/// The ball is driven straight down from `BALL_START_Y` at this speed (units/s).
const BALL_SPEED: f64 = 1.2;
const BALL_START_Y: f64 = 3.0;
/// Where the ball stops, just below the cloth, letting the cloth settle draped over it.
const BALL_STOP_Y: f64 = 1.4;

/// The cloth-plus-ball system's full state.
#[derive(Clone)]
struct ClothState {
    curr: Vec<DVec3>,
    prev: Vec<DVec3>,
    springs: Vec<(usize, usize, f64, f64)>, // (i, j, rest_len, stiffness)
    ball_y: f64,
    ball_mesh: MeshItem,
    cloth_indices: Vec<u32>,
}

impl ClothState {
    fn new() -> Self {
        let width = (COLS - 1) as f64 * SPACING;
        let mut curr = Vec::with_capacity(ROWS * COLS);
        for r in 0..ROWS {
            for c in 0..COLS {
                curr.push(dvec3(
                    -width / 2.0 + c as f64 * SPACING,
                    CLOTH_Y,
                    -width / 2.0 + r as f64 * SPACING,
                ));
            }
        }

        let mut springs = Vec::new();
        for r in 0..ROWS {
            for c in 0..COLS {
                let i = r * COLS + c;
                if c + 1 < COLS {
                    springs.push((i, i + 1, SPACING, K_STRUCTURAL));
                }
                if r + 1 < ROWS {
                    springs.push((i, i + COLS, SPACING, K_STRUCTURAL));
                }
                if c + 1 < COLS && r + 1 < ROWS {
                    springs.push((i, i + COLS + 1, SPACING * 2.0f64.sqrt(), K_SHEAR));
                }
                if c + 2 < COLS {
                    springs.push((i, i + 2, 2.0 * SPACING, K_BEND));
                }
                if r + 2 < ROWS {
                    springs.push((i, i + 2 * COLS, 2.0 * SPACING, K_BEND));
                }
            }
        }

        // Grid quads -> two triangles each, wound for +Y normals on the flat cloth.
        let mut cloth_indices = Vec::with_capacity(6 * (ROWS - 1) * (COLS - 1));
        for r in 0..ROWS - 1 {
            for c in 0..COLS - 1 {
                let a = (r * COLS + c) as u32;
                let b = (r * COLS + c + 1) as u32;
                let d = ((r + 1) * COLS + c) as u32;
                let e = ((r + 1) * COLS + c + 1) as u32;
                cloth_indices.extend_from_slice(&[a, e, b, a, d, e]);
            }
        }

        let ball_mesh = MeshItem::from(
            Surface::from(
                Sphere::new(BALL_RADIUS)
                    .with_resolution((20, 12))
                    .with_fill_color(manim::RED_C),
            )
            .with_smooth_normals(),
        );

        Self {
            prev: curr.clone(),
            curr,
            springs,
            ball_y: BALL_START_Y,
            ball_mesh,
            cloth_indices,
        }
    }

    fn step(&mut self, dt: f64, global_secs: f64) {
        let dt2 = dt * dt;
        let n = self.curr.len();

        // Spring forces (zero gravity: the cloth floats freely and stays flat).
        let mut ax = vec![0.0f64; n];
        let mut ay = vec![0.0f64; n];
        let mut az = vec![0.0f64; n];
        for &(i, j, rest, k) in &self.springs {
            let d = self.curr[j] - self.curr[i];
            let dist = d.length();
            if dist < 1e-9 {
                continue;
            }
            let f = k * (dist - rest) / dist;
            ax[i] += f * d.x;
            ay[i] += f * d.y;
            az[i] += f * d.z;
            ax[j] -= f * d.x;
            ay[j] -= f * d.y;
            az[j] -= f * d.z;
        }

        // Self-collision repulsion (keeps the wrapped cloth from folding onto itself).
        for i in 0..n {
            for j in (i + 1)..n {
                let d = self.curr[j] - self.curr[i];
                let dist = d.length();
                if dist < REPULSION_CUTOFF && dist > 1e-9 {
                    let f = REPULSION_K * (REPULSION_CUTOFF - dist) / dist;
                    ax[i] -= f * d.x;
                    ay[i] -= f * d.y;
                    az[i] -= f * d.z;
                    ax[j] += f * d.x;
                    ay[j] += f * d.y;
                    az[j] += f * d.z;
                }
            }
        }

        // Cloth Verlet integration (no gravity).
        for i in 0..n {
            let vx = (self.curr[i].x - self.prev[i].x) * DAMPING;
            let vy = (self.curr[i].y - self.prev[i].y) * DAMPING;
            let vz = (self.curr[i].z - self.prev[i].z) * DAMPING;
            self.prev[i] = self.curr[i];
            self.curr[i].x += vx + ax[i] * dt2;
            self.curr[i].y += vy + ay[i] * dt2;
            self.curr[i].z += vz + az[i] * dt2;
        }

        // Ball position: driven straight down by the global (unwarped) clock,
        // then held below the cloth. It is kinematic state: updated here in
        // `step` and projected by `extract`.
        self.ball_y = (BALL_START_Y - BALL_SPEED * global_secs).max(BALL_STOP_Y);
        let ball_center = dvec3(0.0, self.ball_y, 0.0);

        // Ball-cloth collision: push cloth particles out of the ball.
        for curr in self.curr.iter_mut().take(n) {
            let d = *curr - ball_center;
            let dist = d.length();
            if dist < BALL_RADIUS && dist > 1e-9 {
                *curr += d / dist * (BALL_RADIUS - dist);
            }
        }
    }
}

impl Extract for ClothState {
    type Target = CoreItem;

    fn extract_into(&self, buf: &mut Vec<CoreItem>) {
        let points: Vec<DVec3> = self.curr.clone();

        // Smooth normals: accumulate face normals, then normalize.
        let mut normals = vec![DVec3::ZERO; points.len()];
        for [a, b, c] in self.cloth_indices.as_chunks::<3>().0 {
            let (a, b, c) = (*a as usize, *b as usize, *c as usize);
            let face = (points[b] - points[a]).cross(points[c] - points[a]);
            normals[a] += face;
            normals[b] += face;
            normals[c] += face;
        }
        for n in &mut normals {
            *n = n.normalize_or_zero();
        }

        let vertex_colors = vec![Rgba::from(manim::WHITE.with_alpha(1.0)); points.len()];
        let cloth = MeshItem {
            points: points.into(),
            triangle_indices: self.cloth_indices.clone(),
            vertex_colors: vertex_colors.into(),
            vertex_normals: normals.into(),
        };

        let ball = self
            .ball_mesh
            .clone()
            .transformed(Translation(dvec3(0.0, self.ball_y, 0.0)));

        cloth.extract_into(buf);
        ball.extract_into(buf);
    }
}

#[scene]
#[wasm_demo_doc]
#[output(dir = "./output/cloth_wrap")]
fn cloth_wrap(r: &mut RanimScene) {
    let sim_secs = 7.0;
    let mut camera = CameraFrame::default();
    // Oblique downward view (~40° below horizontal, slightly asymmetric azimuth).
    camera.pos = dvec3(-4.0, 5.0, 2.5);
    camera.facing = (dvec3(0.0, 1.0, 0.0) - camera.pos).normalize();
    camera.up = DVec3::Y;
    camera.perspective_blend = 1.0;
    camera.fovy = 45.0f64.to_radians();

    r.play(camera.show().with_duration(sim_secs));
    r.play(
        Iterative::from_fn(
            ClothState::new(),
            move |state: &mut ClothState, alpha: f64, delta_alpha: f64| {
                state.step(sim_secs * delta_alpha, sim_secs * alpha);
            },
        )
        .with_steps((sim_secs * 60.0) as usize)
        .with_duration(sim_secs),
    );
    r.insert_time_mark(sim_secs, TimeMark::Capture("preview.png".to_string()));
}