bworld/client/rendering/item_renderer.ts

import { TEXTURE_SIZE } from "$/common/constants.ts";
import { BlockRegistry, EverythingRegistry, ItemRegistry } from "$/common/everything_registry.ts";
import { AssetManager } from "$/client/assets.ts";
import { get_sprite_region } from "$/client/sprites.ts";
import type { ItemEntity } from "$/client/entity/item_entity.ts";
import { push_vertex, Texture } from "$/client/renderer/mod.ts";

// how big items on the ground are drawn, minecraft's ground transform
const BLOCK_SCALE = 0.25;
const ITEM_SCALE = 0.5;
// keeps texture lookups off the sprite's edge
const UV_PAD = 0.5;

// each face's corners in drawing order (counter clockwise from outside) on a unit cube, with its shade.
// top, bottom, front, back, left, right, like the chunk mesher
const CUBE_FACES = [
	{ corners: [[0, 1, 1], [1, 1, 1], [1, 1, 0], [0, 1, 0]], shade: 1.0, texture: "top" },
	{ corners: [[0, 0, 0], [1, 0, 0], [1, 0, 1], [0, 0, 1]], shade: 0.5, texture: "bottom" },
	{ corners: [[0, 0, 1], [1, 0, 1], [1, 1, 1], [0, 1, 1]], shade: 0.8, texture: "front" },
	{ corners: [[1, 0, 0], [0, 0, 0], [0, 1, 0], [1, 1, 0]], shade: 0.8, texture: "side" },
	{ corners: [[0, 0, 0], [0, 0, 1], [0, 1, 1], [0, 1, 0]], shade: 0.6, texture: "side" },
	{ corners: [[1, 0, 1], [1, 0, 0], [1, 1, 0], [1, 1, 1]], shade: 0.6, texture: "side" },
] as const;
// sprite end each corner gets, u then v
const CORNER_UVS = [[0, 1], [1, 1], [1, 0], [0, 0]] as const;

// where the extra copies of a bigger stack sit, as fractions of the model's size
const COPY_OFFSETS = [[0, 0, 0], [0.35, 0.2, -0.25], [-0.3, 0.4, 0.2], [0.15, 0.6, 0.35], [-0.2, 0.8, -0.3]];

// minecraft draws more copies of the model the bigger the stack is
function copies(amount: number) {
	if (amount > 48) return 5;
	if (amount > 32) return 4;
	if (amount > 16) return 3;
	if (amount > 1) return 2;
	return 1;
}

// the atlas has to be the current texture
export function render_item_entity(entity: ItemEntity, partial_tick: number) {
	const atlas = AssetManager.instance.get<Texture>("bworld:textures");
	const item_info = EverythingRegistry.get<ItemRegistry>("items", entity.item.type_id);
	const block_info = item_info?.block_id
		? EverythingRegistry.get<BlockRegistry>("blocks", item_info.block_id)
		: undefined;

	// spinning and bobbing, like minecraft's ItemEntityRenderer
	const time = entity.age + partial_tick;
	const angle = time / 20 + entity.bob_offset;
	const bob = Math.sin(time / 10 + entity.bob_offset) * 0.1 + 0.1;
	const { x, y, z } = entity.render_position(partial_tick);

	const scale = block_info ? BLOCK_SCALE : ITEM_SCALE;
	for (let i = 0; i < copies(entity.item.amount); i++) {
		const [ox, oy, oz] = COPY_OFFSETS[i];
		const base = { x: x + ox * scale, y: y + bob + oy * scale * 0.5, z: z + oz * scale };
		if (block_info) {
			push_block(atlas, block_info, base, scale, angle);
		} else {
			push_sprite(atlas, item_texture(entity, item_info), base, scale, angle);
		}
	}
}

// a small cube turned around its middle
function push_block(
	atlas: Texture,
	block: BlockRegistry,
	base: { x: number; y: number; z: number },
	size: number,
	angle: number,
) {
	const sin = Math.sin(angle);
	const cos = Math.cos(angle);
	const corner = (cx: number, cy: number, cz: number) => {
		const lx = (cx - 0.5) * size;
		const lz = (cz - 0.5) * size;
		return [base.x + lx * cos - lz * sin, base.y + cy * size, base.z + lx * sin + lz * cos];
	};

	for (const face of CUBE_FACES) {
		const region = get_sprite_region(block_face_texture(block, face.texture));
		push_textured_quad(atlas, region, face.corners.map(([cx, cy, cz]) => corner(cx, cy, cz)), face.shade);
	}
}

// a flat sprite standing up and turning, drawn from both sides
function push_sprite(
	atlas: Texture,
	texture_id: string,
	base: { x: number; y: number; z: number },
	size: number,
	angle: number,
) {
	const region = get_sprite_region(texture_id);
	const dx = Math.cos(angle) * size / 2;
	const dz = Math.sin(angle) * size / 2;
	const bottom = base.y;
	const top = base.y + size;
	const front = [
		[base.x - dx, bottom, base.z - dz],
		[base.x + dx, bottom, base.z + dz],
		[base.x + dx, top, base.z + dz],
		[base.x - dx, top, base.z - dz],
	];
	push_textured_quad(atlas, region, front, 1);
	// the back is the same quad the other way round, mirrored so it isn't drawn backwards
	push_textured_quad(atlas, region, [front[1], front[0], front[3], front[2]], 1);
}

function push_textured_quad(atlas: Texture, region: { x: number; y: number }, corners: number[][], shade: number) {
	const u0 = (region.x * TEXTURE_SIZE + UV_PAD) / atlas.width;
	const v0 = (region.y * TEXTURE_SIZE + UV_PAD) / atlas.height;
	const u1 = ((region.x + 1) * TEXTURE_SIZE - UV_PAD) / atlas.width;
	const v1 = ((region.y + 1) * TEXTURE_SIZE - UV_PAD) / atlas.height;

	for (const i of [0, 1, 2, 0, 2, 3]) {
		const [px, py, pz] = corners[i];
		const [cu, cv] = CORNER_UVS[i];
		push_vertex(px, py, pz, cu ? u1 : u0, cv ? v1 : v0, shade, shade, shade, 1);
	}
}

function block_face_texture(block: BlockRegistry, face: "top" | "bottom" | "front" | "side") {
	const textures = block.textures;
	if (typeof textures === "string") {
		return textures;
	}
	if ("top" in textures) {
		return face === "top" ? textures.top : face === "bottom" ? textures.bottom : textures.side;
	}
	return face === "front" ? textures.front : textures.side;
}

function item_texture(entity: ItemEntity, item_info: ItemRegistry | undefined) {
	const texture_id = item_info?.texture_id;
	if (typeof texture_id === "function") {
		return texture_id(entity.item);
	}
	return texture_id ?? "engine:missing";
}