diff --git a/src/lib/components/App.svelte b/src/lib/components/App.svelte
index 758e406..bd1bfc5 100644
--- a/src/lib/components/App.svelte
+++ b/src/lib/components/App.svelte
@@ -1,8 +1,22 @@
 <script lang="ts">
 	import { Canvas } from '@threlte/core';
 	import Scene from './Scene.svelte';
+
+	let progress = 1;
 </script>
 
 <Canvas>
-	<Scene />
+	<Scene bind:progress />
 </Canvas>
+
+<div class="controls">
+	<input type="range" min="0" max="1" step="0.01" bind:value={progress} />
+</div>
+
+<style>
+	.controls {
+		position: absolute;
+		top: 0;
+		right: 0;
+	}
+</style>
diff --git a/src/lib/components/Scene.svelte b/src/lib/components/Scene.svelte
index 6ad2be5..8336bdf 100644
--- a/src/lib/components/Scene.svelte
+++ b/src/lib/components/Scene.svelte
@@ -10,31 +10,34 @@
 		Mesh,
 		DoubleSide,
 		Color,
-		Box3,
-		Matrix4
+		Plane,
+		Line3,
+		Float32BufferAttribute,
+		Box3
 	} from 'three';
 	import { ExtendedTriangle, MeshBVH } from 'three-mesh-bvh';
 	import type { Readable } from 'svelte/store';
-	import type { HitPointInfo } from 'three-mesh-bvh';
-	import { mergeGeometries } from 'three/examples/jsm/utils/BufferGeometryUtils.js';
 
 	export let buildSurface = [300, 300, 300];
 	export let layerHeight = 0.2;
 	export let nozzleSize = 0.4;
 	export let tolerance = 0.005;
+	export let progress = 1;
 
 	export let maxNonPlanarAngle = MathUtils.degToRad(20);
 	export let bedNormal = new Vector3(0, 0, 1);
+	export let extruderNormal = new Vector3(0, 0, -1);
 	export let origin = new Vector3(150, 150, 0);
 
 	const stl: Readable<BufferGeometry> = useLoader(STLLoader).load('/benchy.stl');
 
 	let mesh: Mesh;
-	let layers: BufferGeometry[] = [];
+	let layers: { type: 'line' | 'surface'; geometry: BufferGeometry }[] = [];
 
 	$: if ($stl) {
 		const bvh = new MeshBVH($stl);
 		const positions = $stl.getAttribute('position');
+		const normals = $stl.getAttribute('normal');
 		const index = $stl.index!;
 
 		const qualifyingTriangles = Array.from({ length: index.count / 3 }, () => false);
@@ -50,11 +53,14 @@
 			);
 			triangle.getNormal(normal);
 			const angle = normal.angleTo(bedNormal);
-			if ((angle > Math.PI / 2 ? Math.PI - angle : angle) < maxNonPlanarAngle) {
+			// TODO: bottom layers
+			if (angle < maxNonPlanarAngle) {
 				qualifyingTriangles[i] = true;
 				qualifyingTrianglesCount++;
 			}
 		}
+		const includedTriangles = [...qualifyingTriangles];
+		const includedTrianglesCount = qualifyingTrianglesCount;
 
 		const surfaces: number[][] = [];
 		while (qualifyingTrianglesCount > 0) {
@@ -92,10 +98,10 @@
 			surfaces.push(surface);
 		}
 
-		layers = surfaces.map((surface) => {
+		const nonPlanarSurfaces = surfaces.map((surface) => {
 			const geometry = new BufferGeometry();
 			geometry.setAttribute('position', positions);
-			geometry.setAttribute('normal', $stl.getAttribute('normal'));
+			geometry.setAttribute('normal', normals);
 			const indices: number[] = Array.from({ length: surface.length * 3 });
 			for (let i = 0; i < surface.length; i++) {
 				const pos = surface[i] * 3;
@@ -104,8 +110,104 @@
 				indices[i * 3 + 2] = $stl.index!.array[pos + 2];
 			}
 			geometry.setIndex(indices);
-			return geometry;
+			return new MeshBVH(geometry);
 		});
+		const activeNonPlanarSurfaces: [number, MeshBVH][] = [];
+		const consumedNonPlanarSurfaces = nonPlanarSurfaces.map(() => false);
+		const withheldLines: number[][][] = nonPlanarSurfaces.map(() => [[]]);
+		const blacklist = Array.from({ length: index.count / 3 }).map(() => false);
+
+		const line = new Line3();
+		const targetVector1 = new Vector3();
+		const targetVector2 = new Vector3();
+		const targetVector3 = new Vector3();
+		const layerPlane = new Plane();
+		const layerBox = new Box3();
+		for (let layer = 0; layer < $stl.boundingBox!.max.z; layer += layerHeight) {
+			layerPlane.set(bedNormal, -layer);
+			layerBox.set(
+				new Vector3(0, 0, layer),
+				new Vector3(buildSurface[0], buildSurface[1], layer + layerHeight)
+			);
+			const layerGeometry = new BufferGeometry();
+			const positions: number[] = [];
+			for (let i = 0; i < nonPlanarSurfaces.length; i++) {
+				if (consumedNonPlanarSurfaces[i]) continue;
+				if (layerBox.intersectsBox(nonPlanarSurfaces[i].geometry.boundingBox!)) {
+					activeNonPlanarSurfaces.push([i, nonPlanarSurfaces[i]]);
+					consumedNonPlanarSurfaces[i] = true;
+				}
+			}
+			for (let i = 0; i < activeNonPlanarSurfaces.length; i++) {
+				const [index, surface] = activeNonPlanarSurfaces[i];
+				withheldLines[index].push([]);
+				if (!layerBox.intersectsBox(surface.geometry.boundingBox!)) {
+					activeNonPlanarSurfaces.splice(i, 1);
+					i--;
+					layers.push({ type: 'surface', geometry: surface.geometry });
+					for (const lines of withheldLines[index]) {
+						if (lines.length === 0) continue;
+						const additionalGeometry = new BufferGeometry();
+						additionalGeometry.setAttribute('position', new Float32BufferAttribute(lines, 3));
+						layers.push({ type: 'line', geometry: additionalGeometry });
+					}
+					delete withheldLines[index];
+				}
+			}
+
+			bvh.shapecast({
+				intersectsBounds(box, _isLeaf, _score, _depth, _nodeIndex) {
+					return layerPlane.intersectsBox(box);
+				},
+				intersectsTriangle(target, triangleIndex, _contained, _depth) {
+					if (includedTriangles[triangleIndex] || blacklist[triangleIndex]) return;
+					function intersect(a: Vector3, b: Vector3, targetVector: Vector3) {
+						line.set(a, b);
+						return layerPlane.intersectLine(line, targetVector);
+					}
+					const a = intersect(target.a, target.b, targetVector1);
+					const b = intersect(target.b, target.c, targetVector2);
+					const c = intersect(target.c, target.a, targetVector3);
+
+					function add(a: Vector3, b: Vector3) {
+						for (let i = 0; i < activeNonPlanarSurfaces.length; i++) {
+							const [index, surface] = activeNonPlanarSurfaces[i];
+							const h1 = surface.closestPointToPoint(a);
+							if (
+								h1 &&
+								h1.point.z < a.z &&
+								h1.point.clone().sub(a).angleTo(bedNormal) > maxNonPlanarAngle
+							) {
+								withheldLines[index].at(-1)!.push(a.x, a.y, a.z, b.x, b.y, b.z);
+								return;
+							}
+							const h2 = surface.closestPointToPoint(b);
+							if (
+								h2 &&
+								h2.point.z < b.z &&
+								h2.point.clone().sub(b).angleTo(bedNormal) > maxNonPlanarAngle
+							) {
+								withheldLines[index].at(-1)!.push(a.x, a.y, a.z, b.x, b.y, b.z);
+								return;
+							}
+						}
+						positions.push(a.x, a.y, a.z, b.x, b.y, b.z);
+					}
+
+					if (a && b) {
+						add(a, b);
+					} else if (b && c) {
+						add(b, c);
+					} else if (c && a) {
+						add(c, a);
+					}
+				}
+			});
+			layerGeometry.setAttribute('position', new Float32BufferAttribute(positions, 3));
+			layers.push({ type: 'line', geometry: layerGeometry });
+			console.log('layer', Math.round(layer / layerHeight), positions.length);
+		}
+		layers = [...layers];
 	}
 </script>
 
@@ -126,10 +228,18 @@
 	gridSize={[buildSurface[0], buildSurface[1]]}
 />
 
-{#each layers as surface}
-	<T.Mesh geometry={surface}>
-		<T.MeshMatcapMaterial color={new Color(Math.random() * 0xffffff)} side={DoubleSide} />
-	</T.Mesh>
+{#each layers as { geometry, type }, i}
+	{@const visible = progress >= i / layers.length}
+	{@const color = new Color(0, i / layers.length, 0.2)}
+	{#if type === 'line'}
+		<T.LineSegments {geometry} {visible}>
+			<T.LineBasicMaterial {color} />
+		</T.LineSegments>
+	{:else if type === 'surface'}
+		<T.Mesh {geometry} {visible}>
+			<T.MeshMatcapMaterial {color} side={DoubleSide} />
+		</T.Mesh>
+	{/if}
 {/each}
 
 {#if $stl && false}