darkfi/bin/app/src/ui/vector_art/shadow.rs

/* This file is part of DarkFi (https://dark.fi)
 *
 * Copyright (C) 2020-2026 Dyne.org foundation
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

use super::shape::sexpr_add;
use crate::expr::{Op, SExprCode};

/// Gaussian CDF `0.5 * (1 + erf(dist * sqrt(2) / blur))`: coverage of the
/// box interior at `dist` px from an edge.
fn edge_cdf(dist: f32, blur: f32) -> f32 {
    if blur <= 0. {
        return if dist >= 0. { 1. } else { 0. };
    }
    let z = dist * std::f32::consts::SQRT_2 / blur;
    // This magic is Abramowitz & Stegun 7.1.26, max abs error 1.5e-7
    let t = 1. / (1. + 0.327_591_1 * z.abs());
    let approx = 1. -
        t * (0.254_829_592 +
            t * (-0.284_496_736 +
                t * (1.421_413_741 + t * (-1.453_152_027 + t * 1.061_405_429)))) *
            (-z * z).exp();
    let erf = if z < 0. { -approx } else { approx };
    0.5 * (1. + erf)
}

/// Knot offsets from one edge, one ring every `blur / 4` px. The fade is
/// gone by ~2 * blur, so 8 rings cover it.
pub(crate) fn ring_knots(blur: f32) -> Vec<f32> {
    let n = 8;
    let spacing = blur / 4.;
    (0..=n).map(|k| k as f32 * spacing).collect()
}

/// Inset shadow alpha at a knot.
pub(crate) fn inset_alpha(dx: f32, dy: f32, blur: f32, base: f32) -> f32 {
    let a = base * (1. - edge_cdf(dx, blur) * edge_cdf(dy, blur));
    if a < 1e-4 * base {
        0.
    } else {
        a
    }
}

/// Symbolic position for every knot on one axis. Returns `(position, off)`
/// so callers know each knot's distance from its edge.
pub(crate) fn knot_sexprs(offs: &[f32], e1: &SExprCode, e2: &SExprCode) -> Vec<(SExprCode, f32)> {
    let center = sexpr_add(
        e1.clone(),
        Op::Mul((
            Box::new(Op::Sub((Box::new(e2[0].clone()), Box::new(e1[0].clone())))),
            Box::new(Op::ConstFloat32(0.5)),
        )),
    )
    .unwrap();
    let n = offs.len();
    let near = (0..n).map(|k| {
        let raw = sexpr_add(e1.clone(), Op::ConstFloat32(offs[k])).unwrap();
        let head = raw.into_iter().next().unwrap();
        (vec![Op::Min((Box::new(head), Box::new(center[0].clone())))], offs[k])
    });
    let far = (0..n).rev().map(|k| {
        let raw = sexpr_add(e2.clone(), Op::ConstFloat32(-offs[k])).unwrap();
        let head = raw.into_iter().next().unwrap();
        (vec![Op::Max((Box::new(head), Box::new(center[0].clone())))], offs[k])
    });
    near.chain(far).collect()
}