fcb_wasm/README.md

FlatCityBuf is a library for reading and writing CityJSON with FlatBuffers.

# FlatCityBuf WASM API

This package provides WebAssembly bindings for the FlatCityBuf library, allowing for efficient CityJSON processing in the browser.

## Features

- Read FlatCityBuf files via HTTP
- Query spatial and attribute data
- Convert CityJSON to OBJ format for 3D visualization

## Usage Examples

### Converting CityJSON to OBJ

```javascript
// Import the wasm module
import * as fcb from "fcb_wasm";

// Example CityJSON object
const cityJsonObject = {
  type: "CityJSON",
  version: "1.1",
  transform: {
    scale: [1.0, 1.0, 1.0],
    translate: [0.0, 0.0, 0.0],
  },
  vertices: [
    [0, 0, 0],
    [1, 0, 0],
    [1, 1, 0],
    [0, 1, 0],
    [0, 0, 1],
    [1, 0, 1],
    [1, 1, 1],
    [0, 1, 1],
  ],
  CityObjects: {
    "id-1": {
      type: "Building",
      geometry: [
        {
          type: "Solid",
          lod: "2",
          boundaries: [
            // Cube faces as example
            [[[0, 1, 2, 3]]], // bottom face
            [[[4, 5, 6, 7]]], // top face
            [[[0, 1, 5, 4]]], // front face
            [[[1, 2, 6, 5]]], // right face
            [[[2, 3, 7, 6]]], // back face
            [[[3, 0, 4, 7]]], // left face
          ],
        },
      ],
    },
  },
};

// Convert CityJSON to OBJ
const objContent = fcb.cjToObj(cityJsonObject);

// Create a blob and download link
const blob = new Blob([objContent], { type: "text/plain" });
const url = URL.createObjectURL(blob);

// Create download link
const a = document.createElement("a");
a.href = url;
a.download = "citymodel.obj";
a.textContent = "Download OBJ";
document.body.appendChild(a);
```

The conversion process internally:

1. Takes your JavaScript CityJSON object
2. Deserializes it into a Rust CityJSON struct
3. Processes the 3D geometry to generate OBJ format
4. Returns the OBJ content as a string

### Reading FlatCityBuf via HTTP

```javascript
import * as fcb from "fcb_wasm";

async function loadFcb() {
  // Create an HTTP FlatCityBuf reader
  const reader = await new fcb.HttpFcbReader(
    "https://example.com/path/to/model.fcb"
  );

  // Get CityJSON metadata
  const metadata = await reader.cityjson();
  console.log("CityJSON metadata:", metadata);

  // Select all features and iterate
  const iter = await reader.select_all();
  const count = iter.features_count();
  console.log(`Found ${count} features`);

  // Process features
  let feature;
  while ((feature = await iter.next()) !== null) {
    console.log("Feature:", feature);
  }
}

loadFcb().catch(console.error);
```

## API Reference

### OBJ Conversion

- `cjToObj(cityJsonObject)`: Converts a CityJSON object to OBJ format string. Expects a valid CityJSON object as input.

### FlatCityBuf Reading

- `HttpFcbReader`: Class for reading FlatCityBuf files over HTTP
- `WasmSpatialQuery`: Spatial query helper class
- `WasmAttrQuery`: Attribute query helper class

### For more examples, the demo application is available at <https://github.com/HideBa/flatcitybuf-web-prototype>