bworld/client/workers/translucent_sort.ts

// translucent quad sorting, the way sodium does it (its "translucency sorting"), simplified for quads
// that all face along an axis.
// every chunk's translucent mesh gets a sort type when it's meshed:
// - none: the order can't matter, one quad or all of them in a single plane
// - static: all quads face along one axis, so sorting them by distance along their normal once is right
//   from anywhere the camera can see them
// - dynamic: sorted by distance to the camera, and sorted again only when the camera crosses one of the
//   planes the chunk's quads lie on, the only time the order between them can change (sodium's GFNI)

export type SortType = "none" | "static" | "dynamic";

// same order as the mesher's faces
export const FACE_NORMALS = [
	[0, 1, 0], // top
	[0, -1, 0], // bottom
	[0, 0, 1], // front
	[0, 0, -1], // back
	[-1, 0, 0], // left
	[1, 0, 0], // right
] as const;

export const FACE_AXIS = [1, 1, 2, 2, 0, 0] as const;

// for each quad: its center, and which face it is
export interface TranslucentQuads {
	centers: Float32Array;
	faces: Uint8Array;
	count: number;
}

export function choose_sort_type(quads: TranslucentQuads): SortType {
	if (quads.count <= 1) {
		return "none";
	}

	let axes = 0;
	for (let q = 0; q < quads.count; q++) {
		axes |= 1 << FACE_AXIS[quads.faces[q]];
	}
	// more than one axis, the order depends on where the camera is
	if (axes & (axes - 1)) {
		return "dynamic";
	}

	// quads facing opposite ways on the same axis are never both visible over each other, so only
	// the distance along the axis matters. if they all share a plane, nothing can overlap at all
	const axis = FACE_AXIS[quads.faces[0]];
	const plane = quads.centers[axis];
	for (let q = 1; q < quads.count; q++) {
		if (quads.centers[q * 3 + axis] !== plane) {
			return "static";
		}
	}
	return "none";
}

// the unique coordinates of the planes the quads lie on, per axis, sorted. the camera crossing one of
// these is what triggers a dynamic sort
export function quad_planes(quads: TranslucentQuads): [Float32Array, Float32Array, Float32Array] {
	const sets = [new Set<number>(), new Set<number>(), new Set<number>()];
	for (let q = 0; q < quads.count; q++) {
		const axis = FACE_AXIS[quads.faces[q]];
		sets[axis].add(quads.centers[q * 3 + axis]);
	}
	return sets.map((set) => Float32Array.from(set).sort()) as [Float32Array, Float32Array, Float32Array];
}

// whether moving the camera from a to b crosses any of the planes
export function crosses_planes(planes: [Float32Array, Float32Array, Float32Array], a: number[], b: number[]) {
	for (let axis = 0; axis < 3; axis++) {
		if (a[axis] === b[axis] || planes[axis].length === 0) {
			continue;
		}
		if (count_below(planes[axis], a[axis]) !== count_below(planes[axis], b[axis])) {
			return true;
		}
	}
	return false;
}

function count_below(sorted: Float32Array, value: number) {
	let lo = 0;
	let hi = sorted.length;
	while (lo < hi) {
		const mid = (lo + hi) >> 1;
		if (sorted[mid] < value) {
			lo = mid + 1;
		} else {
			hi = mid;
		}
	}
	return lo;
}

// back to front, for the camera at camera_x/y/z when the sort type is dynamic
export function sort_quads(
	quads: TranslucentQuads,
	sort_type: SortType,
	camera_x: number,
	camera_y: number,
	camera_z: number,
): Uint32Array {
	const { centers, faces, count } = quads;
	if (sort_type === "dynamic") {
		return quad_indices(sort_by_distance(centers, count, camera_x, camera_y, camera_z));
	}

	const order = new Uint32Array(count);
	for (let q = 0; q < count; q++) {
		order[q] = q;
	}
	if (sort_type === "static") {
		// for quads facing the camera, the ones further along their normal are closer to it
		const keys = new Float32Array(count);
		for (let q = 0; q < count; q++) {
			const [nx, ny, nz] = FACE_NORMALS[faces[q]];
			keys[q] = centers[q * 3] * nx + centers[q * 3 + 1] * ny + centers[q * 3 + 2] * nz;
		}
		order.sort((a, b) => keys[a] - keys[b]);
	}
	return quad_indices(order);
}

// dynamic sorting only needs the centers, so resorting doesn't need the whole mesh
export function sort_by_distance(
	centers: Float32Array,
	count: number,
	camera_x: number,
	camera_y: number,
	camera_z: number,
): Uint32Array {
	const order = new Uint32Array(count);
	const distances = new Float32Array(count);
	for (let q = 0; q < count; q++) {
		order[q] = q;
		const dx = centers[q * 3] - camera_x;
		const dy = centers[q * 3 + 1] - camera_y;
		const dz = centers[q * 3 + 2] - camera_z;
		distances[q] = dx * dx + dy * dy + dz * dz;
	}
	return order.sort((a, b) => distances[b] - distances[a]);
}

// two triangles per quad, drawn in the given order
export function quad_indices(order: Uint32Array): Uint32Array {
	const indices = new Uint32Array(order.length * 6);
	for (let i = 0; i < order.length; i++) {
		const v = order[i] * 4;
		const o = i * 6;
		indices[o] = v;
		indices[o + 1] = v + 1;
		indices[o + 2] = v + 2;
		indices[o + 3] = v;
		indices[o + 4] = v + 2;
		indices[o + 5] = v + 3;
	}
	return indices;
}