commit f46196e90329daf6c16e3ced32c568815aac2883
Author: numzero <numzer0@yandex.ru>
Date:   Sun Apr 28 22:16:25 2024 +0300

    Some refraction

diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..a886d81
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,19 @@
+[package]
+name = "refraction"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[profile.dev]
+opt-level = 2
+
+[profile.dev.package."*"]
+opt-level = 3
+
+[dependencies]
+glm = "0.2.3"
+flo_draw = "0.3.1"
+flo_canvas = "0.3.1"
+num-traits = "0.2.18"
+itertools-num = "0.1.3"
diff --git a/src/bin/flat.rs b/src/bin/flat.rs
new file mode 100644
index 0000000..d2656b3
--- /dev/null
+++ b/src/bin/flat.rs
@@ -0,0 +1,161 @@
+use flo_draw::*;
+use flo_canvas::*;
+use glm::*;
+use num_traits::identities::Zero;
+
+pub fn main() {
+	let space = Coil {
+		coil_scale: 2.0,
+		coil_r: 300.0,
+		coil_w: 50.0,
+		coil_m: 10.0,
+	};
+	with_2d_graphics(move || {
+		let canvas = create_drawing_window("Refraction");
+		canvas.draw(|gc| {
+			gc.canvas_height(1000.0);
+
+			gc.new_path();
+			gc.circle(0.0, 0.0, space.coil_r + space.coil_w);
+			gc.circle(0.0, 0.0, space.coil_r - space.coil_w);
+			gc.line_width(3.0);
+			gc.stroke_color(Color::Rgba(0.8, 0.8, 0.8, 1.0));
+			gc.stroke();
+
+			gc.line_width(1.0);
+			gc.stroke_color(Color::Rgba(1.0, 0.5, 0.0, 1.0));
+			for y in itertools_num::linspace(-1.0, 1.0, 101) {
+				let base = vec2(-500.0, 0.0);
+				let dir = vec2(1.0, y);
+				let path = trace(&space, base, dir, 1000.0, 1.0);
+				gc.new_path();
+				gc.move_to(base.x, base.y);
+				for pt in path {
+					gc.line_to(pt.x, pt.y);
+				}
+				gc.stroke();
+			}
+		});
+	});
+}
+
+struct Coil {
+	coil_m: f32,
+	coil_scale: f32,
+	coil_r: f32,
+	coil_w: f32,
+}
+
+impl Metric for Coil {
+	fn halfmetric(&self, pos: Vec2) -> Decomp2 {
+		let r = length(pos);
+		let dir = normalize(pos);
+
+		let s = smoothbox (r, vec2(self.coil_r - self.coil_w, self.coil_r + self.coil_w), self.coil_m);
+		let t = mix(1.0, self.coil_r / r / self.coil_scale, s);
+
+		Decomp2{
+			ortho: mat2(
+				dir.x, - dir.y,
+				dir.y, dir.x,
+			),
+			diag : vec2(1.0, t),
+		}
+	}
+}
+
+struct Decomp2 {
+	ortho: Mat2,
+	diag: Vec2,
+}
+
+type Tens2 = [Mat2; 2];
+
+trait Metric {
+	fn halfmetric(&self, pos: Vec2) -> Decomp2;
+
+	fn metric(&self, pos: Vec2) -> Mat2 {
+		let h = self.halfmetric(pos);
+		transpose(&h.ortho) * diagonal(h.diag * h.diag) * h.ortho
+	}
+
+	fn dmetric(&self, pos: Vec2) -> Tens2 {
+		part_deriv(|p| self.metric(p), pos, 1.0e-3)
+	}
+
+	fn globalize(&self, at: Vec2, v: Vec2) -> Vec2 {
+		let h = self.halfmetric(at);
+		transpose(&h.ortho) * diagonal( Vec2::from_s(1.0) / h.diag) * h.ortho * v
+	}
+}
+
+fn trace(space: &impl Metric, base: Vec2, dir: Vec2, distance: f32, dt: f32) -> Vec<Vec2> {
+	let steps = floor(distance / dt) as usize;
+	let mut result = Vec::with_capacity(steps);
+	let mut p = base;
+	let mut v = normalize(dir);
+	for _ in 0..steps {
+		let a: Vec2 = -convolute(krist(space, p), v);
+		v = v + a * dt;
+		p = p + v * dt;
+		result.push(p);
+	}
+	result
+}
+
+fn krist(space: &impl Metric, pos: Vec2) -> Tens2 {
+	// Γ^i_k_l = .5 * g^i^m * (g_m_k,l + g_m_l,k - g_k_l,m)
+	let g = inverse(&space.metric(pos)); // с верхними индексами
+	let d = space.dmetric(pos);
+	let mut ret: Tens2 = [Mat2::zero(); 2];
+	// ret[i][l][k] = sum((m) => .5f * g[m][i] * (d[k][l][m] + d[l][k][m] - d[m][k][l]))
+	for i in 0..2 {
+		for l in 0..2 {
+			for k in 0..2 {
+				let mut v = 0.0;
+				for m in 0..2 {
+					v += g[m][i] * (d[l][k][m] + d[k][m][l] - d[m][k][l]);
+				}
+				ret[i][l][k] = 0.5 * v;
+			}
+		}
+	}
+	ret
+}
+
+fn dir_deriv(f: impl Fn(Vec2) -> Mat2, pos: Vec2, delta: Vec2) -> Mat2 {
+	(f(pos + delta) - f(pos - delta)) / (2.0 * length(delta))
+}
+
+fn part_deriv(f: impl Fn(Vec2) -> Mat2, pos: Vec2, eps: f32) -> Tens2 {
+	[
+		dir_deriv(&f, pos, vec2(eps, 0.0)),
+		dir_deriv(&f, pos, vec2(0.0, eps)),
+	]
+}
+
+fn convolute(G: Tens2, v: Vec2) -> Vec2 {
+	vec2(
+		dot(v, G[0] * v),
+		dot(v, G[1] * v)
+	)
+}
+
+fn diagonal(v: Vec2) -> Mat2 {
+	mat2(v.x, 0.0, 0.0, v.y)
+}
+
+fn sqr(x: f32) -> f32 {
+	return x * x;
+}
+
+fn smoothstep(x: f32) -> f32 {
+	return 3.0 * x * x - 2.0 * x * x * x;
+}
+
+fn smoothbox(val: f32, range: Vec2, pad: f32) -> f32 {
+	let slope1 = 1.0 + (val - range.x) / pad;
+	let slope2 = 1.0 - (val - range.y) / pad;
+	let lin = min(slope1, slope2);
+	smoothstep(glm::clamp(lin, 0.0, 1.0))
+}